Security for the NFSv4 Protocols
draft-dnoveck-nfsv4-security-11
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| Document | Type | Active Internet-Draft (individual) | |
|---|---|---|---|
| Author | David Noveck | ||
| Last updated | 2024-08-29 | ||
| RFC stream | (None) | ||
| Intended RFC status | (None) | ||
| Formats | |||
| Stream | Stream state | (No stream defined) | |
| Consensus boilerplate | Unknown | ||
| RFC Editor Note | (None) | ||
| IESG | IESG state | I-D Exists | |
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draft-dnoveck-nfsv4-security-11
NFSv4 D. Noveck, Ed.
Internet-Draft NetApp
Updates: 8881, 7530 (if approved) 29 August 2024
Intended status: Standards Track
Expires: 2 March 2025
Security for the NFSv4 Protocols
draft-dnoveck-nfsv4-security-11
Abstract
This document describes the core security features of the NFSv4
family of protocols, applying to all minor versions. The discussion
includes the use of security features provided by RPC on a per-
connection basis. Important aspects of the authorization model,
related to the use of Access Control Lists, will be specified in a
separate document.
The current version of the document is intended, in large part, to
result in working group discussion regarding existing NFSv4 security
issues and to provide a framework for addressing these issues and
obtaining working group consensus regarding necessary changes.
When the resulting documents (i.e. this document and one derived from
the separate ACL specification) are eventually published as RFCs,
they will, by updating these documents, supersede the description of
security appearing in existing minor version specification documents
such as RFC 7530 and RFC 8881,
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at https://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on 2 March 2025.
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Table of Contents
1. Overview . . . . . . . . . . . . . . . . . . . . . . . . . . 5
1.1. Document Motivation . . . . . . . . . . . . . . . . . . . 5
1.1.1. Original Motivation . . . . . . . . . . . . . . . . . 5
1.1.2. Need for Respecification of ACL-related
Functionality . . . . . . . . . . . . . . . . . . . . 6
1.1.3. Further Issues that Needed to be Addressed . . . . . 7
1.2. Document Annotation . . . . . . . . . . . . . . . . . . . 9
1.3. Compatibility and Compliance Issues . . . . . . . . . . . 10
1.3.1. Dealing with Recognized Mistakes . . . . . . . . . . 13
1.3.2. Dealing with Pervasive Uncertainty . . . . . . . . . 14
2. Requirements Language . . . . . . . . . . . . . . . . . . . . 15
2.1. Keyword Definitions . . . . . . . . . . . . . . . . . . . 15
2.2. Special Considerations . . . . . . . . . . . . . . . . . 15
3. Introduction to this Update . . . . . . . . . . . . . . . . . 16
3.1. Per-connection Security Features . . . . . . . . . . . . 16
3.2. Handling of Multiple Minor Versions . . . . . . . . . . . 17
3.3. Handling of Minor-version-specific features . . . . . . . 18
3.4. Features Needing Extensive Clarification . . . . . . . . 20
3.5. Process Going Forward . . . . . . . . . . . . . . . . . . 23
4. Introduction to NFSv4 Security . . . . . . . . . . . . . . . 25
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4.1. NFSv4 Security Terminology . . . . . . . . . . . . . . . 27
4.2. NFSv4 Security Scope Limitations . . . . . . . . . . . . 30
5. Authorization-related Attributes . . . . . . . . . . . . . . 33
5.1. Format of Id Strings in Authorization-related
Attributes . . . . . . . . . . . . . . . . . . . . . . . 36
5.2. Table of Authorization-related Attributes . . . . . . . . 41
5.3. POSIX-oriented Authorization-related Attributes . . . . . 43
5.3.1. The Mode Attribute (v4.0) . . . . . . . . . . . . . . 43
5.3.2. The Mode_set_masked Attribute (v4.1) . . . . . . . . 46
5.3.3. The Owner Attribute (v4.0) . . . . . . . . . . . . . 46
5.3.4. The Owner_group Attribute (v4.0) . . . . . . . . . . 46
5.3.5. Issues with Named Attribute Directories . . . . . . . 46
5.3.6. POSIX Authorization for Named Attribute
Directories . . . . . . . . . . . . . . . . . . . . . 49
5.4. ACL-based Authorization-related Attributes . . . . . . . 50
5.5. Authorization-related Attributes for MAC . . . . . . . . 50
5.5.1. The Seclabel Attribute (v4.2) . . . . . . . . . . . . 50
6. Introduction to Access Control List . . . . . . . . . . . . . 50
6.1. Overview of Existing NFSv4 ACLs . . . . . . . . . . . . . 50
6.2. Previous Treatment of ACLs . . . . . . . . . . . . . . . 51
6.3. New Approach to Treatment of ACLs . . . . . . . . . . . . 52
7. Authorization in General . . . . . . . . . . . . . . . . . . 53
8. User-based File Access Authorization . . . . . . . . . . . . 54
8.1. Attributes for User-based File Access Authorization . . . 54
8.2. Handling of Multiple Parallel File Access Authorization
Models . . . . . . . . . . . . . . . . . . . . . . . . . 55
8.3. POSIX Authorization Model . . . . . . . . . . . . . . . . 57
8.4. ACL-based Authorization Model . . . . . . . . . . . . . . 57
9. Common Considerations for Both File Access Models . . . . . . 58
9.1. Handling of ACCESS and OPEN Operations . . . . . . . . . 58
9.2. Server Considerations . . . . . . . . . . . . . . . . . . 60
9.3. Client Considerations . . . . . . . . . . . . . . . . . . 65
10. Combining Authorization Models . . . . . . . . . . . . . . . 66
11. Labelled NFS Authorization Model . . . . . . . . . . . . . . 66
12. State Modification Authorization . . . . . . . . . . . . . . 67
13. Other Uses of Access Control Lists . . . . . . . . . . . . . 68
14. Identification, Authentication, and Related Services . . . . 68
14.1. RPC Security Flavors . . . . . . . . . . . . . . . . . . 68
14.1.1. RPCSEC_GSS and Security Services . . . . . . . . . . 68
14.2. Identification vs. Authentication . . . . . . . . . . . 71
14.3. Items to be Identified . . . . . . . . . . . . . . . . . 71
14.4. Authentication Provided by specific RPC Auth Flavors . . 73
14.5. Authentication Provided by other RPC Security
Services . . . . . . . . . . . . . . . . . . . . . . . . 73
15. Security of Data in Flight . . . . . . . . . . . . . . . . . 73
15.1. Data Security Provided by Services Associated with Auth
Flavors . . . . . . . . . . . . . . . . . . . . . . . . 74
15.2. Data Security Provided for a Connection by RPC . . . . . 74
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16. Security Negotiation . . . . . . . . . . . . . . . . . . . . 74
16.1. Security Triple Negotiation . . . . . . . . . . . . . . 77
16.1.1. NFSv4Security Tuples . . . . . . . . . . . . . . . . 77
16.1.2. SECINFO and SECINFO_NO_NAME . . . . . . . . . . . . 77
16.1.3. Security Error . . . . . . . . . . . . . . . . . . . 78
16.2. Dealing with Multiple Connections . . . . . . . . . . . 82
17. Future Security Needs . . . . . . . . . . . . . . . . . . . . 83
17.1. Desirable Additional Security Facilities . . . . . . . . 83
18. Security Considerations . . . . . . . . . . . . . . . . . . . 83
18.1. Changes in Security Considerations . . . . . . . . . . . 84
18.1.1. Wider View of Threats . . . . . . . . . . . . . . . 85
18.1.2. Connection-oriented Security Facilities . . . . . . 86
18.1.3. Necessary Security Changes . . . . . . . . . . . . . 86
18.1.4. Compatibility and Maturity Issues . . . . . . . . . 87
18.1.5. Discussion of AUTH_SYS . . . . . . . . . . . . . . . 87
18.2. Security Considerations Scope . . . . . . . . . . . . . 88
18.2.1. Discussion of Potential Classification of
Environments . . . . . . . . . . . . . . . . . . . . 88
18.2.2. Discussion of Environments . . . . . . . . . . . . . 89
18.2.3. Insecure Environments . . . . . . . . . . . . . . . 90
18.3. Major New Recommendations . . . . . . . . . . . . . . . 90
18.3.1. Recommendations Regarding Security of Data in
Flight . . . . . . . . . . . . . . . . . . . . . . . 90
18.3.2. Recommendations Regarding Client Peer
Authentication . . . . . . . . . . . . . . . . . . . 91
18.3.3. Recommendations Regarding Superuser Semantics . . . 92
18.3.4. Issues Regarding Valid Reasons to Bypass
Recommendations . . . . . . . . . . . . . . . . . . . 92
18.4. Threat Analysis . . . . . . . . . . . . . . . . . . . . 93
18.4.1. Threat Analysis Scope . . . . . . . . . . . . . . . 93
18.4.2. Threats based on Credential Compromise . . . . . . . 93
18.4.3. Threats Based on Rogue Clients . . . . . . . . . . . 95
18.4.4. Threats Based on Rogue Servers . . . . . . . . . . . 96
18.4.5. Data Security Threats . . . . . . . . . . . . . . . 97
18.4.6. Authentication-based threats . . . . . . . . . . . . 97
18.4.7. Disruption and Denial-of-Service Attacks . . . . . . 100
19. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 101
19.1. New Authstat Values . . . . . . . . . . . . . . . . . . 101
19.2. New Authentication Pseudo-Flavors . . . . . . . . . . . 102
20. References . . . . . . . . . . . . . . . . . . . . . . . . . 102
20.1. Normative References . . . . . . . . . . . . . . . . . . 102
20.2. Informative References . . . . . . . . . . . . . . . . . 104
Appendix A. Changes Being Made . . . . . . . . . . . . . . . . . 105
A.1. Motivating Security Changes . . . . . . . . . . . . . . . 105
A.1.1. Fundamental Security Changes . . . . . . . . . . . . 105
A.1.2. ACL-Related Changes . . . . . . . . . . . . . . . . . 108
A.2. Need for Clarifying Changes . . . . . . . . . . . . . . . 108
A.3. Addressing the Need for Clarifying Changes . . . . . . . 111
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Appendix B. Issues for which Consensus Needs to be
Ascertained . . . . . . . . . . . . . . . . . . . . . . . 114
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 123
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 124
1. Overview
These documents, including this document and the companion ACL
document [I-D.dnoveck-nfsv4-acls], are intended to form the basis for
a new description of NFSv4 security applying to all current NFSv4
minor versions. The motivation for these new documents and the need
for major improvements in the description of NFSv4 security are
explained in Section 1.1.
Because these documents anticipate making major changes in material
covered in previous standards-track RFCs, extensive working group
discussion will be necessary to make sure that there is a working
group consensus to make the changes being proposed. The changes
needed include the major improvements mentioned in Section 1.1.1 and
the changes necessary to suitably describe features currently
described in a way that is inappropriate in Standards Track
documents, for reasons laid out in Section 3.4. A large part of the
material necessary to accomplish this set of goals will appear in
[I-D.dnoveck-nfsv4-acls] and its successors.
The need to make major changes in the security approach for three
Proposed Standards ([RFC7530] for NFSv4.0, [RFC8881] for NFSv4.1, and
[RFC7862] for NFSv4.2) raises troubling issues. These changes are
necessary for reasons explained in Section 1.1. These troubling
issues often concern compatibility and compliance issues as described
in Section 1.3.
1.1. Document Motivation
1.1.1. Original Motivation
A new treatment of security is necessary because:
* Previous treatments paid insufficient attention to security issues
regarding data in flight, assuming that security could reasonably
be provided on an optional basis, to secure particular portions of
the server namespace.
* The presentation of AUTH_SYS as an "OPTIONAL means of
authentication" obscured the significant security problems that
come with its use.
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* The security considerations sections of existing minor version
specifications contain no threat analyses and focus on particular
security issues in a way that obscures, rather than clarifying,
the issues that need to be addressed, while implying, often
incorrectly, that the existing security features are adequate to
the need.
* The availability of connection-oriented RPC security features,
such as those provided by RPC-with-TLS (described in [RFC9289])
provides facilities that NFSv4 clients and servers will need to
use to provide security for data in flight and mitigate the lack
of user authentication when AUTH_SYS is used.
1.1.2. Need for Respecification of ACL-related Functionality
[Consensus Meeded (Item #67a), Through end of section]:
Review of the existing specifications has made it apparent that the
handling of ACLs has not been described in the detail normally
necessary to make it possible to implement interoperating clients and
servers. Because of the broad license granted by previous
specifications to allow server implementations to choose how to
behave, clients are forced to accept a broad range of server
behaviors, with no way of reliably determining the server behavior
actually implemented.
One important reason that such extensive changes are now necessary
derives from a disagreement among working group participants as to
the purpose of ACL support with one group of participants requiring
new functionality matching that of Windows ACLs with another major
group uninterested in such features and unwilling to devote the
effort involved in providing server-side support for them.
Within the NFSv4 architecture, such situations are normally dealt by
defining one or more optional features, allowing different servers to
provide different levels of support, with the client able to
determine whether a selected server has the desired level of support.
Unfortunately, for reasons that remain unclear, this approach was not
followed in writing RFCs 3530, 5661, 7530, and 8881. Instead, the
full definition of the NFSv4 ACL was defined as an OPTIONAL attribute
with no clear definition of useful support subsets or way of testing
the support level. Essentially, each ACE mask bit was made its own
optional feature. To further complicate things, the keyword "SHOULD"
was frequently used, raising the possibility that the support might
be other than described. This made it impossible for clients to
determine the level of ACL support provided, or to choose whether to
use a server based on the level of support provided.
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As a result, we now need to provide a core ACL model that all clients
can rely upon while providing the ability for server to implement
useful extensions. This work will be done within the companion
document devoted to ACLs [I-D.dnoveck-nfsv4-acls]. Given the need to
thoroughly revise the discussion of ACLs while avoiding prohibited
XDR changes and troublesome implementation incompatibilities, the
working group will need information about implementations of the vast
range of possibilities allowed, inadvertently or not, by previous
treatments of the matter.
In view of these difficulties in the existing specification of acl-
related semantics a new approach toward the specification will be
adopted in [I-D.dnoveck-nfsv4-acls] and its successors, although the
goal, of allowing a range of potential ACL implementations, will
remain the same, as will the XDR used to represent the relevant
protocol elements. This XDR encompasses many OPTIONAL extensions to
the UNIX ACLs whose core semantics need to be supported by all
servers supporting ACLs.
1.1.3. Further Issues that Needed to be Addressed
As work has proceeded, additional important issues were discovered.
Of prime importance are following issues related to the
classification of attributes:
* [Consensus Needed (Item #57a), Through end of bulleted item]: The
attributes Owner, Owner_group, and Mode needed to be made REQUIRED
since clients need this information and not supporting these
attributes would create troublesome interoperability issues.
Previous specifications explicitly allowed servers to support none
of these and even discussed a supposed need to support use of
servers that supported none of the above attributes and none of
the acl-related attributes either. To continue in this way, would
overcomplicate the specification and create difficult
interoperability issues to support implementations that have
little practical purpose.
* [Consensus Needed (Item #59a), Through end of bulleted item]: The
change in the format of specification of user ids and group ids,
made as part of transition from NFSv3 to NFSv4, requires
significant modification, in order to address a serious
underspecification that creates the possibility of additional
security vulnerabilities.
The change to a string-format representation of these ids was
intended to provide a way to allow the protocol to escape the
restrictions inherent in the previous representation of these ids
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by 32-bit unsigned integers. However, as things have developed,
for practical purposes, these restrictions remain in effect since
the associated ids within POSIX are still 32-bit unsigned integers
and the working group has no way of prompting changes necessary to
implement a more flexible approach..
While the use of ids of the form "name@domain" might help achieve
the original goal if used together with multiple domains, that
would require server support to simultaneously support multiple
Kerberos realms, not yet available.
In the case in which AUTH_SYS is used, there is no reliable way to
effect the mapping between names and numeric ids. In the existing
specifications, the provision of this mapping is treated as an
implementation matter without protocol support. This has proved
unacceptable because any implementation advice would require
compatible implementations on the client and server, and would
allow attacks via interference with the mapping. Although use of
numeric ids instead is possible, it has been unfairly stigmatized
in previous specifications, with the suggestion made that use of
numeric strings somehow compromises the original intent of the
shift, which nevertheless, is in no way undercut by the use of
strings having numeric values as long as the "name@domain" format
is still available when useful and necessary.
In the new treatment (in Section 5.1), the mapping between name
and numeric ids is the responsibility of Kerberos and the use of
numeric strings is available for the AUTH_SYS case, avoiding any
need for mapping between names and numeric ids in this common
case. In order to accommodate previous implementations, such
mapping is allowed, although it creates some unfortunate security
vulnerabilities that are best avoided.
* [Consensus Needed (Item #67b), Through end of bulleted item]:
Because the original intention, to somehow allow support for draft
POSIX ACLs together with features derived from Windows ACLs not
usable by POSIX, turned out not to be realizable, we are
restructuring the description of ACLs to support multiple ACL
models.
For existing minor versions, the NFSv4 ACL model will remain the
only supported form of ACLs and will be supported using the acl,
dacl, and sacl attributes. It will be possible to support other
ACL models, such as those supporting draft POSIX ACLs, using other
attributes with their own forms of Access Control Lists and Access
Control Elements, as defined by future standards-track extension
documents.
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In addition to these two major issues, the following other issues
were found and needed to be addressed in this document:
* Although involved in the inadequate specification of NFSv4 ACL
handling, a separate source of issues that need to be addressed
involves use of the term "SHOULD", sometimes meaning "should" and
sometimes meaning MAY, but never including discussion of the harm
caused by not following the recommendation, or a discussion of
what might be valid reasons to ignore the recommendation.
* The discussion of handling of modes is limited to forward-slope
modes and ignores the possibility of reverse-slope modes.
For definitions of these terms, see Section 4.1.
For examples of the necessary changes, see Sections 10.3 and 10.7
(with included subsections) of [I-D.dnoveck-nfsv4-acls]
* There are a number of clarifications/corrections to the
description of ACE mask bits appearing in Section 7.2 of
[I-D.dnoveck-nfsv4-acls] and its constituent sub-sections.
1.2. Document Annotation
In order to make progress on difficult issues which will require that
changes be made in the existing handling of security issues,
including many whose resolution would potentially involve
compatibility issues with existing implementations, the author has
tried his best to resolve these issues, even though there is no
assurance that the resolution adopted by consensus will match the
author's current best efforts. To provide possible resolutions that
might be the basis of discussion while not foreclosing other
possibilities, proposed changes are organized into a series of
consensus items, which are listed in Appendix B and in a
corresponding Appendix in the acl document.
For such pending issues, the following annotations will be used:
* A paragraph headed "[Author Aside]:", provides the author's
comments about possible changes and will probably not appear in an
eventual RFC.
This paragraph can specify that certain changes within the current
section are to be implicitly considered as part of a specific
consensus item.
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The paragraph can indicate that all unannotated material in the
current section is to be considered either the previous treatment
or the proposed replacement text for a specific consensus item.
* A paragraph headed "[Consensus Needed (Item #NNx)]:", provides the
author's preferred treatment of the matter and will only appear in
the eventual RFC if working group consensus on the matter is
obtained, allowing the necessary changes to be made permanent,
without being conditional on a future consensus.
The item id, represented above by "NNx" consists of a number
identifying the specific consensus item and letter which is unique
to appearance of that consensus item in a particular section. In
cases in which a pending item is cited with no part of the
discussion appearing in the current section, an item id of the
form "#NN" is used.
* A paragraph headed "[Previous Treatment]:", indicates text that is
provided for context but which the author believes, need not
appear in the eventual RFC, because it is expected to be
superseded by a corresponding consensus item.
The corresponding consensus item is often easily inferred, but can
be specified explicitly, as it is for items associated with the
consensus item itself.
Each of the annotations above can be modified by addition of the
phrase, "Including List" to indicate that it applies to a following
bulleted list as well as the current paragraph or the phase "Entire
Bulleted Item" to indicate it applies to all paragraphs within a
specific bulleted item.
1.3. Compatibility and Compliance Issues
Changes that need to be adopted in this document might need to
eventually change the behavior of clients and servers that were
written to conform to earlier protocol specifications. There are two
important classes of such changes discussed in Sections 1.3.1 and
1.3.2 below.
As [RFC2119] was originally conceived, compliance and compatibility
issues were tightly bound together so that a change to compliance
specifications would inevitably give rise to compatibility issues.
However, over time, behaviors have come to be denigrated by use of
the terms "SHOULD NOT" and "MUST NOT" to warn implementors of a harm
deriving from insecure operation rather than peer incompatibility.
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When making changes in compliance requirements/recommendations we
need to deal with the possibility that, in changing the
specification, we might cause a previously compliant implementation
to become non-compliant. Some implementers take the view that the
compliance status of their implementations is of less importance than
other considerations such as compatibility with local file system
semantics. Others feel it is important to maintain the compliance
status of existing implementations. In any case, the working group
has been reluctant, when making such necessary changes, to make
previously compliant implementations non-compliant, and will try not
to do in cases in which it is known or can reasonably be expected
that such implementations exist.
Although there is no way to be sure about the non-existence of such
implementations, the working group has made judgments about what
implementations are likely to exist. For example, when
internationalization for NFSv4.0 was changed in the transition to
[RFC7530], many previously non-compliant server implementations
became officially compliant and there was a potential for conflict
with implementations compliant with RFC3530 [RFC3530], if any such
implementations existed.
In that particular case, it was decided that no such
RFC3530-compliant server implementations existed and there was no
need to accommodate servers whose internationalization was written to
conform to [RFC3530] since no such servers existed and there were no
client implementations expecting such behavior. As a result, no
actual implementations became non-compliant as a result of this
necessary shift.
In the corresponding cases dealt with in this document, the situation
is more difficult since it may be harder to determine the actual
behavior of all existing implementations, since the authors might no
longer be actively involved with implementation issues. However, it
will be necessary to warn implementors of the negative consequences
of certain behaviors without going so far as to declare these
practices non-compliant. For the most part, this document will use
the terms "SHOULD" and "SHOULD NOT" to draw attention to the problems
with troublesome behaviors that previous specifications mentioned
with no indication of the problems with them. In such contexts, it
is made clear that the need to maintain existing patterns of
interoperation is a valid reason to bypass the normative term. The
intention is to continue to allow previously acceptable
implementations to be considered compliant while not placing the
troublesome behaviors on the same levels as other alternatives as
would happen if we used the terms "MAY" or "OPTIONAL" or continued to
use some of the unfortunate practices discussed in Section 1.3.2.
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This approach allows implementations to accept input from peers
written in accord with previous specification while not obligating
them to do so. The intent is to allow a transition to newer, better
behaviors over time as client and server policies evolve. However,
the specifics of this anticipated transition will vary:
* In the case of the issues dealt with in Section 1.3.1, the focus
is on making implementers aware of the security problems with
practices previously considered acceptable.
It is specified that these practices SHOULD NOT be used by clients
or allowed by servers in order to draw attention to the security
problems with them. At the same time, the discussion of valid
reasons to bypass these recommendations allows them to continue to
be used while the infrastructure is developed to make their
replacements easy to use.
Servers are encouraged to adopt policies foreclosing client use
but not obligated to do so.
* In the case of the issues dealt with in Section 1.3.2, the focus
is different. In these cases, while we anticipate making changes
in compliance specifications, there is no need to address a
specific set of troublesome practices. Instead, the problem to be
addressed is the vast range of allowable server behaviors
previously defined as allowed, although not necessarily
explicitly. This server-centered approach has made compatibility
a hit-or-miss matter, requiring serious consideration of the
question of what specific instances of multiple server behaviors
need to be allowed and how clients can find out the choices that
servers have made and possibly affect them, also making
appropriate provision, as with other optional features, about how
to adapt to the server's behavioral choices or to decide that they
do not meet its needs.
It appears that this approach was motivated, at least in large
part, by the desire to fully support much of Windows ACL semantics
while accommodating Unix servers incapable of providing much of
that functionality. As a result, the working group will need to
provide a way for the server to explicitly opt out of providing
Windows functionality that it cannot provide and that Linux
clients are not prepared to use.
In addition, the working group will have to restrict, or at least
better organize, sever behavioral choices related to the handling
of NFsv4 ACLs.
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1.3.1. Dealing with Recognized Mistakes
As an example, we consider the handling of AUTH_SYS (presumably in
the clear, without client peer authentication), described in previous
specifications as "OPTIONAL". While the authors might have only been
indicating that servers could choose not to support it, and that
clients had to be prepared for it not to be supported by the server,
the likely import of this designation, for clients, was to indicate
to implementers that they could choose to use AUTH_SYS and that the
authors of the spec were, by not recommending otherwise, as we might
now feel they should have done, indicating there were no issues
whereby using AUTH_SYS in this form had the capacity to cause harm.
As a result, clients using AUTH_SYS in this way were to be considered
specification- compliant and were not warned of the real security
issues created by this use.
As the protocol was implemented and further developed in subsequent
minor versions, the specifications, which had Security Considerations
sections that did not contain threat analyses, had no place to
indicate to users and implementers of NFSv4 the security problems
that come with AUTH_SYS use and it continued to be used heavily while
encryption was only available to clients using RPCSEC_GSS and left as
a choice that was not frequently used, despite the security issues
that this raised.
We are now at a point at which we have to recognize that a mistake
was made in this regard and have to be clear about the security
issues present in many common implementations of the protocol. As we
seek to do this, it is important to understand the compliance effects
of doing so. This document, when adopted, will supersede previous
specifications which took a different approach. Although it might,
given the security issues with AUTH_SYS, make sense to say that it
"MUST NOT" be used in that way, the working group is very reluctant
to retroactively declare previously compliant behavior non-compliant,
even in this case where there is good reason to do so. A more likely
approach is to say that clients "SHOULD NOT" do this while making it
clear that the difficulty of changing existing implementations and
potential compatibility with existing peers are valid reasons to
bypass the recommendation. Servers are, as before, allowed to
support AUTH_SYS but "SHOULD" only do so when using additional
security facilities that make this safe. The effect would be to
create a clear set of recommendations to new implementations while
providing for continued use of previously compliant implementations
to continue as needed,
This approach gives rise to compatibility issues, but leaves them to
implementors and users to resolve, while making clear the security
issues with the old approach.
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1.3.2. Dealing with Pervasive Uncertainty
Addressing the issues described in Section 3.4 raises similar issues.
In this case as well, we will need to make changes in implementation
behavior going forward and try to do so without declaring existing
behavior non-compliant. However, we cannot, as we did in the example
above, identify a specific set of bad choices, and try to come up
with replacements, that reflect our new understanding of security
issues.
In this case, it is necessary, as has been done in other cases in
which NFSv4 tried to accommodate the needs of both UNIX and Windows,
to decide what part of the non-UNIX semantics is required and which
part is an optional extension, which UNIX-oriented clients would not
be likely to use and UNIX-oriented servers might not support. When
this sort of issue is not given the attention it needs, problems can
result, although the nature and severity of the problems depend on
the specifics of feature.
In the case of byte-range locks, servers were given a choice as to
implement byte-range locks in an advisory or mandatory fashion.
Although servers could choose to do either, there was no way for a
client to determine which of these two incompatible semantic models
the server implemented. As a result, UNIX-based clients and
applications assumed the advisory model and could not interoperate
successfully with servers implementing the mandatory model.
Applications requiring mandatory semantics could only interoperate
with a small set of servers which chose to support the mandatory
model that has very few users. The normal way of dealing with
situations like this is to make the server's behavioral choice
available to the client as an attribute, as is provided for in
[RFC8178]
[Consensus Needed (Item #67c)]: In the case of ACLs, we have a
difficult situation to resolve. Instead of having a small set of
individual mistakes which can now be recognized as such, we have a
situation in which the existing specifications have created an
unacceptable interoperability situation in relation to ACL
implementations. Existing specifications have not paid proper
attention to the need to make decisions in the face of disagreements
regarding proper server behavior and have in various ways avoided the
need to compromise and reach a reasonable consensus but instead have
made it the job of the specifications to consider valid any remotely
similar server implementations as valid, leaving clients little that
could do other than to accept a wide range of server behavior as
valid, simply because it was chosen by the server. How this set of
issues is to be addressed is discussed in Section 1.3 of
[I-D.dnoveck-nfsv4-acls].
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2. Requirements Language
2.1. Keyword Definitions
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as specified in BCP 14 [RFC2119]
[RFC8174] when, and only when, they appear in all capitals, as shown
here.
2.2. Special Considerations
Because this document needs to revise previous treatments of its
subject, it will need to cite previous treatments of issues that now
need to be dealt with in a different way. This will take the form of
quotations from documents whose treatment of the subject is being
obsoleted, most often as direct quotation but sometimes as indirect
ones as well.
Paragraphs headed "[Previous Treatment] or otherwise annotated as
having that status, as described in Section 1, can be considered
quotations in this context.
Such treatments in quotations will involve use of these BCP14-defined
terms in two noteworthy ways:
* The term may have been used inappropriately (i.e not in accord
with [RFC2119]), as has been the case for the "RECOMMENDED"
attributes, which are in fact OPTIONAL.
In such cases, the surrounding text will make clear that the
quoted text does not have a normative effect.
Some specific issues relating to this case are described below in
Section 8.1.
* The term may been used in accord with [RFC2119], although the
resulting normative statement is now felt to be inappropriate.
In such cases, the surrounding text will need to make clear that
the text quoted is no longer to be considered normative, often by
providing new text that conflicts with the quoted, previously
normative, text.
An important instance of this situation is the description of
AUTH_SYS as an "OPTIONAL" means of authentication". For detailed
discussion of this case, see Sections 14 and 18.1.5
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3. Introduction to this Update
There are a number of noteworthy aspects to the updated approach to
NFSv4 security presented in this document:
* There is a major rework of the security framework to take
advantage of work done in [RFC9289], as described in Section 1.1.
NFSv4 security is still built on RPC, as had been done previously.
However, it is now able to take advantage of security-related
facilities provided on a per-connection basis. For more
information about this transformation, see Section 3.1.
For an overview of changes made so far as part of this rework, see
Appendix A.1.
* This document deals with all minor versions together, although
there is a need for exceptions to deal with, for example, pNFS
security.
For more detail about how minor version differences will be
addressed, see Sections 3.2 and 3.3.
* There is a new Security Considerations section including a threat
analysis.
* There has been extensive work to clarify the multiple types of
authorization within NFSv4 and deal more completely with the co-
ordination of ACL-based and mode-based file access authorization.
this work is discussed in Section 3.4
3.1. Per-connection Security Features
There are a number of security-related facilities that can be
provided on a per-connection basis, eliminating the need to provide
such support on a per-request basis, based on the RPC auth-flavor
used.
These will initially be provided, in most cases, by RPC-with-TLS but
similar facilities might be provided by new versions of existing
transports or new RPC transports.
* The transport or a layer above it might provide encryption of
requests and replies, eliminating the need for privacy and
integrity services to be negotiated later and applied on a per-
request basis.
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While clients might choose to establish connections that provide
such encryption, servers can establish policies allowing access to
certain pieces of the namespace using such security facilities, or
limiting access to those providing privacy, allowing the use of
either per-connection encryption or privacy services provided by
RPCSEC_GSS.
* The transport or a layer above it might provide mutual
authentication of the client and server peers as part of the
establishment of the connection This authentication is distinct
from the mutual authentication of the client user and server peer,
implemented within the RPCSEC_GSS framework.
This form of authentication is of particular importance when the
server allows the use of the auth-flavors AUTH_SYS and AUTH_NONE,
which have no provision for the authentication of the user
requesting the operation.
While clients might choose, on their own, to establish connections
without such peer authentication, servers can establish policies
limiting access to certain pieces of the namespace without such
peer authentication or only allowing it when using RPCSEC_GSS.
To enable server policies to be effectively communicated to clients,
the security negotiation framework now allows connection
characteristics to be specified using pseudo-flavors returned as part
of the response to SECINFO and SECINFO_NONAME. See Section 16 for
details.
3.2. Handling of Multiple Minor Versions
In some cases, there are differences between minor versions in that
there are security-related features, not present in all minor
versions.
To deal with this issue, this document will focus on a few major
areas listed below which are common to all minor versions.
* File access authorization (discussed in Section 8) is the same in
all minor versions together with the identification/
authentication infrastructure supporting it (discussed in
Section 14) provided by RPC and applying to all of NFS.
An exception is made regarding labelled NFS, an optional feature
within NFSv4.2, described in [RFC7862]. This is discussed as a
version-specific feature in this document in Section 11.
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* Features to secure data in-flight, all provided by RPC, together
with the negotiation infrastructure to support them are common to
all NFSv4 minor versions, are discussed in Section 16.
However, the use of SECINFO_NONAME, together with changes needed
for connection-based encryption, paralleling those proposed here
for SECINFO, is treated as a version-specific feature and, while
mentioned here, will be fully documented in new NFSv4.1
specification documents.
* The protection of state data from unauthorized modification is
discussed in Section 12) is the same in all minor versions
together with the identification/ authentication infrastructure
supporting it (discussed in Section 14 by security services such
as those provided by RPC-with-TLS.
It needs to be noted that state protection based on RPCSEC_GSS is
treated as a version-specific feature and will continue to be
described by [RFC8881] or its successors. Also, it needs to be
noted that the use of state protection was not discussed in
[RFC7530].
3.3. Handling of Minor-version-specific features
There are a number of areas in which security features differ among
minor versions, as discussed below. In some cases, a new feature
requires specific security support while in others one version will
have a new feature related to enhancing the security infrastructure.
How such features are dealt with in this document depends on the
specific feature.
* In addition to SECINFO, NFSv4.1 added a new SECINFO_NONAME
operation, useful for pNFS file as well as having some non-pNFS
uses.
While the enhanced description of SECINFO mentions SECINFO_NONAME,
this is handled as one of a number of cases in which the
description has to indicate that different actions need to be
taken for different minor versions.
The definitive description of SECINFO_NONAME, now appearing in
[RFC8881] needs to be modified to match the description of SECINFO
appearing in this document. It is expected that this will be done
as part of the rfc5661bis process.
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The security implications of the security negotiation facilities
as a whole will be addressed in the security considerations
section of this document.
* The OPTIONAL pNFS feature added in NFSv4.1 has its own security
needs which parallel closely those of non-pNFS access but are
distinct, especially when the storage access protocol used are not
RPC protocols. As a result, these needs and the means to satisfy
them are not discussed in this document.
The definitive description of pNFS security will remain in
[RFC8881] and its successors (i.e. the eventual rfc5661bis
document). However, because pNFS security relies heavily on the
infrastructure discussed here, it is anticipated that the new
treatment of pNFS security will deal with many matters by
referencing the overall NFS security document.
The security considerations section of rfc5661bis will deal with
pNFS security issues.
* In addition to the state protection facilities described in this
document, NFS has another set of such facilities that are only
implemented in NFSv4.1.
While this document will discuss the security implications of
protection against state modification, it will not discuss the
details of the NFSv4.1-specific features to accomplish it.
* The additional NFSv4.1 acl attributes, sacl and dacl, are
discussed in this document, together with the ACL inheritance
features they enable.
As a result, the responsibility for the definitive description of
these attributes will move to overall NFS security document, with
the fact that they are not available in NFSv4.0 duly noted. While
these attributes will continue to be mentioned in NFSv4.1
specification documents, the detailed description appearing in
[RFC8881] will be removed in successor documents.
* Both NFSv4.0 and NFSv4.1 specifications discussed the coordination
of the values the mode and ACL-related attributes. While the
treatment in [RFC8881] is more detailed, the differences in the
approaches are quite minor.
[Consensus Item #25a]: This document will provide a unified
treatment of these issues, which will note any differences of
treatment that apply to NFSv4.0. Changes applying to NFSv4.2 will
also be noted.
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As a result, this document will override the treatment within
[RFC7530] and [RFC8881]. This material will be removed in the
rfc5661bis document suite and replaced by a reference to the
treatment in the NFSv4 security RFC.
* The protocol extension defined in [RFC8257], now part of NFSv4.2,
is also related to the issue of co-ordination of acl and mode
attributes and will be discussed in that context.
Nevertheless, the description in [RFC8257] will remain definitive.
* The NFSv4.1 attribute set-mode-masked attribute is mentioned
together with the other attributes implementing the POSIX
authorization model.
Because this attribute. while related to security, does not
substantively modify the security properties of the protocol, the
full description of this attribute, will continue to be the
province of the NFSv4.1 specification proper.
* There is a brief description of the v4.2 Labelled NFS feature in
Section 11. Part of that description discusses the limitations in
the description of that feature within [RFC7862].
Because of some limitations in the description, it is not possible
to provide an appropriate security considerations section for that
feature in this document.
As a result, the responsibility for providing an appropriate
Security Considerations section remains, unrealized for now, with
the NFSv4.2 specification document and its possible successors.
3.4. Features Needing Extensive Clarification
For a number of authorization-related features, the existing
descriptions are inadequate for various reasons:
* In the description of the use of the mode attribute in
implementing the POSIX-based authorization model, critical pieces
of the semantics are not mentioned, while, ironically, the
corresponding semantics for ACL-based authorization are discussed.
This includes the authorization of file deletion and of
modification of the mode, owner and owner-group attributes. For
ACL-based authorization, there is an attempt to provide the
corresponding description.
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* The description of authorization for ACLs is more complete but it
needs further work, because the previous specifications make
extensive efforts, that now appear misguided, to allow an enormous
range of server behaviors, making it hard for a client to know
what the effect of many actions, and the corresponding security-
related consequences, might be.
Troublesome in this connection were the discussion of ACE mask
bits which essentially treats every mask bit, as its own OPTIONAL
feature, the use of "SHOULD" and "SHOULD NOT" in situations which
it is unclear what valid reasons to ignore the recommendation
might be, and cases in which it is simply stated that some servers
do some particular thing, leaving the unfortunate implication that
clients need to be prepared for a vast range of server behaviors.
This approach essentially treated ACLs in a manner appropriate to
an experimental feature even though in appeared in a Proposed
Standard.
* Similar issues apply to descriptions related to the need to co-
ordinate the values of the mode attribute and the ACL-related
attributes.
Although the need for such coordination is recognized. There are
multiple modes of mapping an ACL to a corresponding mode together
with multiple sources of uncertainty about the reverse mapping.
In addition, certain of the mapping algorithms have flaws in that
their behavior under unusual circumstances providing results that
appear erroneous.
Dealing with these issues is not straightforward, because the
appropriate resolution will depend on:
* The actual existence of server implementations with non-preferred
semantics.
In some cases in which "SHOULD" was used, there may not have been
any actual severs choosing to ignore the recommendation,
eliminating the possibility of compatibility issues when changing
the "SHOULD" to a formulation that restricts the server's choices.
* The difficulty of modifying server implementations to eliminate or
narrow the effect of non-standard semantics.
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One aspect of that difficulty might be client or application
expectations based on existing server implementations, even if the
existing specifications give the client no assurance that that
server's behavior is mandated by the standard.
* Whether the existing flaw in some existing recommended actions to
be performed by the server is sufficiently troublesome to justify
changing the specification at this point.
This sort of information will be used in deciding whether to:
* Narrow the scope of allowable server behavior to those actually
used by existing severs.
* Limiting the negative effects of unmotivated SHOULDs by limiting
valid reasons to ignore the recommendation to the difficulty of
changing existing implementations.
This would give significant guidance to future implementations,
while forcing clients to live with the uncertainty about existing
servers
* Tie a more restricted set of semantics to nominally unrelated
OPTIONAL features such as implementation of dacl and sacl.
This would provide a way to allow the development of newer servers
to proceed on a firmer basis, without requiring changes on older
servers that do not support these SMB-oriented attributes.
* Provide means that clients could use to determine, experimentally,
what semantics are provided by the server.
Would need to be supported by a requirement/assurance that a
server behave uniformly, at least within the scope of a single
file system.
* Allow the provision of other ways for the client to know the
semantics choices made by the server or the file system.
Despite the difficulty of addressing these issues, if the protocol is
to be secure and ACLs are to be widely available, these problems have
to be addressed. While there has not been significant effort to
provide client-side ACL APIs and there might not be for a while, we
cannot have a situation in which the security specification makes
that development essentially impossible.
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3.5. Process Going Forward
Because of the scope of this document, and the fact that it is
necessary to modify previous treatments of the subject previously
published as Proposed Standards, it is necessary that the process of
determining whether there is Working Group Consensus to submit it for
publication be more structured than that used for the antecedent
documents.
In order to facilitate this process, the necessary changes which need
to be made, beyond those clearly editorial in nature, are listed in
Appendix B. As working group review and discussion of this document
and its successors proceeds, there will be occasion to discuss each
of these changes, identified by the annotations described in
Section 1.2.
Based on working group discussions, successive document versions will
do one of the following for some set of consensus items:
* Deciding that the replacement text is now part of a new working
group consensus.
When this happens, future drafts of the document will be modified
to remove the previous treatment, treat the proposed text as
adopted, and remove Author Asides or replace them by new text
explaining why a new treatment of the matter has been adopted or
pointing the reader to an explanation in Appendix A.
At this point, the consensus item will be removed from Appendix B
and an explanation for the change will be added to Appendix A.
* Deciding that the general approach to the issue, if not
necessarily the specific current text has reached the point of
"general acceptance" as defined in Appendix B
In this case, to facilitate discussion of remaining issues, the
text of the document proper will remain as it is.
At this point, the consensus item will be marked within the table
in Appendix B as having reached general acceptance, indicating the
need to prioritize discussion in the next document cycle, aimed at
arriving at final text to address the issue.
In addition, an explanation for the change will be added to
Appendix A.
* Deciding that modification of the existing text is necessary to
facilitate eventual consensus, based on the working group's input.
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In this case, there will be changes to the document proper in the
next draft revision. In some cases, because of the need for a
coherent description, text outside the consensus item may be
affected.
The table in Appendix B will be updated to reflect the new item
status while Appendix A is not expected to change.
* Deciding that the item is best dropped in the next draft.
In this case, the changes to the document proper will be the
inverse of those when a change is accepted by consensus. The
previous treatment will be restored as the current text while the
proposed new text will vanish from the document at the next draft
revision. The Author Aside will be the basis for an explanation
of the consequences of not dealing with the issue.
At this point, the consensus item will be removed from Appendix B.
The changes that the working group will need to reach consensus on,
either to accept (as-is or with significant modifications) or reject
can be divided into three groups.
* A large set of changes, all addressing issues mentioned in
Section 1.1, were already present in the initial I-D so that there
has been the opportunity for working group discussion of them,
although that discussion has been quite limited so far.
As a result, a small set of these changes is marked, in
Appendix B, as having reached general acceptance.
That subset of these changes, together with the organizational
changes to support them are described in Appendix A.1.
* Another large set of changes were made in draft -02. These mostly
concern the issues mentioned in Section 3.4 None of these changes
is yet considered to have reached general acceptance.
The issues that need to be addressed are described in Appendix A.2
while the possible approaches that might be taken to resolve these
issues are described in Appendix A.3.
* There remain a set of potential changes for which a need is
expected but for which no text is yet available.
Such changes have associated Author Asides and are listed in
Appendix B.
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The text for these changes is expected to be made available in
future document revisions and they will be processed then, in the
same way as other changes will be processed now.
If and when such changes reach general acceptance, they will be
explained in the appropriate subsection of Appendix A.
4. Introduction to NFSv4 Security
Because the basic approach to security issues is so similar for all
minor versions, this document applies to all NFSv4 minor versions.
The details of the transition to an NFSv4-wide document are discussed
in Sections 3.2 and 3.3.
NFSv4 security is built on facilities provided by the RPC layer,
including various auth-flavors and other security-related services
provided by RPC.
Support for multiple auth flavors can be provided. Not all of these
actually provide authentication, as discussed in Section 14.
* Support for RPCSEC_GSS is REQUIRED, although use of other auth-
flavors is provided for.
This auth-flavor provides for mutual authentication of the
principal making the request and the server performing it.
This auth-flavor allows the client to request the provision of
encryption-based services to provide privacy or integrity for
specific requests. Although such services are often provided, on
a per-connection basis, by RPC, this support is useful, when such
services are not supported or are otherwise unavailable.
* AUTH_SYS, provides identification of the principal making the
request but SHOULD NOT be used unless the client peer sending the
request can be authenticated and there is protection against the
modification of the request in flight.
Both of the above require specific RPC support such as that
provided by RPC-with-TLS [RFC9289].
* AUTH_NONE does not provide identification of the principal making
the request so would only be used for requests for which there is
no such principal or for which it would irrelevant.
The restrictions mentioned above for AUTH_SYS apply to AUTH_NONE
as well.
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There are important services that can be provided by RPC, when RPC-
with-TLS or similar transport-level facilities are available.
* Such services can provide data security to all requests on the
connection. This is to be preferred to data security provided by
the RPC auth flavor because it provides protection to the request
headers, because it applies to requests using all authentication
flavors, and because it is more likely to be offloadable.
* These services can authenticate the server to the client peer.
This is desirable since that authentication applies even when
AUTH_SYS or AUTH_NONE is used.
* The client-peer can be authenticated to the server at the time the
connection is set up. This is essential to allow AUTH_SYS to be
used with a modicum of security, based on the server's level of
trust with regard to the client peer.
Because important security-related services depend on the security
services, rather than the auth flavor, the process of security
negotiation, described in Section 16, has been extended to provide
for the negotiation of appropriate connection characteristics at
connection time if the server's policy limits the range of transports
being used and also when use of a particular auth flavor on a
connection with inappropriate security characteristics causes
NFS4ERR_WRONGSEC to be returned.
The authentication provided by RPC, is used to provide the basis of
authorization, which is discussed in general in Section 7. This
includes file access authorization, discussed in Sections 8 through
10 and state modification authorization, discussed in Section 12
File access is controlled by the server support for and client use of
certain recommended attributes, as described in Section 8.1.
Multiple file access model are provided for and the considerations
discussed in Section 9 apply to all of them.
* The mode attribute provides a POSIX-based authorization model, as
described in Section 8.3
* The ACL-related attributes acl, sacl, and dacl (the last two
introduced in NFSv4.1) support a finer grained authorization model
and provide additional security-related services. The structure
of these ACLs is described in new ACL document
[I-D.dnoveck-nfsv4-acls].
The ACL-based authorization model for NFSv4 ACLs is described in
Section 8.4
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The additional security-related services are described in
Section 13. These also rely on the authentication provided by
RPC.
* Because there are two different approaches to file-access
authorization, servers might implement both, in which case the
associated attributes need to be coordinated as described in
Section 10.
* NFSv4.2 provides a file access authorization model oriented toward
Mandatory Access Control. It is described in Section 11. For
reasons described there, its security properties are hard to
analyze in detail and this document will not consider it as part
of the NFSv4 threat analysis.
Authorization of locking state modification is discussed in
Section 12. This form of authorization relies on the authentication
of the client peer as opposed to file access authorization, which
relies on authentication of the client principal.
4.1. NFSv4 Security Terminology
In this section, we will define the security-related terminology used
in this document. This is particularly important for NFSv4 because
many of the terms related to security in previous specification may
be hard to understand because their meanings have changed or they
have been used inconsistently, leading to confusion.
The following terms are listed in alphabetical order:
* "Access Control" denotes any control implemented by a server peer
to limit or regulate file system access to file system objects.
It includes but is not limited to authorization decisions. Access
control features can be divided into those which are
"Discretionary" or "Mandatory" as described below.
* "ACL" or "Access Control List" denotes a structure used, like the
mode (see below), to defines the privileges that individual users
have with respect to a given file. These structures provide more
options than modes with regard to the association of privileges
with specific users or group and often provide a finer-grained
privilege structure as well. This specification will have need to
refer to two types of ACLs.
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The ACLs intended to be presented in the acl, sacl, and dacl
attributes are called "NFSv4 ACLs". This ACL format, was modeled
on the semantics of the SMB ACL format which provide a privilege
model substantially finer-grained than that provided by POSIX
modes.
[Consensus needed (Item #56a)]: Another ACL type derives from an
attempt to define, within POSIX, a UNIX-oriented approach to ACLs
which was published as a draft (POSIX 1003.1e draft 17), but
subsequently withdrawn. Despite the withdrawal of this draft and
the working group's decision to adopt a native NFsv4 ACL format
based on SMB ACLs, this document will have to discuss these ACLs,
which we will term "draft POSIX ACLs" because many server file
systems do not support the finer-grained privilege model needed by
the NFSv4 ACL model and because many clients are built on systems
whose only ACL-related API is based on the draft POSIX ACL model.
* "authentication" refers to a reliable determination that one
making a request is in fact who he purports to be. Often this
involves cryptographic means of demonstrating identity.
This is to be distinguished from "identification" which simply
provides a specified identity without any evidence to verify that
the identification is accurate.
In the past, these terms have been confused, most likely because
of confusion engendered by the use of the term "authentication
flavor" including flavors for which only identification is
provided or which do not provide even identification.
* "authorization" refers to the process of determining whether a
request is authorized, depending on the resources (e.g. files) to
be accessed, the identity of the entity on whose behalf the
request was issued, and the particular action to be performed.
Depending on the type of request, the entity whose identity is
referenced can be a user, a peer, or a combination of both.
Authorization is distinct from authentication. However,
performing authorization based on identities which have not been
authenticated makes secure operation impossible since use of
unauthenticated identities allows acceptance of requests that are
not properly authorized if the sender has the ability, as it
typically does, to pretend to be an authorized user/peer.
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* "client" refers to the entity responsible for setting up a
connection. In most cases the client and the requester reside on
the same node but this not always the case for NFSv4 because of
the possibility of callback requests in which the server makes
some request of the client.
* "confidentiality" refers to the assurance provided, typically
through encryption, that the contents of requests and responses
are not inadvertently disclosed to unauthorized parties.
* "Discretionary Access Control" denotes forms of access control,
that rely on a user, such as the owner, specifying the privileges
that various users are to have.
* "Mandatory Access Control" denotes forms of access control that
reflect choices made by the server peer and based on its policy
and that are typically based on the identity of the client peer
rather than the specific user making a request. While such access
control is discussed in this document, it is important to note
that many forms of mandatory access control are discussed by other
NFsv4 documents and that there are forms that are not
standardized.
* [Consensus Needed, Entire Bulleted Item (Items #21a, #57b)]:
"Mode" designates a set of twelve flag bits used by POSIX-based
systems to control access to the file with which it is associated.
In NFSv4, there are represented by the REQUIRED attribute Mode.
The three high-order flags are generally accessed only by the
client while low-order bits are divided into three three-bit
fields, which give, in order of decreasing numeric value, the
privileges to be associated with, the owner of the file, other
users in the group owning the file, and users not in the above two
categories.
In most cases, the privileges associated with each successive
group are no greater than those for the previous group. Modes
whose privileges are of this form are referred to as "forward-
slope modes" because the privilege level proceeds downward as
successive groups of users are specified. Cases in which the
contrary possibility is realized are referred to as "reverse-slope
modes".
* "peer" refer to the entity which is charged with requesting or
performing a specified request as opposed to the entity on whose
behalf the request is requested or performed, the principal;
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* "principal" refers to the specific entity (e.g. user) on whose
behalf a request is being made.
* "privacy", has in the past been used to refer, to what is now
referred to as "confidentiality".
over time, this usage has changed so that the word most often
refers to applicability of data to a single individual and
person's right to prevent its unauthorized disclosure
As a result, many references to "privacy" in previous are no
longer appropriate and really refer to confidentiality.
The NFSv4 protocol has no way to determine whether particular data
items raise privacy concerns (In the new sense). NFSv4 provides
confidentiality whatever type of data is being accessed so that
private data is kept private.
* "integrity" refers to the assurance that data in a request has not
been modified in the process of transmission. Such an assurance
is generally provided b means of a cryptographic hash of the
requests or response.
* "requester" is the entity making a request, whether that entity is
on the client-side, as it most often is (forward-direction
request) or the server side, in the case of callback (i.e.,
reverse-direction) requests.
* "responder" is the entity performing a request, whether that
entity is on the server side, as it most often is (forward-
direction request) or the client side, in the case of callbacks
(reverse-direction requests.
* "server" refers to the entity to which the client connects. In
most cases the client and the responder reside on the same node
but this not always the case for NFSv4 because of the possibility
of callback requests in which the server makes some request of the
client.
4.2. NFSv4 Security Scope Limitations
This document describes the security features of the NFSv4 protocol
and is unable to address security threats that are inherently outside
the control of the protocol implementors. Such matters as out of
this document's scope.
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As a way of clarifying the threats that this document, and the threat
analysis in Section 18.4 can and cannot deal with, we list below the
potential threats discussed Section 3.1 of [nist-209] and review how,
if at all, it is discussed in the current document. In cases in
which the threat is dealt with in this document, distinctions are to
be made between cases in which the issues have been dealt with
directly or have been delegated to a lower layer on which the
protocol is built and whether the issue has been addressed by the
changes to NFSv4 security made by this document.
* Regarding the possibility of "Credential Theft or Compromise",
this is not a matter that the NFSv4 protocols concern themselves
with or can address directly, despite its importance for security.
Depending on the auth flavor chosen, either the client (for
AUTH_SYS) or a third-party (for RPCSEC_GSS), usually Kerberos,
will be responsible for credential verification.
Since experience has shown that credential compromise (e.g.
through "phishing" attacks) is a common occurrence, this problem
cannot be ignored, even though NFSv4's reliance on RPC facilities
for authentication might be thought to make it out-of-scope as it
would be RPC if had an effective solution to the issue. However,
the urgency of the situation this issue is such that will be
discussed in Section 18.4.2, even though no definitive solutions
to this issue are likely before this document is completed and
published.
Regardless of such issues, the likelihood of such compromise has
had a role in decisions made regarding the acceptance and use of
"superuser" credentials. The possibility of such compromise is
also relevant to implementation of means to synchronize
credentials when they are managed by the client, as described in
Section 18.4.6.1
* Regarding the possibility of "Cracking Encryption", prevention of
this is responsibility of the NFSv4 protocols but it is one which
has been delegated to RPC, so that its discussion in Security
Considerations will rely on RPCSEC_GSS and RPC-with-TLS
implementations to manage the selection and replacement of keys
for encryption so as to limit the possibility of such unwanted
encryption key discovery.
* Regarding the possibility of "Infection of Malware and
Ransomware", NFSv4 has no direct role in preventing such
infection, but does have an important role in limiting its
consequences, by limiting the ability of Malware to access or
modify data, through the file access authorization model supported
by NFSv4 to limit access to authorized users. Of course, malware
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will be able to execute on behalf of the user mistakenly invoking
it but the authorization model will server to limit the potential
damage.
The possibility of vertical privilege escalation is of concern as
regard the possible elevation to "superuser" privileges. For this
reason, this document recommends that any such escalation not be
effective on the server, even if it happens on local clients for
which NFSv4 has no role.
Execution of a ransomeware-based attack requires the attacker to
have the ability to read existing data and replacing it with an
encrypted version together with the ability to temporarily hide
the encryption from ongoing operations by intercepting requests to
read encrypted data and substitute the unencrypted data.
* Regarding the possibility of "Backdoors and Unpatched
Vulnerabilities", it needs to be noted that the NFSv4 protocols do
not specify any backdoors even though it is possible that might
choose to provide such backdoors. Since it is not practical to
specifically prohibit the existence of such backdoors nor would
they be enforceable if written, this document will not attempt to
do so. Instead, Section 18.2.3 will note the possibility of such
backdoors and recommend against any such implementation, and
include implementations containing backdoors in the category of
insecure use that will not be dealt with in Section 18.4.
Although it is expected that vulnerabilities will be due to
incorrect implementations and thus outside the scope of this
document, the possibility of a protocol design errors cannot be
excluded. In dealing with such eventualities, it is likely that
complete remediation would require coordinated changes on the
client and server
* Regarding the possibility of "Privilege Escalation", NFSv4 has
dealt with the possibility of vertical escalation by not allowing
a client-local escalation to superuser privileges to be effective
on the server.
With regard to horizontal "escalation", NFSv4 provides for the use
of various means RPC authentication of principals but relies on
the client operating system to make sure that one user principal
cannot masquerade as another.
* Regarding the possibility of "Human Error and Deliberate
Misconfiguration", the approach taken is to limit the need for the
server to make complicated decisions regarding the security
requirements of each section of its namespace, with many
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opportunities for misconfiguration, if the chosen security
requirements are insufficiently restrictive. This is in contrast
to previous specifications which made such configuration the
centerpiece of the security approach.
Although it is possible to create configurations where certain
data, generally publicly accessible, are to be made available
without encryption, this is expected to be a rarely used option
with the possibility of in-transit modification kept in mind
before adopting such use.
* Regarding the possibility of "Physical Theft of Storage Media",
this a matter which, while of concern to those deploying NFSv4
server, will be considered out-of-scope since there is nothing
that the protocol could do to deal with this threat.
* Regarding the possibility of "Network Eavesdropping", when the
protocol implementation follows the recommendations in this
document, the protocol's use of RPC facilities is designed,
through the consistent use of encryption to make it difficult for
an attacker to have access to the data being transmitted, to
modify it, or inject requests into an existing data stream.
The possibility of an attacker with access to the network creating
a new connection is best considered as a case of the attacker
pretending to be a client and is addressed in Section 18.4.3.
* Regarding the possibility of "Insecure Images, Software and
Firmware", while attention to such matters is important for those
deploying NFSv4, it is important to note that these are matters
outside the control the NFSv4, which has to assume that the
infrastructure it is built is working properly. As a result, this
document will not deal with the possibility of such threats.
5. Authorization-related Attributes
NFSv4 operations are authorized (or not) based on the entity
requesting the operation and the values of the authorization-related
attributes documented in this section. The table in Section 5.2
lists all such attributes with the actual descriptions appearing in
three subsections based on the specific authorization models
supported by various NFSv4 protocols.
* There are a number of attributes derived from file characteristics
defined by POSIX. They are similar to the corresponding NFSv3
attributes, although they are different in form. These are
described in Section 5.3.
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* In order to provide finer-grained control of authorization
decisions, a number of ACL-related attributes are defined. These
are described in Section 5.4.
[Consensus Required (Item #57c)]: Although it might have been
intended that the acl-related and POSIX-derived attributes would
serve as two alternate modes authorization, with each OPTIONAL,
that has not been possible, with the POSIX-derived model becoming
REQUIRED while the ACL-related ones remain OPTIONAL.
As the acl-related model has evolved, it was constrained to work
well with the POSIX-based authorization model, as there were many
clients who required support for POSIX authorization semantics.
* [Consensus Because of important semantic differences between the
existing ACL-related attributes described in Section 5.4 which
support the NFsv4 ACL model and that of the draft POSIX ACL model,
it appears necessary to provide better support for authorization
using the latter ACL model.
This is likely to require creation of new OPTIONAL ACL-related
attributes designed to provide these semantics.
As in the case of the NFSv4 ACL model, there is a need for proper
coordination with POSIX-based authorization. However, in this
case, specification of this co-ordination is the responsibility of
the extensions defining these new attributes.
* In addition, an additional authorization model was made available
in NFSv4.2. It is described in Section 5.5.
This provides a form of Mandatory Access Control in which
authorization decisions derived from the identity of the client
making the request rather than the identity of the specific user/
principal. When enabled, it serves as an additional authorization
step in addition to that specified by POSIX-related attributes or
ACLs.
The NFSv4 protocols have integrated a set authorization-related
attributes within the extensible attribute model, introduced in
[RFC7530]. This extensibility has been the basis of the introduction
of additional OPTIONAL attributes provided for in [RFC8178]
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As a result, different sets of attributes are valid in different
minor versions. Although attributes are described in the
specifications for the minor version in which they are introduced,
and, in some cases, in specifications for later minor versions, the
description in this document is definitive and overrides any other
such specification. This includes the following attributes:
* The attributes owner, group, mode, aclsupport and acl were
introduced in NFSv4.0 and are valid in all minor versions.
* The attribute mode_set_masked, dacl, and sacl were introduced in
NFSv4.1 and are valid in all minor versions except minor version
zero.
* The attribute sec_label was introduced in NFSv4.2 and is only
valid in minor version two.
[Consensus Needed, Including List (Items #57c, #58a)]:", Although
previous specifications have treated all of these as OPTIONAL,
sometimes using the incorrect designation "RECOMMENDED", it has to be
understood that there important exceptions that need to be noted:
* In the interest of providing better assurances of meaningful
interoperability to compliant servers and clients, the attributes
described in Section 5.3 (i.e. owner, group, mode) are to be
considered REQUIRED.
* While it is reasonable to say, as [RFC8881] does, that OPTIONAL
need to be "understood well enough to warrant support", it should
be the case that this understanding is documented sufficiently to
enable clients and servers to interoperate and new implementations
of each to be implemented. Unfortunately, that condition is not
currently met for some attributes which we need to realize are
under-specified, and thus essentially experimental, even though
formally OPTIONAL. The details are discussed in Sections 4.4,
4.7, and 4.8 of [I-D.dnoveck-nfsv4-acls] and in Section 5.5.1 (of
the current document). The intention, not yet fully realized, is
that new features described in the ACL document might, when
adopted, allow the Acl, Dacl and Sacl attributes to be designated
as OPTIONAL in fact, i.e. not under-specified.
[Author Aside (Item #58a)]: It could be that, as discussed
elsewhere, that the inadequate semantic description referred to in
the paragraph above is the result of a flawed approach to the
description of the features, rather than to the attempt to support
two types of ACLs within the same set of attributes.
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5.1. Format of Id Strings in Authorization-related Attributes
Unlike the case in NFSv3, users and groups are represented in NFSv4
by UTF-8-encoded Unicode strings. These strings include:
* [Consensus needed (Item #57d)]: The values of the REQUIRED
attribute Owner, as described in Section 5.3.3.
* [Consensus needed (Item #57d)]: The values of the REQUIRED
attribute Owner_group, as described in Section 5.3.4
* Values within the "who" field within Access Control Entries as
described in Section 7.3.3 of [I-D.dnoveck-nfsv4-acls]. These
entries appear in the Acl, Dacl, and Sacl attributes.
[Author Aside, Including List (Item #59b)]: This section had to be
extensively revised in order to be clearer and to allow more uses of
string representations of numeric ids. The previous treatment, which
is not reproduced here, can be found in Section 5.9 of [RFC8881]. In
the author's opinion, major revisions were necessary because:
* The existing treatment, by leaving the methods of mapping between
strings and local user ids unspecified, created an
interoperability issue. Since providing this mapping in a secure
way is difficult, the gap is likely to be filled in an a way
vulnerable to attack.
While the need for flexibility might make it impossible to specify
mapping fully, the requirements for agreement between client and
server needed to be specified more clearly and avoided where this
can be done without impeding protocol operation.
* There is inadequate attention to the additional implementation
needs to support multiple domain values.
The difficulties inherent in supporting all possible domains are
obscured, with the very real security issues essentially ignored.
* The description of the domain value as "meant to be a DNS domain"
is not clear and makes it harder to focus on the real difficulties
involved in providing coherent secure means of providing necessary
mappings for multiple domains.
* Some negative characterizations of the use of numeric ids appear
to be unjustified, leading to their non-use in situations in which
artificially mapping to the name@domain format creates difficult
issues while providing no benefit.
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[Consensus Needed, Including Rest of Section (Items #52a, #59a)]:
These identifiers are represented by strings which have two possible
formats:
* Strings of the format "name@domain" can be used with a number of
important benefits. They are to be preferred in situations in
which the mapping between names and the 32-bit numeric ids
preferred by typical file systems is securely provided by means of
the RPC auth flavor being used, as is the case when Kerberos
services are made available through the use of RPCSECGSS.
Servers would be able support multiple clients each with users
from one or more domains, without requiring construction of a
single list of users together with associated authentication info.
The number of users would not be limited by existing hard-to-
extend limits such as the POSIX/NFSv3 use of a 32-bit field to
represent a user or group id.
* The string can be the ASCII representation o the 32-bit user or
group id.
This format is to preferred when the auth flavor AUTH_SYS is being
used and in other cases in which the translation between user
names and numeric ids is not available or creates security
vulnerabilities.
These strings are assigned the XDR type utf8str_mixed because the
name and domain portions are treated differently:
* The name portion is a utf8 string that is matched in a case-
sensitive manner without any attempt to treat distinct canonically
equivalent strings as the same.
* The handling of the domain portion, including recognition of
equivalent string as the same is specified by the server.
However, when multiple domains are supported, case-insensitivity
of ASCII characters is mandated by DNS, as well as translations to
and from Punycode-encoded forms.
It is expected that the client and server file system will have their
own local representation of users and user groups that is used for
local storage or presentation to the end user. In addition, it is
expected that when these attributes are transferred between the
client and server and the format used is of the form name@domain, the
local representation is translated to that form.
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Although this allows for a client and server that both use the
name@domain format and do not use the same local representation the
ability to translate to a common format, there will often be cases in
which both client and server use the same internal format. This
often happens with servers that only support a single domain.
However, regardless of whether these forms are, in fact, different,
it is necessary, when such translation is necessary, that client and
server agree on the following:
* The set of valid strings that can appear as the domain portion of
these identifiers. This set must have at least a single member
but when this not the case, the set known to each client must be a
subset of the set recognized by the server.
* For each valid domain string passed by a client and accepted by a
server, the server and client must agree on a means by which the
user designated by each user and group name can be mapped to a
user or group so that each interprets all name@domain strings
identically.
The above applies whether the client and server use the same
internal name representation or not.
When this translation is used, security principals and the groups
they belong to need to be integrated within this arrangement since
these identities need to be presented to the server in a form
compatible with this new format. The specifics depend on the auth
flavor providing the principal identification:
* When auth flavors based on RPCSEC_GSS are used, the principal is
generally identified in a form easily converted to the name@domain
format. For example, with Kerberos, the Kerberos user name can
provide the name portion while the Kerberos realm can serve as the
domain portion.
* When the AUTH_SYS auth flavor is used, principals are identified
by a 32-bit numeric identifier which can be generally treated in
the same manner as numeric id's used locally within the file
system to arrive at a name with the domain being the one assigned
to that filesystem.
The identification of principals MAY be subject to filtering to
eliminate users with a high level of privilege.
In the case of servers with filesystems that support the use of
multiple domains, clients for which use of AUTH_SYS is supported
need to be assigned to a specific domain so that the principal
identifications received are properly dealt with.
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The translation used to interpret owner and group strings is not
specified as part of the protocol. This allows various solutions to
be employed, as long as the requirement mentioned above are satisfied
and additional security vulnerabilities are not created. For
example, a local translation table may be consulted that maps a
numeric identifier to the user@domain syntax. A name service may
also be used to accomplish the translation. A server may provide a
more general service, not limited by any particular translation
(which would only translate a limited set of possible strings) by
storing the owner and owner_group attributes in local storage without
any translation or it may augment a translation method by storing the
entire string for attributes for which no translation is available
while using the local representation for those cases in which a
translation is available.
[Author Aside:] The next sentence previously used the word "SHOULD.
Since the author has been unable to determine valid reasons to do
otherwise and has no reason believe that any exist, the new text uses
"MUST". It is possible that the original author might have been
thinking of "nobody" for this but that case needs to be addressed
separately and will need to be clearly distinguished from
"nobody@domain".
Servers that do not provide support for all possible values of the
owner and owner_group attributes MUST return an error
(NFS4ERR_BADOWNER) when a string is presented that has no
translation, as the value to be set for a SETATTR of the owner,
owner_group, or as the who value in an ACE within the acl, sacl, or
dacl attributes. When a server does accept an owner or owner_group
value as valid on a SETATTR (and similarly for the owner and group
strings in an ACL), it is promising to return that same string when a
corresponding GETATTR is done. Configuration changes (including
changes from the mapping of the string to the local representation)
and ill-constructed name translations (those that include aliasing)
may make that promise impossible to honor. Servers need to make
appropriate efforts to avoid a situation in which these attributes
have their values changed when no real change to ownership has
occurred.
The "domain" portion of the owner string will often be a DNS domain
name, for example, user@example.org. Servers should accept as valid
a set of users for at least one value of the domain portion. A
server may treat other domains as having no valid translations. A
more general service is provided when a server is capable of
accepting users for multiple domains values but the following will
need to be attended to:
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* When use of AUTH_SYS is allowed, the translation of numeric ids to
the form name@domain becomes problematic.
The server will need to determine the domain value to be used
based on the identity of the client peer.
* When numeric ids are used in the file system the handling of those
ids needs to be modified to support multiple sets of users with
one set for each of the various domain values supported.
One option is to add a small (i.e. 8- or 16-bit) field to
represent the chosen domain value will leaving the 32-bit id field
as it is.
It is also possible to map individual 32-bit id spaces into a
single 32-bit id space. However, this involves establishing for
each component id space a restricted range, so that the mapping
can be done without resulting in conflicting reverse mappings.
In cases in which translation is being used and there is no
translation available to the client or server, the attribute value
will be constructed without the "@". The absence of the @ from the
user or group string signifies that no translation was available at
the sender and that the receiver of the attribute should not use that
string as a basis for translation into its own internal format. When
the string consists of a numeric value with no leading zeroes it can
be interpreted as representing the corresponding 32-bit numeric id.
Even though such attribute values cannot be translated, they are
still likely to be useful. In the case of a client, the attribute
string may be used for local display of ownership. However, in the
case in which the server receives such a string, it is less likely to
be useful and might be harmful in that its use by the server might
undercut the value of the translation to the form name@domain. In
order to avoid these negative effects:
* When a numeric value is received in an attribute being set where
an RPCSECGSS-based auth flavor is being used, the SETATTR MUST
return an error (NFS4ERR_BADOWNER).
* When the auth flavor AUTH_SYS is being used, the server MAY accept
user and groups identified by ids using the string numeric form
and return them in that form. However, when it does so it still
MUST return these values, in the name@domain form to those using
other auth flavors.
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[Author Aside, Including List (Items #52a, #60a)]: Something needs to
be done about the failure of the corresponding text in RFC8881 to
deal appropriately with root@domain and nobody@domain, either to
exclude these or include them with appropriate explanations.
Specifically:
* With regard to the case of "nobody", the primary issue is that
"nobody@domain" is not addressed, although "nobody", which should
not occur, is.
A further difficulty is the use of the term "anonymous" with the
intention to include unidentified and unauthenticated users as
well as those which are not appropriately authenticated.
* With regard to the case of "root", the issue is not mentioned at
all, leaving it unclear as to whether this sort of special
handling is gone, or is dependent now on the name "root" or the
numeric id zero despite the general deprecation of numeric ids.
The possibility of support for multiple domains, creates
additional complexity which needs to be addressed.
[Consensus Needed (Item #60a)]: An owner string of the form
"nobody@domain" may be used to designate an anonymous or
unauthenticated user, which will be associated with a file created by
a security principal whose identity is not determinable or cannot be
mapped through normal means to the Owner attribute. Users and
implementations of NFSv4 should avoid the use of the string "nobody"
to identify an actual user.
[Consensus Needed (Item #52a)]: Certain id strings that map to known
numeric values such as zero MAY be assigned special privileges with
regard to operation authorization, allowing operations to be
authorized that POSIX or ACL-based authentication might disallow.
The granting of such privileges MUST NOT be based on the user name
(e.g. "root"). Instead, the server MUST only use the principal id or
information returned by a secure id mapping facility in assigning
such privileges, which MAY be assigned differently based on the
reliability of the authentication method used. When multiple domains
are supported, the privilege assignments might be different for
different domains, but the mapping of ids into a value used
internally by the file system is not to be considered in deciding
about granting such privileges.
5.2. Table of Authorization-related Attributes
The list of authorization-related attributes appears in Table 1.
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The meaning of the columns of the table are:
Name: The name of the attribute.
Id: The number assigned to the attribute. In the event of conflicts
between the assigned number and minor version specification
documents, the former is authoritative, but conflicts should be
resolved with Errata to this document and/or the minor version
specification document. See [errata] for the Errata process.
Data Type: The XDR data type of the attribute.
Acc: Access allowed to the attribute. R means read-only (GETATTR
may retrieve, SETATTR may not set). W means write-only (SETATTR
may set, GETATTR may not retrieve). R W means read/write (GETATTR
may retrieve, SETATTR may set).
Defined in: The section of this specification that describes the
attribute.
+=================+====+===============+=====+===============+
| Name | Id | Data Type | Acc | Defined in: |
+=================+====+===============+=====+===============+
| acl | 12 | nfsace4<> | R W | ACL document |
+-----------------+----+---------------+-----+---------------+
| aclsupport | 13 | uint32_t | R | ACL document |
+-----------------+----+---------------+-----+---------------+
| dacl | 58 | nfsace4<> | R W | ACL document |
+-----------------+----+---------------+-----+---------------+
| mode | 33 | uint32_t | R W | Section 5.3.1 |
+-----------------+----+---------------+-----+---------------+
| mode_set_masked | 74 | mode_masked4 | __W | Section 5.3.2 |
+-----------------+----+---------------+-----+---------------+
| owner | 36 | utf8str_mixed | R W | Section 5.3.3 |
+-----------------+----+---------------+-----+---------------+
| owner_group | 37 | utf8str_mixed | R W | Section 5.3.4 |
+-----------------+----+---------------+-----+---------------+
| sacl | 59 | nfsace4<> | R W | ACL document |
+-----------------+----+---------------+-----+---------------+
| sec_label | 80 | sec_label4 | R W | Section 5.5.1 |
+-----------------+----+---------------+-----+---------------+
Table 1
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5.3. POSIX-oriented Authorization-related Attributes
[Consensus Needed (Item #57e)]: The REQUIRED attributes Owner,
Owning_group, and Mode enable use of a POSIX-based authorization
model, as described in Section 8.3. Given that all of these
attributes MUST be supported, this authorization model is always
available.
These attributes are also of use when ACL-based authorization is in
effect.
* The values of the Owner and Owner_group attributes affect the
interpretation of the special "who" values "@OWNER" AND "@GROUP">
* Changes to the acl and dacl attributes will often result in
corresponding changes to the mode attribute.
* Setting the mode attribute will often override the values in acl
and dacl attributes, even if there has been no changes to the
authorization-related bits.
The set_mode_masked attribute can be set when changing bits in the
mode, if the authorization-related bits are not to be changed and
there is a need not to update authorization-related ACEs.
5.3.1. The Mode Attribute (v4.0)
[Consensus needed (Item #6a)]: This field is limited to twelve bits,
of which only the low-order ten bits are authorization-related. Of
the lowest-order nine bit, each set of three bits controls the
actions authorized for the file object by a particular set of users.
Proceeding from higher-order bits to lower, these sets determine the
privileges of:
* The user that is owner of the file.
* Any user within the owning user group other than the owner of the
file.
* All other users.
Once the appropriate set is determined, based on the principal making
the request, the corresponding three bits determine operation
authorization as follows:
* Authorization for reading of data from the file is controlled by
the highest-order bits, as is reading the contents from a
directory.
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* Authorization for writing of data to the file is controlled by the
second-highest-order bit, as is modifying the contents of a
directory using REMOVE, RENAME, CREATE and OPEN of file specifying
creation of a new file.
* Authorization for read of data from the file for the purposes of
executing it as code (either via loading/execution of machine
instructions or by interpretation of scripts is controlled by the
lowest-order bit. Similarly, in the case of a directory, this bit
controls the searching of the directory to open the file, do a
LOOKUP, or do any other operation that involves searching a
directory for a specific name
The specification of the individual mode bits appears below:
const MODE4_SUID = 0x800; /* set user id on execution */
const MODE4_SGID = 0x400; /* set group id on execution */
const MODE4_SVTX = 0x200; /* save text even after use */
const MODE4_RUSR = 0x100; /* read permission: owner */
const MODE4_WUSR = 0x080; /* write permission: owner */
const MODE4_XUSR = 0x040; /* execute permission: owner */
const MODE4_RGRP = 0x020; /* read permission: group */
const MODE4_WGRP = 0x010; /* write permission: group */
const MODE4_XGRP = 0x008; /* execute permission: group */
const MODE4_ROTH = 0x004; /* read permission: other */
const MODE4_WOTH = 0x002; /* write permission: other */
const MODE4_XOTH = 0x001; /* execute permission: other */
Bits MODE4_RUSR, MODE4_WUSR, and MODE4_XUSR apply to the principal
identified by the owner attribute. Bits MODE4_RGRP, MODE4_WGRP, and
MODE4_XGRP apply to principals belonging to the group identified in
the owner_group attribute but who are not identified by the owner
attribute. Bits MODE4_ROTH, MODE4_WOTH, and MODE4_XOTH apply to any
principal that does not match that in the owner attribute and does
not belong to a group matching that of the owner_group attribute.
These nine bits are used in providing authorization information.
[Previous Treatment]: The bits MODE4_SUID, MODE4_SGID, and MODE4_SVTX
do not provide authorization information and do not affect server
behavior. Instead, they are acted on by the client just as they
would be for corresponding mode bits obtained from local file
systems.
[Consensus needed (Item #6a)]: For objects which are not directories,
the bits MODE4_SUID, MODE4_SGID, and MODE4_SVTX do not provide
authorization information and do not affect server behavior.
Instead, they are acted on by the client just as they would be for
corresponding mode bits obtained from local file systems.
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[Consensus needed (Item #6a)]: For directories, the bits MODE4_SUID
and MODE4_SGID, do not provide authorization information and do not
affect server behavior. Instead, they are acted on by the client
just as they would be for corresponding mode bits obtained from local
file systems. The mode bit MODE_SVTX does have an authorization-
related role as described later in this section
[Consensus Needed, Including List (Item #6a]): When handling REMOVE
or RENAME operations, the check for authorization MAY depend on the
setting of MODE_SVTX for the directory.
* When MODE_SVTX is not set on the directory, authorization requires
write permission on the source directory.
* When MODE_SVTX is set on the directory, authorization can require,
in addition, that the requesting principal be the owner of the
file being removed or renamed or of the source directory. The
authorization of the target directory in the case of a RENAME
typically does not involve MODE_SVTX. However, when a file is
removed as part of the RENAME operation (i.e., in the rename-over
case), authorization for the consequent deletion can involve the
possible presence of MODE4_SVTX for the target directory.
[Consensus needed (Item #6a)]: It needs to be noted that this
approach is similar to the ACL-based approach documented in
Section 7.2.15 of [I-D.dnoveck-nfsv4-acls]. The handling of deletion
for both POSIX-based and ACL-based authorization need to work
together consistently since the set of entities involved will often
include a mixture of objects with and without a corresponding ACL.
[Author Aside]: Bringing the above into more alignment with the
corresponding ACL-based semantics necessary, despite the fact that
previous specifications made no mention of this issue. For tracking
purposes, that realignment will be considered as Consensus Item #20.
Bits within a mode other than those specified above are not defined
by this protocol. A server MUST NOT return bits other than those
defined above in a GETATTR or READDIR operation, and it MUST return
NFS4ERR_INVAL if bits other than those defined above are set in a
SETATTR, CREATE, OPEN, VERIFY, or NVERIFY operation.
[Consensus Needed (Item #21b)]: As will be seen in Sections 10.4,
10.5, and 10.7 of [I-D.dnoveck-nfsv4-acls]. Many straightforward
ways of dealing with mode that work well with forward-slope modes
need adjustment to properly deal with reverse-slope modes, as defined
in Section 4.1
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5.3.2. The Mode_set_masked Attribute (v4.1)
The mode_set_masked attribute is an OPTIONAL write-only attribute
that allows individual bits in the mode attribute to be set or reset,
without changing others. It allows, for example, the bits
MODE4_SUID, MODE4_SGID, and MODE4_SVTX to be modified while leaving
unmodified any of the nine low-order mode bits devoted to
permissions.
When this attribute is not supported, clients' only option in setting
the mode attribute is to set all bits of that attribute using
SETATTR, even if the motivation is only to modify bits which are not
authorization-related.
This has the unfortunate result that some the effects of any
associated ACL attribute are negated as it is assumed that by
changing the mode, the intention is to override all ACL-related
authorization, wherever its effect is different from that specified
by the mode alone.
5.3.3. The Owner Attribute (v4.0)
Defines the user who owns the file, which affects which set of three
bits from the mode attribute controls the bits controlling
authorization.
The interpretation of this string value is described in Section 5.1
5.3.4. The Owner_group Attribute (v4.0)
Defines the group that owns the file, which affects which set of
three bits from the mode attribute controls the bits controlling
authorization.
The interpretation of this string value is described in Section 5.1
5.3.5. Issues with Named Attribute Directories
[Previous Treatment (Item #66a]): Note that the hidden directory
returned by OPENATTR is a convenience for protocol processing. The
client should not make any assumptions about the server's
implementation of named attributes and whether or not the underlying
file system at the server has a named attribute directory.
Therefore, operations such as SETATTR and GETATTR on the named
attribute directory are undefined.
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[Author Aside (Item #66a), Through end of bulleted list]: Despite the
dubious logic of the preceding paragraph, it is probably too late to
significantly revise this decision, which first appeared in
[RFC3530]. Nevertheless, it might be helpful to understand how this
decision was made, why it was not caught in document review, and why
it has not been addressed in the many years since it was first
incorporated in NFSv4. The following factors should be noted:
* Whether the underlying file system at the server has a named
directory attribute or not is an implementation matter and out of
scope for this sort of specification.
Despite this fact, a named attribute directory is required by the
specification because LOOKUP and OPEN operations are done on it
and that remains the case whether it is dismissively characterized
as a "convenience" or not.
It is a fundamental mistake to conflate the protocol choice made
here, to deprive these directories of the need/ability to store
attributes, with the basic architectural point that it is up to
the server to decide on the means by which the protocol's
requirements are met.
* While it is hard to conceive of a situation in which the above
mistake led to this decision, it is much more likely that this
paragraph was assembled at the last minute to try to justify a
decision already arrived at, for other reasons.
It could well be that this decision was made because the working
group was not prepared to resolve issues that might arise from
supporting attributes on these directories in the time available
or feared that many proposed implementations of named attributes
might be unwilling to provide appropriate support for named
attributes if support for attributes of named attribute
directories were included.
It was probably felt that honestly citing the real difficulties
cited above might result in IESG criticism from those prefacing
their remarks by disclaiming NFS expertise, while feeling that,
given the complexity of the explanation actually provided, it was
more protected from such non-expert criticism.
* The difficulties cited above might not have been sufficient to
foreclose these attributes if proper notice was taken of the need
for authorization-related attributes to support the authorization
of actions related to named attributes.
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As things turned out, this need was not appropriately recognized.
This lack of recognition arose mostly because it had never been
important previously, given that POSIX semantics could relied upon
without separate effort from NFSv4. It was not noticed that with
the non-POSIX semantics implied by named attributes, that
situation had fundamentally changed.
* Given that the lack of support for such attributes might be more
appropriately be dealt with by deferral without essentially
foreclosing future implementation of such a feature, it needs to
be understood why such deferral was never considered.
Given this context, it is worth noting that, at the time this
choice was made, there was no extension model for NFSv4 except
minor versioning and that made what would be, in retrospect, the
best choice, i.e deferring the named attribute feature to a later
time, inconceivable.
[Consensus Needed (Item #66a]): Since there is no way to set the
attributes associated with a newly-created named attribute directory
and because operations such as SETATTR and GETATTR are undefined when
applied to named attribute directories, normal approaches to
authorization of operations on named attribute directories are not
available. As a result, the necessary authorization semantics need
to be specified somehow, most likely by deriving values to be used
for Mode, Owner, and Owner_group attributes for the named directory
from the corresponding attributes for the base object.
[Author Aside (Item #66a), Through end of section}: For the most
part, the NFSv4 specifications have avoided the need to describe
authorization semantics, by relying on the POSIX definition and
making it the responsibility of the protocol, as it should be, to
duplicate local semantics when used remotely.
In the case of the OPTIONAL named attributes feature which is outside
the scope of POSIX, that approach is no longer viable and needs to
supplemented by a semantic description. In formulating such
description, the following issued need to be addressed:
* We don't have information about authorization semantics for
implementations of this feature or even if such implementations
exist.
This applies to both potential implementations that do and do not
support ACLs.
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The lack of previous focused discussion of this issue strongly
suggests that implementations are either non-existent, or pay no
serious attention to authorization semantics.
* The privilege structure provided by the REQUIRED attributes Mode,
Owner, and Owner_group seems like it could be adapted to control
access to the named attribute, but there are some troublesome
gaps.
OPENATTR provides no way to set the attributes for the named
attribute directory it creates. Furthermore, GETTATTR and SETATTR
on these directories are undefined, according to [RFC8881]) and
most likely will remain so.
* There are corresponding issues relating ACL-based authorization
that are part of the associated Consensus Item #100 which is
discussed within [I-D.dnoveck-nfsv4-acls].
There will need to be extensive co-ordination in addressing these
two related issues.
5.3.6. POSIX Authorization for Named Attribute Directories
[Consensus Needed (Item #66b), through end of section]: Because named
attribute directories do not have attributes, the values to be used
in place of those attributes in order to support authorization
decisions for operations on named attribute directories are as
follows:
* The value to be used in place of mode attribute is based on the
value of the mode attribute of the base object with the following
modifications:
For each of owner, group, and others, the read and execute
permissions are set iff either the read or execute bit is set in
the corresponding set of three bits in the mode attribute of the
base object.
* The value to be used in place of owner attribute is identical to
the owner attribute for the base object.
* The value to be used in place of owner_group attribute is
identical to the owner_group attribute for the base object.
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5.4. ACL-based Authorization-related Attributes
The following ACL-related attributes (all OPTIONAL are described in
detail in the new document devoted to ACL handling
[I-D.dnoveck-nfsv4-acls].
* The per-fs Aclsupport attribute.
* The per-fs Aclchoice attribute being added as an extension within
NFsv4.1
* The per-object Acl attribute.
* The per-object Dacl attribute added in NFSv4.1.
* The per-object Sacl attribute added in NFSv4.1
5.5. Authorization-related Attributes for MAC
5.5.1. The Seclabel Attribute (v4.2)
This opaque attribute provides authorization-related information to
support Mandatory Access Control. Given the lack of specific
documentation about the contents and the uncertainty regarding
identification of the actors making the requests to be authorized,
client-server interoperability is not available and any authorization
decision are the responsibility of the client itself.
[Consensus needed (Item #58b)]: Although this was intended to be an
OPTIONAL attribute, it is now more appropriate to describe it is an
under-specified one, and thus essentially experimental, although
still formally. OPTIONAL. This is due to the absence of
specifications for the content of the attribute, or descriptions of
the way in which it is to be used to govern authorization.
6. Introduction to Access Control List
6.1. Overview of Existing NFSv4 ACLs
The ACL-related attributes Acl, Sacl, and Dacl, introduced above in
Section 5.4 and described in more detail in the ACL specification
document [I-D.dnoveck-nfsv4-acls], each contain an array of NFSv4
Access Control Entries. These ACEs:
* Define the operations that are authorized for particular users or
user groups.
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* Define the users and use groups for which various security-
related reporting (i.e., audit and alarm events) is to be
performed.
ACL-related attributes and the structure of ACEs used to support the
NFSv4 ACL model are described in detail in a companion document
[I-D.dnoveck-nfsv4-acls].
Although these attributes and data structures are the same those
defined in previous NFSv4 specification document, there are important
differences in the ACL architecture that have been necessary. For
more detail, see Section 6.3.
6.2. Previous Treatment of ACLs
The description of ACL-related attributes and ACEs that appeared in
previous documents embodied a flawed approach to protocol description
that makes in unsuitable for use as a basis for a description
intended to appear and in a standards-track document. As the goal of
this effort is to produce a helpful standards-track document, these
item will be described in [I-D.dnoveck-nfsv4-acls] as part of a new
approach to ACL architecture, summarized in Section 6.3.
This same flawed approach was used in a number of documents published
as Proposed Standards. These included RFCs now obsoleted ([RFC3010],
[RFC3530], and [RFC5661]) and a number RFCs to be obsoleted when this
document is published as an RFC ([RFC7530] and [RFC8881]). Although
some changes were made in the transition between minor versions, the
essence of the approach remained the same.
The underlying goal that led to this flawed approach remains as it
was. There was need to support an extended ACL model, with a fine-
grained permission model and other helpful extensions together with
providing support for a more limited ACL model with a more direct
connection to POSIX semantics, very similar to that defined in the
withdrawn POSIX ACL draft implemented by a number of file systems
implemented on UNIX systems.
The approach actually taken to this need had the following elements:
* The definition of the Acl attribute was based on the extended ACL
model, rather than the simpler, more POSIX-oriented core.
* To allow servers implement to implement the core UNIX ACL subset
to be considered compliant, the specification was written
providing an unreasonable degree of leeway for the server to
implement many elements in pretty much whatever way it chose.
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This approach allowed core UNIX ACLs, NFSv4 ACLs and hybrids of
the two to be considered compliant, as well as many unanticipated
variants. In any case, the client had no way of determining the
ACL semantics being implemented.
* It was assumed that the draft POSIX ACL model, being more
restricted could be accommodated within the NFSv4 ACL model
without clear choices being provided for the implementation of
OPTIONAL features.
Since the draft POSIX ACL model was more restricted in that the
privilege set was more coarse-gained and there were no DENY ACEs,
this assumption appeared plausible, given that it was assumed, at
the time, that detailed consideration of operation semantics was
out of the question.
Once suitable attention was paid to the details of authorization
semantics, it became clear that server implementations of support
for draft POSIX ACLs could not support the core UNIX ACL subset of
the NFSv4 ACL model and that client implementations would be
restricted or would require hard-to-specify ACL mappings.
6.3. New Approach to Treatment of ACLs
The new approach, which will be explained in more detail in the
companion document [I-D.dnoveck-nfsv4-acls] takes a very different
approach, with the following characteristics:
* The architecture supports, at least potentially, the possibility
of supporting multiple ACL models, with authorization for each
file system object being controlled by a single ACL model.
* The UNIX ACL subset is the basis of the canonical description
which can assume is available, if any of the attributes defining
authorization based on the NFSv4 ACL model is supported (i.e Acl
or Dacl).
* Additions to this core are treated as OPTIONAL extensions. each of
which might or might not be supported by a server implementing the
Acl attribute.
* A new per-fs OPTIONAL attribute is defined to allow clients to
determine which NFsv4 ACL model extensions are supported, if any.
This new approach is embodied in the companion document
[I-D.dnoveck-nfsv4-acls]. As a result, that document has three
functions:
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* It defines the role of Access Control Lists in the security
architecture including the definition of the NFSv4 ACL model.
Other models are not specified but can be added as extensions.
* Together with this document it defines security for all current
minor versions, updating [RFC7530] and, together with the other
rfc5661bis documents, obsoleting [RFC8881]. This applies to all
NFSv4 current minor versions.
* Defining an extension that allows clients to determine the NFSv4
ACL extensions supported by any given file system. This applies
to NFSv4.2 and subsequent minor versions.
7. Authorization in General
There are three distinct methods of checking whether NFSv4 requests
are authorized:
* The most important methods of authorization is used to effect
user-based file access control, as described in Section 8. These
methods are often termed "Discretionary access control" because
they rely on attributes set by particular users, to control
acceptable file access.
This requires the identification of the user making the request.
Because of the central role of such access control in providing
NFSv4 security, server implementations SHOULD NOT use such
identifications when they are not authenticated. In this context,
valid reasons to do otherwise are limited to the compatibility and
maturity issues discussed in Section 18.1.4
* NFSv4.2, via the labelled NFS feature, provides an additional
potential requirement for request authorization. The labelled NFS
provides "Mandatory access control" not under the control of
individual users.
For reasons made clear in Section 11, there is no realistic
possibility of the server using the data defined by existing
specifications of this feature to effect request authorization.
While it is possible for clients to provide this authorization,
the lack of detailed specifications makes it impossible to
determine the nature of the identification used and whether it can
appropriately be described as "authentication".
* Since undesired changes to server-maintained locking state (and,
for NFSv4.1, session state) can result in denial of service
attacks (see Section 18.4.7), server implementations SHOULD take
steps to prevent unauthorized state changes. This can be done by
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implementing the state authorization restrictions discussed in
Section 12. Because these restrictions apply on a per-peer basis
rather than being affected by the identity of the user making the
request, it is better to consider them as part of "Mandatory
access control".
8. User-based File Access Authorization
8.1. Attributes for User-based File Access Authorization
NFSv4 provides for multiple authentication models, controlled by the
support for particular recommended attributes implemented by the
server, as discussed below:
* Consensus Needed (Item #57f)]: The REQUIRED attributes owner,
owning_group, and mode enable use of a POSIX-based authorization
model, as described in Section 8.3. Since these attributes are
always supported, this authorization model is aways available.
* [Consensus Needed (Item #67d)]: The acl attribute (or the
attribute dacl in NFSv4.1) can provide an ACL-based authorization
model as described in Section 8.4.
[Consensus Needed (Items #67d)]: When one of these two ACE types
are not supported, this authorization model is, to a degree,
incompatible with the mode-based one, since there are some modes
that cannot be represented as a corresponding NFSv4 ACL, when
using only a single ACE type. See Sections 10.2 and 10.7 of
[I-D.dnoveck-nfsv4-acls] for details
* [Consensus Needed (Items #67d), Through end of bulleted item]:
When attributes associated with other ACL models are supported,
these provide an ACL-based authorization model as described in
Section 8.4.
When these attributes are supported together with other attributes
that affect ACL-based file access authorization (e.g., Acl or
Dacl), only one ACL-based authorization model can be effective for
a give file object at a time. The specification of how this
restriction is effected is the responsibility of the extension
defining the new ACL model.
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8.2. Handling of Multiple Parallel File Access Authorization Models
ACLs and modes represent two well-established models for specifying
user-based file access permissions. NFSv4 provides support for
either or both depending on the attributes supported by the server
and, in cases in which both ACLs and the mode attribute are
supported, the actual attributes set for a particular object.
When the ACLs in question implement the NFsv4 ACL model, interaction
with POSIX authorization is as described below. The description of
such interaction for other ACL models is the responsibility of the
document defining the new ACL model.
* Given that the attributes mode, owner, owner group are REQUIRED
the POSIX-based authorization model, described in Section 8.3 can
be used.
* [Consensus Needed (Items #67e, #57g)]: When the acl (or dacl)
attribute is supported together with one of the ACE types ALLOW
and DENY, the acl based authorization model, described in
Section 8.4 can be used
When both authorization models can be used, there are difficulties
that can arise because the ACL-based model provides finer-grained
access control than the POSIX model. The ways of dealing with these
difficulties appear later in this section while more detail on the
appropriate handling of this situation, which might depend on the
particular ACL model or the minor version used, appears in
Section 10.
The following describes NFSv4's handling in supporting multiple
authorization models for file access.
* A server needs to provide the appropriate POSIX semantics if no
ACL-based attributes have ever been assigned to object. These
semantics include the restriction of the ability to modify the
mode, owner and owner-group to the current owner of the file.
* If a server supports ACL attributes, it needs to provide NFSv4 ACL
semantics as described in this document for all objects for which
the ACL attributes have actually been set. This includes the ACL-
based restrictions on the authorization to modify the mode, owner
and owner_group attributes.
* If ACL attributes have never been set on an object, via
inheritance or explicitly, the behavior is to be the behavior
mandated by POSIX, including those provisions that restrict the
setting of authorization-related attributes.
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* If the ACL attributes have been previously set on an object,
either explicitly or via inheritance:
- [Previous Treatment]: Setting only the mode attribute should
effectively control the traditional UNIX-like permissions of
read, write, and execute on owner, owner_group, and other.
[Author Aside]: It isn't really clear what the above paragraph
means, especially as it governs the handling of ACEs
designating specific users and groups which are not the owner
and have no overlap with the owning group
{Consensus Needed (Item #19a)]: Setting only the mode
attribute, will result in the access of the file being
controlled just it would be if the existing acl did not exist,
with file access decisions as to read made in accordance with
the mode set. The ALLOW and DENY ACEs in the ACL will reflect
the modified security although there is no need to modify AUDIT
and ALARM aces or mask bits not affected by the mode bits, such
as SYNCHRONIZE.
- [Previous Treatment]: Setting only the mode attribute should
provide reasonable security. For example, setting a mode of
000 should be enough to ensure that future OPEN operations for
OPEN4_SHARE_ACCESS_READ or OPEN4_SHARE_ACCESS_WRITE by any
principal fail, regardless of a previously existing or
inherited ACL.
[Author Aside]: We need to get rid of or provide some
replacement for the subjective first sentence. While the
specific example given is unexceptionable, it raises questions
in other cases as to what would constitutes "reasonable
semantics". While the resolution of such questions would be
subject to dispute, the author believes that consensus item
#19a deals with the matter adequately. As a result he
proposes, that the that this bullet be removed and that the
second-level list be collapsed to a single paragraph.
* Although [RFC7530] and [RFC8881] present different descriptions of
the specific semantic requirements relating to the interaction of
mode and ACL attributes, the differences are quite small, with the
most important ones deriving from the absence of the
set_mode_masked attribute. The unified treatment in Section 10
will indicate where version-specific differences exist.
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8.3. POSIX Authorization Model
Uses the REQUIRED attributes Mode, Owner, and Owner_group to
authorize actions applying to files and directories. Aside from the
conversion of Owner and Owner_group to the forms of strings, this
mirrors authorization in NFSv3, in being based on semantics defined
by POSIX, originally used to control local file access.
There are some potential differences to be noted:
* The use of "owner-override" semantics is allowed. This variant of
authorization semantics was necessary in earlier NFS versions, to
allow continued access to open files even when the mode had been
changed subsequent to the open.
Even though it is no longer necessary because NFSv4 is a stateful
protocol, it is still allowed.
* [Consensus needed (Item #6b)]: The bit MODE_SVTX within the Mode
attribute on directories affects the authorization of REMOVE and
RENAME.
8.4. ACL-based Authorization Model
While the relevant details regarding the NFsv4 ACL model appear in
[I-D.dnoveck-nfsv4-acls], this model can be helpfully summarized as
follows:
The use of NFSv4 ACLs, via setting the OPTIONAL attributes Acl and
Dacl, allows a more flexible approach to authorization in that the
individual entries (ACEs) within the ACL can assign different
privileges to different users or groups of users.
The ACEs within an ACL are examined in sequence with those
designating the user requesting or a group to which that user belongs
affecting the authorization decision to see if a needed authorization
for the requested operation is granted, or, depending the ACE type,
denied.
Once all requested authorization are granted, the operation is
authorized and can proceed. If any requested authorization is
denied, the operation is not authorized. In either case, this
terminates the processing of ACEs.
[Consensus Needed (Item #67f)]: For other ACL models, the specifics
of the authorization process are specified in the document defining
the associated ACL attributes.
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9. Common Considerations for Both File Access Models
[Author Aside, Including List]: The subsections within this section
are derived from Section 6.3 of 8881, entitled "Common Methods.
However, its content is different because of the following:
[Consensus Needed (Item #67g), Through end of section]:
* This section has been rewritten to deal with issues common to both
file access models, which now appears to have not been the
original intention.
* This section only deal ADL-based authorization when the NFSv4 ACL
model is being used.
When support for other sorts of ACL models have been added to the
protocol, the appropriate discussion is the responsibility of the
extension adding support for that ACL model.
In any case, the following changes have been made:
* The section "Server Considerations" has been revised to deal with
both the mode and acl attributes, since the points being made
apply, in almost all cases, to both attributes.
* The section "Client Considerations" has been heavily revised,
since what had been there did not make any sense to me.
* The section "Computing a Mode Attribute from an ACL" has been
moved to Section 10.3 of [I-D.dnoveck-nfsv4-acls] since it deals
with the co-ordination of the POSIX and ACL authorization models.
9.1. Handling of ACCESS and OPEN Operations
The primary means by which client-side users find out whether
particular operations are authorized is to attempt those operations
and have them executed successfully or rejected with an error.
However, clients can use the ACCESS operation to determine in advance
whether operations are permitted done without actually attempting
those operations.
The use of the ACCESS operation is of particular importance when ACLs
exist. This is due in part to the complexity of the ACLs but also
derives from cases in which the client is incapable of determining
whether a specific ACE applies to a particular request, as when some
of the special who-field values are used in an ACE.
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An additional difficulty precluding the client performing its own
interpretation of an ACL concerns the existing specifications' lack
of clear requirements for server support, making it difficult for the
client to determine how a server might choose to behave in some
cases. Even if this lack of specificity were remedied by clarifying
the specification servers would still exist that reflected the
previously valid scope of acceptable server behavior. As a result,
it is impossible for the client to predict determine the choices a
server might make without using ACCESS.
Similarly, the OPEN operation can determine access rights in advance
of actual access. There are two differences from ACCESS that clients
need to be aware of, since they might require use of ACCESS in
addition to OPEN or make the results of OPEN incompatible with the
results of attempting IO operations.
* The permission model of OPEN is coarser-grained than that provided
for by ACCESS or the ACE masks defined as part of the definition
of NFSv4 ACLs.
When reading files, the fact that OPEN treats requests to open a
file to read it and to execute it together means that the client
will need to do an ACCESS operation to determine whether
particular reads are to be allowed.
When writing files, the fact OPEN that does not distinguish
between WRITEs which extend the file and those that modify bytes
already written means that the client will need to get the
permission information using ACCESS or be prepared to have some
set of WRITES on a file open for Write rejected as unauthorized.
* [Consensus needed, Entire Bulleted Item (Item #23a)]: Just as with
ACCESS, the granting of permission does not foreclose subsequent
permission changes, there are good reasons for servers to provide
ways of allowing IO allowed at OPEN time to continue even in the
face of permission changes that would normally be expected to make
the IO operation invalid. These reasons derive from the fact that
local operations work this way and that it is often necessary for
requests over NFSv4 to behave just as they would if done locally.
As a result, it is desirable for servers to provide this expected
behavior in some way since application programs have come to
depend on it.
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One way of handling this situation is described in Section 9.2.
It deals with the most common of these situations in which the
owner of the is both writing a file and seeking to make it
subsequently unwritable, and while it does not deal with all such
situations, it has proven satisfactory for many NFS protocols over
a long time period
Another way of dealing with this situation involves the server
explicitly using the stateid to reference a particular open made
by a user, and avoiding reverification of access for READs and
WRITEs made by that user since that verification was made at OPEN
time. To do this safely, the server needs to have authenticated
principals and client peers and, in order to prevent man-in-middle
attacks, it s necessary for all connections on which stateids are
sent to provide encryption.
9.2. Server Considerations
The server uses the mode attribute or the acl attribute applying the
algorithm described in Section 9 of [I-D.dnoveck-nfsv4-acls] to
determine whether an ACL allows access to an object.
[Author Aside, Including List]: The list previously in this section
(now described as "Previous Treatment" combines two related issues in
a way which obscures the very different security-related consequences
of two distinct issues:
* In some cases an operation will be authorized but is not allowed
for reasons unrelated to authorization.
This has no negative effect on security.
* The converse case can have troubling effects on security which are
mentioned in this section and discussed in more detail in
Section 18
[Author Aside, Including List]: The items in that list have been
dealt with as follows:
* The first and sixth items fit under the first (i.e. less
troublesome) of these issues. They have been transferred into an
appropriate replacement list.
* The third item is to be deleted since it does not manifest either
of these issues. In fact, it refers to the semantics described in
Section 7.2. of [I-D.dnoveck-nfsv4-acls].
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* The second, fourth and fifth items need to be addressed in a new
list dealing with the potentially troublesome issues arising from
occasions in which the access semantics previously described are
relaxed, for various reasons.
Included are cases in which previous specifications explicitly
allowed this by using the term "MAY" and others in which the
existence of servers manifesting such behavior was reported, with
the implication that clients need to be prepared for such
behavior.
[Previous Treatment, Including List (Items #22a, #41a, #52b)]:
However, these attributes might not be the sole determiner of access.
For example:
* In the case of a file system exported as read-only, the server
will deny write access even though an object's file access
attributes would grant it.
* Server implementations MAY grant ACE4_WRITE_ACL and ACE4_READ_ACL
permissions to prevent a situation from arising in which there is
no valid way to ever modify the ACL.
* All servers will allow a user the ability to read the data of the
file when only the execute permission is granted (e.g., if the ACL
denies the user the ACE4_READ_DATA access and allows the user
ACE4_EXECUTE, the server will allow the user to read the data of
the file).
* Many servers implement "owner-override semantics" in which the
owner of the object is allowed to perform accesses that are denied
by the ACL or mode bits This may be helpful, for example, to allow
users continued access to open files on which the permissions have
changed.
* Many servers provide for the existence of a "superuser" that has
privileges beyond an ordinary user. The superuser may be able to
read or write data or metadata in ways that would not be permitted
by the ACL or mode attributes.
* A retention attribute might also block access otherwise allowed by
ACLs (see Section 5.13 of [RFC8881]).
[Consensus Needed, Including List (Item #22a)]: It needs to be noted
that, even when an operation is authorized, it may be denied for
reasons unrelated to authorization. For example:
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* In the case of a file system exported as read-only, the server
will deny write access even though an object's file access
attributes would authorize it.
* A retention attribute might also block access otherwise allowed by
ACLs (see Section 5.13 of [RFC8881]).
[Author aside, including List, (Item #22a)]: Unlike other cases in
which previous specs have granted permission to the server to expand
allowed behavior, in the case of "owner-override semantics", the need
for supersession does not arise from security issues but instead
derives from:
* The unacceptable implication in the superseded text that the fact
that a server chooses to do something, means that the client needs
to accept that behavior.
* The lack of any definition of the term "owner-override semantics.
Subsequent investigation has led to the conclusion that the
semantics being referred to derive from the following material in
[RFC1813]:
Another problem arises due to the usually stateful open
operation. Most operating systems check permission at open
time, and then check that the file is open on each read and
write request. With stateless servers, the server cannot
detect that the file is open and must do permission checking on
each read and write call. UNIX client semantics of access
permission checking on open can be provided with the ACCESS
procedure call in this revision, which allows a client to
explicitly check access permissions without resorting to trying
the operation. On a local file system, a user can open a file
and then change the permissions so that no one is allowed to
touch it, but will still be able to write to the file because
it is open. On a remote file system, by contrast, the write
would fail. To get around this problem, the server's
permission checking algorithm should allow the owner of a file
to access it regardless of the permission setting. This is
needed in a practical NFS version 3 protocol server
implementation, but it does depart from correct local file
system semantics. This should not affect the return result of
access permissions as returned by the ACCESS
As a result, it appears that NFSv4 servers have implemented
semantics different from that provided for by POSIX that were
defined in order to deal with lack of an open operation in NFsv3
in the context of NFSv4, which does have an open operation.
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* The need to make it clear that an alternate approach to the issue
of permission change after open is to allow the OPEN's permission
check to be dispositive as long as the appropriate security
infrastructure is in place to allow client stateids to be trusted.
[Consensus Needed, (Item #22a)]: There are also cases in which the
converse issue arises, so that an operation which is not authorized
as specified by the mode and ACL attributes is, nevertheless,
executed as if it were authorized. Because previous NFSv4
specifications have cited the cases listed below without reference to
the security problems that they create, it is necessary to discuss
them here to provide clarification of the security implications of
following this guidance, which might now be superseded. These cases
are listed below and discussed in more detail in Section 18.1.3.
[Consensus Needed, Including List (Item #22a, #41a, #52b)]: In the
following list, the treatment used in [RFC8881] is quoted, while the
corresponding text in [RFC7530] is essentially identical.
* [RFC8881] contains the following, which is now superseded:
Server implementations MAY grant ACE4_WRITE_ACL and
ACE4_READ_ACL permissions to prevent a situation from arising
in which there is no valid way to ever modify the ACL.
While, as a practical matter, there do need to be provisions to
deal with this issue, the "MAY" above is too broad, in that it
describes the motivation without any limits providing appropriate
restriction on the steps that might be taken to deal with the
issue. See Section 18.1.3 for the updated treatment of this
issue.
* [RFC8881] contains the following, which is now superseded:
Many servers implement owner-override semantics in which the
owner of the object is allowed to override accesses that are
denied by the ACL. This may be helpful, for example, to allow
users continued access to open files on which the permissions
have changed.
The principal problem with the above statement is the lack of a
clear definition of the term "owner-override semantics". Also, it
needs to be made clear that the fact that a server manifests a
particular behavior does not imply that it is valid according to
the protocol specification.
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In this case, investigation has led to the conclusion that the
semantics being referred to derive from the discussion of NFSv3
semantics appearing Section 4.4 of [RFC1813] and that this
handling has been implemented in a number of NFSv4 servers,
despite the fact that NFSv4, unlike NFSv3, does have an open
operation.
With regard to the second sentence of the quotation above, it is
not clear whether it is helpful or hurtful to allow continued
access to open files which have become inaccessible due to changes
in security and it is not clear that the working group will make a
decision on the matter in this document, despite the obvious
security implications. In any case, the resolution is unlikely to
depend on whether the owner is involved.
Since this divergence from POSIX semantics is unlikely to result
in security issues, we can clarify the above by saying that a
server MAY diverge from POSIX semantics by always allowing READ or
WRITE to be done by the file owner but that this divergence MUST
NOT affect the handling of OPEN and ACCESS. It also needs to be
noted that servers could address the issue of preventing
permission change affecting that handling of READs and WRITEs of
open files as described in Section 9.1.
* [RFC8881] contains the following, which is now superseded:
Many servers have the notion of a "superuser" that has
privileges beyond an ordinary user. The superuser may be able
to read or write data or metadata in ways that would not be
permitted by the ACL or mode attributes.
While many (or almost all) systems in which NFSv4 servers are
embedded, have provisions for such privileged access to be
provided, it does not follow that NFSv4 servers, as such, need to
have provision for such access.
Providing such access as part of the NFSv4 protocols, would
necessitate a major revision of the semantics of ACL including
such troublesome matters as the proper handling of AUDIT and ALARM
ACEs in the face of such privileged access.
Because of the effect such unrestricted access might have in
facilitating and perpetuating attacks, Section 18.1.3 will the new
approach to this issue, while Section 18.4.1, will explain how
such access is addressed in the threat analysis.
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9.3. Client Considerations
[Previous Treatment]: Clients SHOULD NOT do their own access checks
based on their interpretation of the ACL, but rather use the OPEN and
ACCESS operations to do access checks. This allows the client to act
on the results of having the server determine whether or not access
is to be granted based on its interpretation of the ACL.
[Author Aside]: With regard to the use of "SHOULD NOT" in the
paragraph above, it is not clear what might be valid reasons to
bypass this recommendation. Perhaps "MUST NOT" or "are not advised
to" would be more appropriate.
[Consensus Needed (Item #23b)]: Clients are not expected to do their
own access checks based on their interpretation of the ACL, but
instead use the OPEN and ACCESS operations to do access checks, where
this is possible. This allows the client to act on the results of
having the server determine whether or not access is to be granted
based on its interpretation of the ACL, rather then the client's
which might be different. For a full discussion of limitations on
the use of ACCESS and appropriate client approaches to deal these
limitations, see Section 9.1.
[Previous Treatment]: Clients must be aware of situations in which an
object's ACL will define a certain access even though the server will
not enforce it. In general, but especially in these situations, the
client needs to do its part in the enforcement of access as defined
by the ACL.
[Author Aside]: Despite what is said later, the only such case I know
of is the use of READ and EXECUTE where the client, but not the
server, has any means of distinguishing these. I don't know of any
others. If there were, how could ACCESS or OPEN be used to verify
access?
[Consensus Needed (Item #23b)]; Clients need to be aware of
situations in which an object's ACL will define a certain access even
though the server is not in position to enforce it because the server
does not have the relevant information, such as knowing whether a
READ is for the purpose of executing a file. Because of such
situations, the client needs to do be prepared to do its part in the
enforcement of access as defined by the ACL.
To do this, the client will send the appropriate ACCESS operation
prior to servicing the request of the user or application in order to
determine whether the user or application is to be granted the access
requested.
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[Previous Treatment (Item #24a)]: For examples in which the ACL may
define accesses that the server doesn't enforce, see Section 9.2.
[Author Aside]: The sentence above is clearly wrong since that
section is about enforcement the server does do. The expectation is
that it will be deleted as part of Consensus Item #24a.
10. Combining Authorization Models
[Consensus Needed (Item #67h), Through end of section]:
The existence of multiple authorization models where ACLs are
supported raises a number of issue regarding how these two models are
to interact. For the NFSv4 ACL model, these are deal with in
[I-D.dnoveck-nfsv4-acls], while for other ACL models, they are the
responsibility of the extension defining the new ACL model. These
include:
* How the value of the mode attribute is to computed from the ACL.
* How the case of the mode and ACL both being set in the same
SETATTR are to be dealt with.
* How ACLs are modified in response to changes in the mode
attribute.
* How the ACL mode is to be modified when the mode attribute is set.
11. Labelled NFS Authorization Model
The attribute sec_label was intended to enable an authorization model
focused on Mandatory Access Control.
Not much can be said about this feature because the specification, in
the interest of flexibility, has left important features undefined in
order to allow future extension. As a result, we have something that
is a framework to allow Mandatory Access Control rather than one to
provide it. In particular,
* The sec_label attribute, which provides the objects label has no
existing specification.
* There is no specification of the format of the subject labels or
way to authenticate them.
* As a result, all authorization takes place on the client, and the
server simply accepts the client's determination.
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This arrangements shares important similarities with AUTH_SYS. As
such it makes sense:
* To require/recommend that an encrypted connection be used.
* To require/recommend that client and server peers mutually
authenticate as part of connection establishment.
* That work be devoted to providing a replacement without the above
issues.
12. State Modification Authorization
Modification of locking and session state data are not be done by a
client other than the one that created the lock. For this form of
authorization, the server needs to identify and authenticate client
peers rather than client users.
Such authentication is not directly provided by any RPC auth flavor.
However, RPC can, when suitably configured, provide this
authentication on a per-connection basis.
NFSv4.1 defines a number of ways to provide appropriate authorization
facilities. These will not be discussed in detail here but the
following points need to be noted:
* NFSv4.1 defines the MACHCRED mechanism which uses the RPCSEC_GSS
infrastructure to provide authentication of the clients peer.
However, this is of no value when AUTH_SYS is being used.
* NFSv4.1 also defines the SSV mechanism which uses the RPCSEC_GSS
infrastructure to enable it to be reliably determined whether two
different client connections are connected to the same client. It
is unclear whether the word "authentication" is appropriate in
this case. As with MACHCRED, this is of no value when AUTH_SYS is
being used.
* Because of the lack of support for AUTH_SYS and for NFSv4.0, it is
quite desirable for clients to use and for servers to require the
use of client-peer authentication as part of connection
establishment.
When unauthenticated clients are allowed, their state is exposed to
unwanted modification as part of disruption or denial-of-service
attacks. As a result, the potential burdens of such attacks are felt
principally by clients who choose not to provide such authentication.
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13. Other Uses of Access Control Lists
Whether the Acl or Sacl attributes are used, AUDIT and ALARM ACEs
provide security-related facilities separate from the file access
authorization provided by ALLOW and DENY ACEs
* AUDIT ACEs provide a means to audit attempts to access a specified
file by specified sets of principals.
* ALARM ACEs provide a means to draw special attention to attempts
to access specified files by specified sets of principals.
Whichever attribute is used to store the relevant ACEs, only these
ACE types are processed while other types are ignored. While the
processing of successive ACEs is similar to that described in
Section 8.4, these two scans cannot be combined. The authorization-
related scan needs to be done first and the result of that check,
whether success or failure is needed to govern the processing (or
not) of AUDIT and ALARM ACEs.
14. Identification, Authentication, and Related Services
Various objects and subjects need to be identified for a protocol to
function. For it to be secure, many of these need to be
authenticated so that incorrect identification is not the basis for
attacks.
14.1. RPC Security Flavors
As described in "Authentication", Section 7 of [RFC5531], RPC
security is encapsulated in the RPC header, via a security or auth
flavor, and information specific to the specified security flavor.
Every RPC header potentially conveys information used to identify and
authenticate a client and server. As discussed in Section 14.1.1,
some security flavors provide additional security services.
NFSv4 clients and servers MUST implement RPCSEC_GSS. (This
requirement to implement is not a requirement to use.) Other
flavors, such as AUTH_NONE and AUTH_SYS, MAY be implemented as well.
14.1.1. RPCSEC_GSS and Security Services
RPCSEC_GSS [RFC2203] uses the functionality of GSS-API. This allows
for the use of multiple security mechanisms by the RPC layer while
reducing the scope of implementation otherwise needed when adding RPC
security flavors.
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14.1.1.1. Identification, Authentication, Integrity, Privacy
Via the GSS-API, RPCSEC_GSS can be used to identify and authenticate
users on clients to servers, and servers to users. It can also
perform integrity checking on the entire RPC message, including the
RPC header, and on the arguments or results. Finally, privacy,
usually via encryption, is a service available with RPCSEC_GSS.
Privacy is performed on the arguments and results. Note that if
privacy is selected, integrity, authentication, and identification
are enabled. If privacy is not selected, but integrity is selected,
authentication and identification are enabled. If integrity and
privacy are not selected, but authentication is enabled,
identification is enabled. RPCSEC_GSS does not provide
identification as a separate service.
Although GSS-API has an authentication service distinct from its
privacy and integrity services, GSS-API's authentication service is
not used for RPCSEC_GSS's authentication service. Instead, each RPC
request and response header is integrity protected with the GSS-API
integrity service, and this allows RPCSEC_GSS to offer per-RPC
authentication and identity. See [RFC2203] for more information.
NFSv4 client and servers MUST support RPCSEC_GSS's integrity and
authentication service. NFSv4 servers MUST support RPCSEC_GSS's
privacy service. NFSv4.1 clients SHOULD support RPCSEC_GSS's privacy
service, while NFSv4.0 clients support RPCSEC_GSS's privacy service.
14.1.1.2. Security Mechanisms for NFSv4
RPCSEC_GSS, via GSS-API, normalizes access to mechanisms that provide
security services. To enable access to those services, NFSv4 clients
and servers MUST support the Kerberos V5 security mechanism.
The use of RPCSEC_GSS requires selection of mechanism, quality of
protection (QOP), and service (authentication, integrity, privacy).
For the mandated security mechanisms, NFSv4 specifies that a QOP of
zero is used, leaving it up to the mechanism or the mechanism's
configuration to map QOP zero to an appropriate level of protection.
Each mandated mechanism specifies a minimum set of cryptographic
algorithms for implementing integrity and privacy. NFSv4 clients and
servers MUST be implemented on operating environments that comply
with the REQUIRED cryptographic algorithms of each REQUIRED
mechanism.
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14.1.1.2.1. Kerberos V5
The Kerberos V5 GSS-API mechanism as described in [RFC4121] MUST be
implemented with the RPCSEC_GSS services as specified in the
following table:
column descriptions:
1 == number of pseudo flavor
2 == name of pseudo flavor
3 == mechanism's OID
4 == RPCSEC_GSS service
5 == support requirements
1 2 3 4 5
------------------------------------------------------------------
390003 krb5 1.2.840.113554.1.2.2 rpc_gss_svc_none [both]
390004 krb5i 1.2.840.113554.1.2.2 rpc_gss_svc_integrity [both]
390005 krb5p 1.2.840.113554.1.2.2 rpc_gss_svc_privacy [priv}
Regarding the contents of column 5:
* [both] indicates that client and server support are both REQUIRED.
* For [priv], server support is REQUIRED while client support
depends on the minor version:
For NFSv4.0, client support is RECOMMENDED.
For NFSv4.1 and beyond, client support is REQUIRED.
Note that the number and name of the pseudo flavor are presented here
as a mapping aid to the implementor. Because the NFSv4 protocol
includes a method to negotiate security and it understands the GSS-
API mechanism, the pseudo flavor is not needed. The pseudo flavor is
needed for NFSv3 since security negotiation is done via the MOUNT
protocol as described in [RFC2623].
At the time NFSv4.1 was originally specified, the Advanced Encryption
Standard (AES) with HMAC-SHA1 was a REQUIRED algorithm set for
Kerberos V5. In contrast, when NFSv4.0 was originally specified,
weaker algorithm sets were REQUIRED for Kerberos V5, and were
REQUIRED in the original NFSv4.0 specification, because the Kerberos
V5 specification at the time did not specify stronger algorithms.
The NFSv4 specification does not specify REQUIRED algorithms for
Kerberos V5, and instead, the implementor is expected to track the
evolution of the Kerberos V5 standard if and when stronger algorithms
are specified.
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14.1.1.2.1.1. Security Considerations for Cryptographic Algorithms in
Kerberos V5
When deploying NFSv4, the strength of the security achieved depends
on the existing Kerberos V5 infrastructure. The algorithms of
Kerberos V5 are not directly exposed to or selectable by the client
or server, so due diligence is required by those deploying NFSv4 to
ensure that security is sufficient.
14.1.1.3. GSS Server Principal
Regardless of what security mechanism under RPCSEC_GSS is being used,
the NFS server MUST identify itself in GSS-API via a
GSS_C_NT_HOSTBASED_SERVICE name type. GSS_C_NT_HOSTBASED_SERVICE
names are of the form:
service@hostname
For NFS, the "service" element is
nfs
Implementations of security mechanisms will convert nfs@hostname to
various different forms. For Kerberos V5, the following form is
RECOMMENDED:
nfs/hostname
14.2. Identification vs. Authentication
It is necessary to be clear about this distinction which has been
obscured in the past, by the use of the term "RPC Authentication
Flavor" in connection with situation in which identification without
authentication occurred or in which there was neither identification
nor authentication involved. As a result, we will use the term "Auth
Flavors" instead
14.3. Items to be Identified
Some identifier are not security-relevant and can used be used
without authentication, given that, in the authorization decision,
the object acted upon needs only to be properly identified
* File names are of this type.
Unlike the case for some other protocols, confusion of names that
result from internationalization issues, while an annoyance, are
not relevant to security. If the confusion between LATIN CAPITAL
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LETTER O and CYRILLIC CAPITAL LETTER O, results in the wrong file
being accessed, the mechanisms described in Section 8 prevent
inappropriate access being granted.
Despite the above, it is desirable if file names together with
similar are not transferred in the clear as the information
exposed may give attackers useful information helpful in planning
and executing attacks.
* The case of file handles is similar.
Identifiers that refer to state shared between client and server can
be the basis of disruption attacks since clients and server
necessarily assume that neither side will change the state corpus
without appropriate notice.
While these identifiers do not need to be authenticated, they are
associated with higher-level entities for which change of the state
represented by those entities is subject to peer authentication.
* Unexpected closure of stateids or changes in state sequence values
can disrupt client access as no clients have provision to deal
with this source of interference.
While encryption may make it more difficult to execute such
attacks attackers can often guess stateid's since server generally
not randomize them.
* Similarly, modification to NFSv4.1 session state information can
result in confusion if an attacker changes the slot sequence by
assuring spurious requests. Even if the request is rejected, the
slot sequence is changed and clients may a difficult time getting
back in sync with the server.
While encryption may make it more difficult to execute such
attacks attackers can often guess slot id's and obtain sessionids
since servers generally do not randomize them.
it is necessary that modification of the higher-level entities be
restricted to the client that created them.
* For NFSv4.0, the relevant entity is the clientid.
* for NFSv4.1, the relevant entity is the sessionid.
Identifiers describing the issuer of the request, whether in numeric
or string form always require authentication.
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14.4. Authentication Provided by specific RPC Auth Flavors
Different auth flavors differ quite considerably, as discussed below;
* When AUTH_NONE is used, the user making the request is neither
authenticated nor identified to the server.
Also, the server is not authenticated to the client and has no way
to determine whether the server it is communicating with is an
imposter.
* When AUTH_SYS is used, the user making is the request identified
but there no authentication of that identification.
As in the previous case, the server is not authenticated to the
client and has no way to determine whether the server it is
communicating with is an imposter.
* When RPCSEC_GSS is used, the user making the request is
authenticated as is the server peer responding.
14.5. Authentication Provided by other RPC Security Services
Depending on the connection type used, RPC may provide additional
means of authentication. In contrast with the case of RPC auth
flavors, any authentication happens once, at connection
establishment, rather than on each RPC request. As a result, it is
the client and server peers, rather than individual users that are
authenticated.
* For many types of connections such as those created TCP without
RPC-with-TLS and RPC-over-RDMA version 1, there is no provision
for peer authentication.
As a result use of AUTH_SYS using such connections is inherently
problematic.
* Some connection types provide for the possibility of mutual peer
authentication. These currently include only those established by
RPC-with-TLS. However, given the value of peer authentication,
there is reason to believe further means of providing such
services will be defined.
15. Security of Data in Flight
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15.1. Data Security Provided by Services Associated with Auth Flavors
The only auth flavor providing these facilities is RPCSEC_GSS. When
this auth flavor is used, data security can be negotiated between
client and server as described in Section 16. However, when data
security is provided for the connection level, as described in
Section 15.2, the negotiation of privacy and integrity support,
provided by the auth flavor, is unnecessary,
Other auth flavors, such as AUTH_SYS and AUTH_NONE have no such data
security facilities. When these auth flavors are used, the only data
security is provided on a per-connection basis.
15.2. Data Security Provided for a Connection by RPC
RPC, in many case, provide data security for all transactions
performed on a connection, eliminating the need for that security to
be provided or negotiated by the selection of particular auth
flavors, mechanisms, or auth-flavor-associated services.
16. Security Negotiation
[Author Aside]: All unannotated paragraphs in this section are
considered part of Consensus Item #32a.
Because of the availability of security-related services associated
with the transport layer, the security negotiation process has been
enhanced so that the server can indicate the connection-level
services needed together with the determination of the appropriate
security triples to use that were previously the only subject of
negotiation (See Section 16.1 for discussion of negotiation of
appropriate sets of security triples.
Without the addition of pseudo-flavors defining connection-level
services needed, use of SECINFO would be limited to selection of a
particular auth flavor and security triple with connection
characteristics selected by a process of trial and error.
When the SECINFO response does not indicate the connection
characteristics needed, either because it does not contain any of the
pseudo-flavors described below or because the only such pseudo-flavor
is PFL_ANY, the client only knows the available auth flavors together
with the auth-flavor-associated services and needs to use trial and
error to determines the server requirements for connection-provided
services, as described below:
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* For each SECINFO entry, the client can attempt to make a
connection with whatever connection characteristics that its (i.e.
the client's) security policy requires. When that connection is
successfully established, it can be used to access the current
portion of the server, as indicated by SECINFO or SECINFO_NONAME.
In many cases, this will be a connection without rpc-with-tls,
while in other cases the client will require rpc-with-tls to be
assured of encryption or of encryption with mutual peer
authentication.
* If that connection cannot be established, because the server does
not provide support for the requested characteristics, then the
client moves on to the next auth-flavor entry. If there are no
further entries, this portion of the server namespace cannot be
accessed.
* If the connection cannot be established because it is rejected by
the server because of its security policy, the client takes note
and proceeds to request additional connection-level services on
the next connection request.
For example, if the client attempts an RPC connection of the
server's security policy does not allow that, the connection
rejection will indicate to the client that use of rpc-with-tls is
required. If the server's policy allows non-tls connection but
restricts the auth-flavors that may use such connections, the
requests made with those flavors may be rejected and the client
will also try to establish an rpc-with-tls connection.
Similarly, when an rpc-tls-connection is attempted without mutual
peer authentication, its rejection will result in the client
attempting to establish a connection with mutual peer
authentication.
When the client and server are unable to establish a mutually
acceptable connection to use with a specific auth-flavor, the
client moves on to the next auth-flavor in the SECINFO response.
In order for the server to better communicate its security policies
to the client and limit the cases in which connections are
established only to be rejected due to inappropriate connection
characteristics, the following pseudo-flavor can be used in the
response to SECINFO and SECINFO_NONAME requests.
* PFL_CRYPT indicates that encryption, such as that provided by rpc-
with-tls is to be required on any connection to support access to
the current portion of the server namespace.
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* PFL_CRYPTMPA indicates that encryption such as that provided by
rpc-with-tls is to be required together with mutual peer
authentication on any connection to support access to the current
portion of the server namespace.
* PFL_ANY indicates that, for subsequent auth-flavors, there are no
connection-based restrictions to be assumed by the client,
allowing connections that do not use encryption to be used as well
as those that do.
When any of these flavors are included in a SECINFO or SECINFO_NONAME
response, the procedure described above is modified as follows:
* When scanning the response, if pseudo-flavor entries are
encountered, the client takes note of them, keeping track of the
last such entry encountered.
* When processing a flavor entry in the response, the most recent
pseudo-flavor is used in establishing the connection to be used,
to avoid using connection that the server's security policy will
not allow to be used. The initial connection characteristics used
will be those that meets the requirements specified by the pseudo-
flavor and is consistent with the client's security policy.
As an example of how these pseudo-flavors can be used, consider a
situation in which it is the server's security to only accept
encrypted requests, whether the encryption is provide by RPCSEC_GSS
or by rpc-with-tls. A response consisting of the following entries
will specify this behavior so the client can connect appropriately.
* PFL_CRYPT to indicate that the following auth-flavor entry, in
this case AUTH_SYS, is to be limited to use on connection that
provide encryption of requests and responses.
* AUTH_SYS indicates that use of the auth-flavor AUTH_SYS is
acceptable to access the current portion of the server namespace.
Because of the preceding PFL_CRYPT, the client knows that this is
only accepted on encrypted connections.
If the server supports rpc-with-tls and allows its use together
with AUTH_SYS, this connection is used. Otherwise processing
proceeds to use subsequent entries.
* PFL_ANY to indicate that the subsequent auth-flavor entry, in this
case RPCSEC_GSS, is not limited to any particular connection type.
* RPC_SECGSS indicating the requirement for encryption provided by
rpcsec_gss.
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16.1. Security Triple Negotiation
With the NFSv4 server potentially offering multiple security
mechanisms, the client needs a method to determine or negotiate which
mechanism is to be used for its communication with the server. The
NFS server may have multiple points within its file system namespace
that are available for use by NFS clients. These points can be
considered security policy boundaries, and, in some NFS
implementations, are tied to NFS export points. In turn, the NFS
server may be configured such that each of these security policy
boundaries may have different or multiple security mechanisms in use.
The security negotiation between client and server SHOULD be done
with a secure channel to eliminate the possibility of a third party
intercepting the negotiation sequence and forcing the client and
server to choose a lower level of security than required or desired.
See Section 18 for further discussion.
16.1.1. NFSv4Security Tuples
An NFS server can assign one or more "security tuples" to each
security policy boundary in its namespace. Each security tuple
consists of a security flavor (see Section 14.1) and, if the flavor
is RPCSEC_GSS, a GSS-API mechanism Object Identifier (OID), a GSS-API
quality of protection, and an RPCSEC_GSS service.
16.1.2. SECINFO and SECINFO_NO_NAME
The SECINFO and SECINFO_NO_NAME operations allow the client to
determine, on a per-filehandle basis, what security tuple is to be
used for server access. In general, the client will not have to use
either operation except during initial communication with the server
or when the client crosses security policy boundaries at the server.
However, the server's policies may also change at any time and force
the client to negotiate a new security tuple.
Where the use of different security tuples would affect the type of
access that would be allowed if a request was sent over the same
connection used for the SECINFO or SECINFO_NO_NAME operation (e.g.,
read-only vs. read-write) access, security tuples that allow greater
access should be presented first. Where the general level of access
is the same and different security flavors limit the range of
principals whose privileges are recognized (e.g., allowing or
disallowing root access), flavors supporting the greatest range of
principals should be listed first.
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16.1.3. Security Error
Based on the assumption that each NFSv4 client and server MUST
support a minimum set of security (i.e., Kerberos V5 under
RPCSEC_GSS), the NFS client will initiate file access to the server
with one of the minimal security tuples. During communication with
the server, the client may receive an NFS error of NFS4ERR_WRONGSEC.
This error allows the server to notify the client that the security
tuple currently being used contravenes the server's security policy.
The client is then responsible for determining (see Section 16.1.3.1)
what security tuples are available at the server and choosing one
that is appropriate for the client.
16.1.3.1. Using NFS4ERR_WRONGSEC, SECINFO, and SECINFO_NO_NAME
This section explains the mechanics of NFSv4 security negotiation.
16.1.3.1.1. Put Filehandle Operations
The term "put filehandle operation" refers to PUTROOTFH, PUTPUBFH,
PUTFH, and RESTOREFH. Each of the subsections herein describes how
the server handles a subseries of operations that starts with a put
filehandle operation.
16.1.3.1.1.1. Put Filehandle Operation + SAVEFH
The client is saving a filehandle for a future RESTOREFH, LINK, or
RENAME. SAVEFH MUST NOT return NFS4ERR_WRONGSEC. To determine
whether or not the put filehandle operation returns NFS4ERR_WRONGSEC,
the server implementation pretends SAVEFH is not in the series of
operations and examines which of the situations described in the
other subsections of Section 16.1.3.1.1 apply.
16.1.3.1.1.2. Two or More Put Filehandle Operations
For a series of N put filehandle operations, the server MUST NOT
return NFS4ERR_WRONGSEC to the first N-1 put filehandle operations.
The Nth put filehandle operation is handled as if it is the first in
a subseries of operations. For example, if the server received a
COMPOUND request with this series of operations -- PUTFH, PUTROOTFH,
LOOKUP -- then the PUTFH operation is ignored for NFS4ERR_WRONGSEC
purposes, and the PUTROOTFH, LOOKUP subseries is processed as
according to Section 16.1.3.1.1.3.
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16.1.3.1.1.3. Put Filehandle Operation + LOOKUP (or OPEN of an Existing
Name)
This situation also applies to a put filehandle operation followed by
a LOOKUP or an OPEN operation that specifies an existing component
name.
In this situation, the client is potentially crossing a security
policy boundary, and the set of security tuples the parent directory
supports may differ from those of the child. The server
implementation may decide whether to impose any restrictions on
security policy administration. There are at least three approaches
(sec_policy_child is the tuple set of the child export,
sec_policy_parent is that of the parent).
1. sec_policy_child <= sec_policy_parent (<= for subset). This
means that the set of security tuples specified on the security
policy of a child directory is always a subset of its parent
directory.
2. sec_policy_child ^ sec_policy_parent != {} (^ for intersection,
{} for the empty set). This means that the set of security
tuples specified on the security policy of a child directory
always has a non-empty intersection with that of the parent.
3. sec_policy_child ^ sec_policy_parent == {}. This means that the
set of security tuples specified on the security policy of a
child directory may not intersect with that of the parent. In
other words, there are no restrictions on how the system
administrator may set up these tuples.
In order for a server to support approaches (b) (for the case when a
client chooses a flavor that is not a member of sec_policy_parent)
and (c), the put filehandle operation cannot return NFS4ERR_WRONGSEC
when there is a security tuple mismatch. Instead, it should be
returned from the LOOKUP (or OPEN by existing component name) that
follows.
Since the above guideline does not contradict approach (a), it should
be followed in general. Even if approach (a) is implemented, it is
possible for the security tuple used to be acceptable for the target
of LOOKUP but not for the filehandles used in the put filehandle
operation. The put filehandle operation could be a PUTROOTFH or
PUTPUBFH, where the client cannot know the security tuples for the
root or public filehandle. Or the security policy for the filehandle
used by the put filehandle operation could have changed since the
time the filehandle was obtained.
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Therefore, an NFSv4 server MUST NOT return NFS4ERR_WRONGSEC in
response to the put filehandle operation if the operation is
immediately followed by a LOOKUP or an OPEN by component name.
16.1.3.1.1.4. Put Filehandle Operation + LOOKUPP
Since SECINFO only works its way down, there is no way LOOKUPP can
return NFS4ERR_WRONGSEC without SECINFO_NO_NAME. SECINFO_NO_NAME
solves this issue via style SECINFO_STYLE4_PARENT, which works in the
opposite direction as SECINFO. As with Section 16.1.3.1.1.3, a put
filehandle operation that is followed by a LOOKUPP MUST NOT return
NFS4ERR_WRONGSEC. If the server does not support SECINFO_NO_NAME,
the client's only recourse is to send the put filehandle operation,
LOOKUPP, GETFH sequence of operations with every security tuple it
supports.
Regardless of whether SECINFO_NO_NAME is supported, an NFSv4 server
MUST NOT return NFS4ERR_WRONGSEC in response to a put filehandle
operation if the operation is immediately followed by a LOOKUPP.
16.1.3.1.1.5. Put Filehandle Operation + SECINFO/SECINFO_NO_NAME
A security-sensitive client is allowed to choose a strong security
tuple when querying a server to determine a file object's permitted
security tuples. The security tuple chosen by the client does not
have to be included in the tuple list of the security policy of
either the parent directory indicated in the put filehandle operation
or the child file object indicated in SECINFO (or any parent
directory indicated in SECINFO_NO_NAME). Of course, the server has
to be configured for whatever security tuple the client selects;
otherwise, the request will fail at the RPC layer with an appropriate
authentication error.
In theory, there is no connection between the security flavor used by
SECINFO or SECINFO_NO_NAME and those supported by the security
policy. But in practice, the client may start looking for strong
flavors from those supported by the security policy, followed by
those in the REQUIRED set.
The NFSv4 server MUST NOT return NFS4ERR_WRONGSEC to a put filehandle
operation that is immediately followed by SECINFO or SECINFO_NO_NAME.
The NFSv4 server MUST NOT return NFS4ERR_WRONGSEC from SECINFO or
SECINFO_NO_NAME.
16.1.3.1.1.6. Put Filehandle Operation + Nothing
The NFSv4 server MUST NOT return NFS4ERR_WRONGSEC.
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16.1.3.1.1.7. Put Filehandle Operation + Anything Else
"Anything Else" includes OPEN by filehandle.
The security policy enforcement applies to the filehandle specified
in the put filehandle operation. Therefore, the put filehandle
operation MUST return NFS4ERR_WRONGSEC when there is a security tuple
mismatch. This avoids the complexity of adding NFS4ERR_WRONGSEC as
an allowable error to every other operation.
A COMPOUND containing the series put filehandle operation +
SECINFO_NO_NAME (style SECINFO_STYLE4_CURRENT_FH) is an efficient way
for the client to recover from NFS4ERR_WRONGSEC.
The NFSv4 server MUST NOT return NFS4ERR_WRONGSEC to any operation
other than a put filehandle operation, LOOKUP, LOOKUPP, and OPEN (by
component name).
16.1.3.1.1.8. Operations after SECINFO and SECINFO_NO_NAME
Suppose a client sends a COMPOUND procedure containing the series
SEQUENCE, PUTFH, SECINFO_NONAME, READ, and suppose the security tuple
used does not match that required for the target file. By rule (see
Section 16.1.3.1.1.5), neither PUTFH nor SECINFO_NO_NAME can return
NFS4ERR_WRONGSEC. By rule (see Section 16.1.3.1.1.7), READ cannot
return NFS4ERR_WRONGSEC. The issue is resolved by the fact that
SECINFO and SECINFO_NO_NAME consume the current filehandle (note that
this is a change from NFSv4.0). This leaves no current filehandle
for READ to use, and READ returns NFS4ERR_NOFILEHANDLE.
16.1.3.1.2. LINK and RENAME
The LINK and RENAME operations use both the current and saved
filehandles. Technically, the server MAY return NFS4ERR_WRONGSEC
from LINK or RENAME if the security policy of the saved filehandle
rejects the security flavor used in the COMPOUND request's
credentials. If the server does so, then if there is no intersection
between the security policies of saved and current filehandles, this
means that it will be impossible for the client to perform the
intended LINK or RENAME operation.
For example, suppose the client sends this COMPOUND request:
SEQUENCE, PUTFH bFH, SAVEFH, PUTFH aFH, RENAME "c" "d", where
filehandles bFH and aFH refer to different directories. Suppose no
common security tuple exists between the security policies of aFH and
bFH. If the client sends the request using credentials acceptable to
bFH's security policy but not aFH's policy, then the PUTFH aFH
operation will fail with NFS4ERR_WRONGSEC. After a SECINFO_NO_NAME
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request, the client sends SEQUENCE, PUTFH bFH, SAVEFH, PUTFH aFH,
RENAME "c" "d", using credentials acceptable to aFH's security policy
but not bFH's policy. The server returns NFS4ERR_WRONGSEC on the
RENAME operation.
To prevent a client from an endless sequence of a request containing
LINK or RENAME, followed by a request containing SECINFO_NO_NAME or
SECINFO, the server MUST detect when the security policies of the
current and saved filehandles have no mutually acceptable security
tuple, and MUST NOT return NFS4ERR_WRONGSEC from LINK or RENAME in
that situation. Instead the server MUST do one of two things:
* The server can return NFS4ERR_XDEV.
* The server can allow the security policy of the current filehandle
to override that of the saved filehandle, and so return NFS4_OK.
16.2. Dealing with Multiple Connections
[Author Aside]: All unannotated paragraphs in this section are
considered part of Consensus Item #32b.
Because effective security will require both an appropriate auth
flavor (and possibly services provide by the auth flavor) together
with appropriate connection characteristics, it is often necessary
that clients and server be aware of connection characteristics:
* When multiple connections with different security-related
characteristics, are used to access a server, the clients needs to
ensure that each request is issued on an appropriate connection.
* Similarly, in such situations, the server needs to be aware of the
security-related characteristics for the connection on which each
request is received, in order to enforce its security policy.
Depending on how the client and server implementations are
structured, implementations may have to be changed to accomplish the
above.
In the case of NFSv4.1 and above, the protocol requires that requests
associated with a given session only be issued on connections bound
to that session and accepted by the server only when that binding is
present. This makes it likely that clients or servers will be able
to correctly associate requests with the appropriate connections
although additional work might be necessary to enable them to
determine, for any given connection, its security characteristics.
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In the case of NFSv4.0, no such binding is present in the protocol so
that, depending on existing implementations' layering, channel
binding functionality might have to be added.
17. Future Security Needs
This section deals with weaknesses in the security of the existing
protocol which might be dealt with in future minor versions or in
extensions to existing minor versions. While potential improvements
to NFSv4 security are discussed in Section 17.1 the details of these
extensions are not specified and will, when the Working Group decides
to implement them, be defined in new standards-track extension
documents.
Not discussed in this section are the extensions provided to inform
of the specifics of the ACL semantics implemented by particular file
system
17.1. Desirable Additional Security Facilities
[Author Aside]: All unannotated paragraphs in this section are to be
considered part of Consensus Item #35a.
[Author Aside]: This section is basically an outline for now, to be
filled out later based on Working Group input, particularly from
Chuck Lever who suggested this section and has ideas about many of
the items in it.
* Security for data-at-rest, most probably based on facilities
defined within SAN.
* Support for content signing.
* Revision/extension of labelled NFS to provide true
interoperability and server-based authorization.
* Work to provide more security for RDMA-based transports. This
would include the peer authentication infrastructure now being
developed as part of RPC-over-RDMA version 2. In addition, there
is a need for an RDMA-based transport that provides for
encryption, which might be provided in number of ways.
18. Security Considerations
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18.1. Changes in Security Considerations
Beyond the needed inclusion of a threat analysis as Section 18.4 and
the fact that all minor versions are dealt with together, the
Security Considerations in this section differ substantially from
those in [RFC7530] and [RFC8881]. These differences derive from a
number of substantive changes in the approach to NFSv4 security
presented in [RFC7530] and [RFC8881] and that appearing in this
document.
These changes were made in order to improve the security of the NFSv4
protocols because it had been concluded that the previous treatment
of these matters was in error, leading to a situation in which
NFSv4's security goals were not met. As a result, this document
supersedes the treatment of security in earlier documents, now viewed
as incorrect. However, it will, for the benefit of those familiar
with the previous treatment of these matters, draw attention to the
important changes listed here.
* There is a vastly expanded range of threats being considered as
described in Section 18.1.1
* New facilities provided by RPC on a per-connection basis can be
used to deal with security issues, as described in Section 18.1.2.
These include the use encryption on a per-connection basis, and
the use of peer mutual authentication, to mitigate the security
problems that come with the use of AUTH_SYS.
* The handling of identities with superuser privileges is no longer
part of NFSv4 semantics, even though many platforms on which NFSv4
servers are implemented continue to depend, for local operation,
on the existence of such identities.
NFSv4 servers SHOULD NOT provide for such unrestricted access
since doing so would provide a means by which an escalation-of-
privilege on a client could be used to compromise a server to
which it was connected, affecting all clients of that server.
In connection with the use of "SHOULD NOT" above, and similar uses
elsewhere, it is to be understood that valid reasons to do other
than recommended are limited to the difficulty of promptly
changing existing server implementations and the need to
accommodate clients that have become dependent upon the existing
handling. Further, those maintaining or using such
implementations need to be aware of the security consequences of
such use as well as the fact that clients who become aware of this
characteristic may not be inclined to store their data on such a
system.
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* The appropriate handling of ACL-based authorization and necessary
interactions between ACLs and modes is now specified in this
standards-track document rather it being assumed that the behavior
of server implementations needs to be accepted and deferred to.
18.1.1. Wider View of Threats
Although the absence of a threat analysis in previous treatments
makes comparison most difficult, the security-related features
described in previous specifications and the associated discussion in
their security considerations sections makes it clear that earlier
specifications took a quite narrow view of threats to be protected
against and placed the burden of providing for secure use on those
deploying such systems with very limited guidance as to how such
secure use could be provided.
One aspect of that narrow view that merits special attention is the
handling of AUTH_SYS, at that time in the clear, with no client peer
authentication.
With regard to specific threats, there is no mention in existing
security considerations sections of:
* Denial-of-service attacks.
* Client-impersonation attacks.
* Server-impersonation attacks.
The handling of data security in-flight is even more troubling.
* Although there was considerable work in the protocol to allow use
of encryption to be negotiated when using RPCSEC_GSS. The
existing security considerations do not mention the potential need
for encryption at all.
It is not clear why this was omitted but it is a pattern that
cannot repeated in this document.
* The case of negotiation of integrity services is similar and uses
the same negotiation infrastructure.
In this case, use of integrity is recommended but not to prevent
the corruption of user data being read or written.
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The use of integrity services is recommended in connection with
issuing SECINFO (and for NFSv4.1, SECINFO_NONAME). The presence
of this recommendation in the associated security considerations
sections has the unfortunate effect of suggesting that the
protection of user data is of relatively low importance.
18.1.2. Connection-oriented Security Facilities
Such RPC facilities as RPC-with-TLS provide important ways of
providing better security for all the NFSv4 minor versions.
In particular:
* The presence of encryption by default deals with security issues
regarding data-in-flight, whether RPCSEC_GSS or AUTH_SYS is used
for client principal identification.
* Peer authentication provided by the server eliminates the
possibility of a server-impersonation attack, even when AUTH_SYS
or AUTH_NONE is used to issue requests
* When mutual authentication is part of connection establishment,
there is a possibility, where an appropriate trust relationship
exists, of treating the uids and gids presented in AUTH_SYS
requests, as effectively authenticated, based on the
authentication of the client peer.
18.1.3. Necessary Security Changes
[Consensus Needed (Items #36a, #37a)]: For a variety of reasons,
there are many cases in which a change to the security approach has
been adopted but for which provisions have been made in order to give
implementers time to adapt to the new approach. In such cases the
words "SHOULD", "SHOULD NOT", and "RECOMMENDED" are used to introduce
the new approach while use of the previous approach is allowed on a
temporary basis, by limiting the valid reasons to bypass the
recommendation. Such instances fall into two classes:
* [Consensus Needed (Item #36a)]: In adapting to the availability of
security services provided by RPC on a per-connection basis,
allowance has been made for implementations for which these new
facilities are not available and for which, based on previous
standards-track guidance, AUTH_SYS was used, in the clear, without
client-peer authentication.
* [Consensus Needed (Item #37a)]: In dealing with server
implementations that support both ACLs and the mode attribute,
previous specifications have allowed a wide range of possible
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server behavior in coordinating these attributes. While this
document now clearly defines the recommended behavior in dealing
with these issues, allowance has been made to provide time for
implementations to conform to the new recommendations.
[Consensus Needed (Items #36a, #37a)]: The threat analysis within
this Security Considerations section will not deal with older servers
for which allowance has been made but will explore the consequences
of the recommendations made in this document.
18.1.4. Compatibility and Maturity Issues
[Author Aside]: All unannotated paragraphs within this section are
considered part of Consensus Item #38a.
Given the need to drastically change the NFSv4 security approach from
that specified previously, it is necessary for us to be mindful of:
* The difficulty that might be faced in adapting to the newer
guidance because the delays involved in designing, developing, and
testing new connection-oriented security facilities such as RPC-
with-TLS.
* The difficulty in discarding or substantially modifying previous
existing deployments and practices, developed on the basis of
previous normative guidance.
For these reasons, we will not use the term "MUST NOT" in some
situations in which the use of that term might have been justified
earlier. In such cases, previous guidance together with the passage
of time may have created a situation in which the considerations
mentioned above in this section may be valid reasons to defer, for a
limited time, correction of the current situation making the term
"SHOULD NOT" appropriate, since the difficulties cited would
constitute a valid reason to not allow what had been recommended
against.
18.1.5. Discussion of AUTH_SYS
[Author Aside]: All unannotated paragraphs within this section are
considered part of Consensus Item #39a.
An important change concerns the treatment of AUTH_SYS which is now
divided into two distinct cases given the possible availability of
connection-oriented support from RPC.
When such support is not available, AUTH_SYS SHOULD NOT be used,
since it makes the following attacks quite easy to execute:
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* The absence of authentication of the server to the client allow
server impersonation in which an imposter server can obtain data
to be written by the user and supply corrupted data to read
requests.
* The absence of authentication of the client user to the server
allow client impersonation in which an imposter client can issue
requests and have them executed as a user designated by imposter
client, vitiating the server's authorization policy.
With no authentication of the client peer, common approaches, such
as using the source IP address can be easily defeated, allowing
unauthenticated execution of requests made by the pseudo-clients
* The absence of any support to protect data-in-flight when AUTH_SYS
is used result in further serious security weaknesses.
In connection with the use of the term "SHOULD NOT" above, it is
understood that the "valid reasons" to use this form of access
reflect the Compatibility and Maturity Issue discussed above in
Section 18.1.4 and that it is expected that, over time, these will
become less applicable.
18.2. Security Considerations Scope
18.2.1. Discussion of Potential Classification of Environments
[Author Aside]: All unannotated paragraphs within this section are
considered part of Consensus Item #40a.
This document will not consider different security policies for
different sorts of environments. This is because,
* Doing so would add considerable complexity to this document.
* The additional complexity would undercut our main goal here, which
is to discuss secure use on the internet, which remain an
important NFSv4 goal.
* The ubiquity of internet access makes it hard to treat corporate
networks separately from the internet per se.
* While small networks might be sufficiently isolated to make it
reasonable use NFSv4 without serious attention to security issues,
the complexity of characterizing the necessary isolation makes it
impractical to deal with such cases in this document.
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18.2.2. Discussion of Environments
[Author Aside]: All unannotated paragraphs within this section are
considered part of Consensus Item #40b.
Although the security goal for Nfsv4 has been and remains "secure use
on the internet", much use of NFSv4 occurs on more restricted IP
corporate networks with NFS access from outside the owning
organization prevented by firewalls.
This security considerations section will not deal separately with
such environments since the threats that need to be discussed are
essentially the same, despite the assumption by many that the
restricted network access would eliminate the possibility of attacks
originating inside the network by attackers who have some legitimate
NFSv4 access within it.
In organizations of significant size, this sort of assumption of
trusted access is usually not valid and this document will not deal
with them explicitly. In any case, there is little point in doing
so, since, if everyone can be trusted, there can be no attackers,
rendering threat analysis superfluous.
In corporate networks, as opposed to the Internet, there is good
reason to be less concerned about denial-of-service attacks, since
there is no tangible benefit to attackers inside the organization,
and the anonymity that makes such attacks attractive to outside
attackers will not be present.
The above does not mean that NFSv4 use cannot, as a practical matter,
be made secure through means outside the scope of this document
including strict administrative controls on all software running
within it, frequent polygraph tests, and threats of prosecution.
However, this document is not prepared to discuss the details of such
policies, their implementation, or legal issues associated with them
and treats such matters as out-of-scope.
Nfsv4 can be used in very restrictive IP network environments where
outside access is quite restricted and there is sufficient trust to
allow, for example, every node to have the same root password. The
case of a simple network only accessible by a single user is similar.
In such networks, many thing that this document says "SHOULD NOT" be
done are unexceptionable but the responsibility for making that
determination is one for those creating such networks to take on.
This document will not deal further with NFSv4 use on such networks.
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18.2.3. Insecure Environments
As noted in Section 18.2.2, NFSv4 is often used in environments of
much smaller scope than the internet, with the assumption often being
made, that the prevention of NFSv4 access from outside the
organization makes the attention to security recommended by this
document unnecessary, the possibility of insider attacks being
explicitly or implicitly disregarded.
As a result, there will be implementations that do not conform to
these recommendations, many of which because the implementations were
based on the RFCs [RFC3530], [RFC7530], [RFC5661], or [RFC8881]. In
addition to these cases in which the disregard of the recommendations
is considered valid because implementors relied on existing normative
guidance, there will be other cases in which implementors choose to
ignore these recommendations,
Despite the original focus of [RFC2119] on interoperability, many
such implementations will interoperate, albeit without effective
security, whether the reasons that the recommendations are not
adhered to are considered valid or not.
When such insecure use is mentioned in this Security Considerations
section it will only be in explaining the need for the
recommendations, by explaining the likely consequences of not
following them. The threat analysis, in Section 18.4 and included
subsections, will not consider such insecure use and will concern
itself with situation in which these recommendations are followed.
18.3. Major New Recommendations
18.3.1. Recommendations Regarding Security of Data in Flight
[Author Aside]: All unannotated paragraphs within this section are
considered part of Consensus Item #41b.
It is RECOMMENDED that client implementer always support data privacy
in some form, whether using connection-based encryption or data
privacy services as provided by RPSCEC_GSS. This is despite the fact
that previous specifications stopped short of requiring this support
on the client.
It is RECOMMENDED that requesters always issue requests with data
security (i.e. with protection from disclosure or modification in
flight) whether provided at the RPC request level or on a per-
connection basis, irrespective of the responder's requirements.
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It is RECOMMENDED that implementers provide servers the ability to
configure policies in which requests without data security will be
rejected as having insufficient security.
It is RECOMMENDED that servers use such policies over either their
entire local namespace or for all file systems except those clearly
designed for the general dissemination of non-sensitive data.
When these recommendations are not followed, data, including data for
which disclosure is a severe [problem is exposed to unwanted
disclosure or modification in flight. Depending on the server to be
aware of the need for confidentiality or integrity, as expected by
previous specifications, has not proved workable, making encryption
by default as provided uniformly by RPC (e.g. through RPC-with-TLS)
necessary.
18.3.2. Recommendations Regarding Client Peer Authentication
[Author Aside]: All unannotated paragraphs within this section are
considered part of Consensus Item #41c.
It is RECOMMENDED that clients provide authentication material
whenever a connection is established with a server capable of using
it to provide client peer authentication.
It is RECOMMENDED that implementers provide servers the ability to
configure policies in which attempts to establish connections without
client peer authentication will be rejected.
It is RECOMMENDED that servers adopt such policies whenever requests
not using RPCSEC_GSS (i.e. AUTH_NONE Or AUTH_SYS) are allowed to be
executed.
When these recommendations are not followed, it is possible for
connections to be established between servers and client peers that
have not been authenticated with the following consequences:
* The server will be in the position of executing requests where the
identity used in the authorization of operations is not
authenticated, including cases in which the identification has
been fabricated by an attacker.
* When no identification of a specific user is needed or present
(i.e AUTH_NO is used) there is no way of verifying that the
request was issued by the appropriate client peer.
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When the recommendations are followed, use of AUTH_SYS can be valid
means of user authentication, so long as due attention is paid to the
discussion in Section 18.4.6.1. Despite this fact, the description
of AUTH_SYS as an "OPTIONAL means of authentication" is no longer
appropriate since choosing to use it requires heightened attention to
security as discussed later in this document.
18.3.3. Recommendations Regarding Superuser Semantics
[Author Aside]: All unannotated paragraphs within this section are
considered part of Consensus Item #52c.
It is RECOMMENDED that servers adhere to the ACL semantics defined in
this document and avoid granting to any remote user, however
authenticated, unrestricted access capable of authorizing access
where the file/directory ACL would deny it.
Servers are free to conform to this recommendation either by
implementing authorization semantics without provisions for
superusers or by mapping authenticated users that would have
superuser privileges to users with more limited privileges (e.g.
"nobody").
It needs to be understood that the second of these choices is
preferable when there are NFsv4-accessible files owned by a special
users (e.g. root) whose compromise might be taken advantage of by
attackers to enable permanent unauthorized access to a server.
18.3.4. Issues Regarding Valid Reasons to Bypass Recommendations
[Author Aside]: All unannotated paragraphs within this section are
considered part of Consensus Item #41d.
Clearly, the maturity and compatibility issues mentioned in
Section 18.1.4 are valid reasons to bypass the proposed
recommendations requiring pervasive use of encryption, as long as
these issues continue to exist.
[Author Aside]: The question the working group needs to address is
whether other valid reasons exist.
[Author Aside]: In particular, some members of the group might feel
that the performance cost of connection-based encryption constitutes,
in itself, a valid reason to ignore the above recommendations.
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[Author Aside]: I cannot agree and feel that accepting that as a
valid reason would undercut Nfsv4 security improvement, and probably
would not be acceptable to the security directorate. However, I do
want to work out an a generally acceptable compromise. I propose
something along the following lines:
In dealing with recommendations requiring pervasive use of
connection-based encryption, it needs to be understood that the
connection-based encryption facilities are designed to be compatible
with facilities to offload the work of encryption and decryption.
When such facilities are not available, at a reasonable cost, to
NFSv4 servers and clients anticipating heavy use of NFSv4, then the
lack of such facilities can be considered a valid reason to bypass
the above recommendations, as long as that situation continues.
18.4. Threat Analysis
18.4.1. Threat Analysis Scope
Because of the changes that have been made in NFSv4 security, it
needs to be made clear that the primary goal of this threat analysis
is to explore the threats that would be faced by implementations that
follow the recommendations in this document.
When the possibility is raised of implementations that do not conform
to these recommendations, the intention is to explain why these
recommendations were made rather that to expand the scope of the
threat analysis to include implementations that bypass/ignore the
recommendations.
The typical audience for threat analyses is client and server
implementers, to enable implementations to be developed that are
resistant to possible threats. While much of the material in
Section 18.4 is of that form, it also contains material that relates
to threats whose success depends primarily on the ways in which the
implementation is deployed, such as the threats discussed in Sections
18.4.2, 18.4.4 and 18.4.3. While it is not anticipated that those
deploying implementations will be aware of the detail of this threat
analysis, it is expected that implementors could use this material to
properly set expectations and provide guidance helpful to making
deployments secure.
18.4.2. Threats based on Credential Compromise
In the past, it had been assumed that a user-selected password could
serve as a credential, the knowledge of which was adequate to
authenticate users and provide a basis for authorization.
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That assumption is no longer valid for a number of reasons:
* The inability or unwillingness of users to remember multiple
passwords has meant that the single password they will remember
controls access to large set of resources, increasing the value of
this knowledge to attackers and the effort that will be expended
to obtain it.
In addition, the common use of a single password for applying to
all of a user's data has resulted in a situation in which the
client is aware of user passwords (since they are used for client
login) that apply to data on many servers. As will be seen later,
this has the effect of changing the considerations appropriate to
comparing the security of AUTH_SYS and RPCSEC_GSS.
* CPU developments have made exhaustive search possible for larger
classes of passwords.
* The success of "phishing" attacks taking advantage of user
gullibility provides an additional path to credential compromise
which need to be addressed in the near-term by those deploying
NFSv4, and will eventually need work in the security
infrastructure on which NFSv4 is built.
In the near term, there are a number of steps, listed below that
those deploying NFSv4 servers can take to mitigate these weaknesses.
These steps are outside the scope of the NFSv4 protocols and
implementors only role with regard to them is to make it clear that
these weaknesses exist and generally require mitigation.
* Limitations on password choice to eliminate weak passwords.
* Requirements to change passwords periodically.
* User education about "phishing" attacks including ways to report
them and effective ways of replacing a compromised password.
From a longer-term perspective, it appears that password-based
credentials need to be either replaced or supplemented by some form
of multi-factor authentication. Since NFSv4's approach to security
relies on RPC, that work would most probably be done within the RPC
layer, limiting the work that implementations and the NFSv4 protocols
would have to do to adapt to these changes once they are available.
While the precise form of these changes is not predictable, the
following points need to be kept in mind.
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* [Verification Needed (Item #53a)]: For those using RPCSEC_GSS
authentication of principals, it appears that RPCSEC_GSS interface
is flexible enough that the addition of a second credential
element, in the form of a one-time code could be added.
[Elaboration/Verification Needed (Item #53a)]: Enhancement of
Kerberos is one possibility to provide multi-factor
authentication. However, work on this is not far enough along to
enable deployment to be discussed now.
If this approach were taken, rogue servers would still have access
to user passwords but their value would be reduced since the
second credential element would have a very limited lifetime.
* For those using AUTH_SYS to identify principals, the client
operating system's authentication of user at login would need to
be enhanced to use multi-factor authentication.
If this were done, the client would retain responsibility for
credential verification with the server needing to trust the
client, as discussed in Section 18.4.6.1.
Although there is need for protocol standardization to enable this
approach to be commonly used, it is not likely to be widely used
until some operating system adopts it for user login.
* One important variant of AUTH_SYS use concerns clients used by a
single user, when, as recommended, client-peer authentication is
in effect For such clients, it is possible for the authentication
of that specific client peer to effectively become the second
factor, in a multi-factor authentication scheme.
Despite the fact that the RPC-with-TLS specification [RFC9289])
does not allow TLS to used for user authentication, this
arrangement in which the user identity is inferred from the peer
authentication, could be used to negate the effects of credential
compromise since an attacker would need both the user password,
and the physical client to gain access.
18.4.3. Threats Based on Rogue Clients
When client peers are not authenticated, it is possible to a node on
the network to pretend to be a client. In the past, in which servers
only checked the from-IP address for correctness, address spoofing
would allow unauthenticated request to be executed, allowing
confidential data to be read or modified.
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Now that such use of AUTH_SYS is recommended against, this cannot
happen. The recommended practice is to always authenticate client
peers making this sort of imposture easily detectable by the server.
Despite this protection, it is possible that an attacker, through a
client vulnerability unrelated to NFSv4, or the installation of
malware, could effectively control the client peer and act as
imposter client would, effectively undercutting the authentication of
the client. This possibility makes it necessary, as discussed in
Section 18.4.6.1 that those deploying NFSv4 clients using AUTH_SYS
takes steps to limit the set of user identifications accepted by a
server and to limit the ability of rogue code running on the server
to present itself as a client entitled to use AUTH_SYS.
18.4.4. Threats Based on Rogue Servers
When server peers are not authenticated, it is possible for a node on
the network to act as if it were an NFSv4 server, with the ability to
save data sent to it and use it or pass it to other, rather than
saving it in the file system, as it needs to do..
When current recommendations are adhered to, this is be prevented as
follows:
* When RPCSEC_GSS is used, the mutual authentication of the server
and client principal provides assurance the server is not an
imposter.
* When AUTH_SYS or AUTH_NONE is used, the mutual authentication of
client and server peers provides assurance the server is not an
imposter.
Despite this protection, it is possible that an attacker, through a
operating system vulnerability unrelated to NFSv4, or the
installation of malware, could effectively control the server peer
and act as an imposter server would, effectively undercutting the
authentication of the server.
The above possibility makes it necessary, that those deploying NFSv4
servers take the following steps, particularly in cases in cases in
which the server has access to user credentials, including, but not
limited to, cases in which AUTH_SYS is supported
When an NFSv4 is implemented as part of a general-purpose operating
system, as it often is, steps need to be taken to limit the ability
of attackers to take advantage of operating system vulnerabilities
that might allow the attacker to obtain privileged access and subvert
the servers operation, turning it, effectively, into a rogue server.
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Such steps include controls on the software installed on the machine
acting as the server, and limitation of the network access to
potentially dangerous sites.
18.4.5. Data Security Threats
When file data is transferred in the clear, it is exposed to unwanted
exposure. As a result, this document recommends that encryption
always be used to transfer NFSv4 requests and responses.
That encryption, whether done on encrypted connections, or on a per-
request basis, using RPCSEC_GSS security services, provides the
necessary confidentiality. In addition, it contributes to security
in other ways as well:
* The ability of an attacker to plan and execute attacks is enhanced
by the monitoring of client-server traffic, even if none of the
data intercepted is actually confidentiality.
An attacker can deduce which users are allowed to read or write a
specific file by examining the results of OPEN and ACCESS
operations allowing later attacks to impersonate users with the
appropriate access.
* All the methods of encryption used with NFS4 provide a checksum,
to enable the detection of unwanted modifications to data being
read or written.
18.4.6. Authentication-based threats
18.4.6.1. Attacks based on the use of AUTH_SYS
Servers, when they allow access using AUTH_SYS, to a specific client
machines using AUTH_SYS are responsible for ensuring that the
principal identifications presented to the server can be relied upon.
The existence of client-peer authentication as recommended in
Section 18.1.5 means that imposter servers can be detected and not
allowed to use AUTH_SYS. However there are an additional number of
issues that need to be addressed to adequately protect against use of
AUTH_SYS enabling attacks:
* The server accepting requests using AUTH_SYS needs to determine
that the authenticated client-peer can be trusted to properly
authenticate the principals that it identifies in requests.
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The specific standards for trustworthiness are up to the server
but they need to take account of the controls in place to prevent
malware from pretending to be a client and thus taking advantage
of the fact that the request is from the expected client machine.
This server MUST NOT accept AUTH_SYS requests from unknown clients
or from unauthenticated clients.
* [Elaboration Needed (Item #54a)]: The client verification
procedure needs to take steps to prevent code on a compromised
client to presenting itself as the successor to a legitimate
client, taking advantage of the fact that the machine is the same.
* Given the inherent vulnerabilities of client operating systems, it
is desirable, to limit the set of users whose identification will
be accepted. The elimination of particular users such as "root'
is one long-standing approach to the issue but it probably isn't
sufficient in most environments. More helpful would be the
ability to exclude multiple sensitive users or group of users or
to limit the user identifications accepted to a user group or a
single user.
Another important that issue that arises when AUTH_SYS is used
concerns the storage of credentials on the clients. While it is
theoretically possible for these not to be of use elsewhere, the
reluctance/inability of users to remember multiple passwords means
that these credentials will be used by many clients and will need to
be updated as users are added or deleted or when passwords are
changed. The propagation of these credentials and their storage on
clients could be the basis for attacks if appropriate step are not
taken to secure this data.
While it is helpful to store a cryptographic hash of the password
rather than the password itself, this does not dispose of the issue,
since possession of the hash would greatly simplify an exhaustive
search for the password, since the attacker could limit login
attempts to guessed password whose hash value matched the value
obtained from the files on the client.
Although it is true that making clients responsible for
authentication of user identities undercuts much of the original
motivation for making RPCSEC_GSS REQUIRED to implement, it needs to
be understood that the situation today is different from that when
this decision was made.
* It has been recommended that servers not allow unauthenticated
clients to issue requests using AUTH_SYS.
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* The identification of a request as issued by the user with uid
zero, no longer provides access without file access authorization.
* Given that users are unaware of where their files are located and
it is desirable that they are able to remain unaware of this, it
is natural that they use the same password to authenticate
themselves for local resource use as for use of files located on
NFSv4 servers.
Support for AUTH_SYS in NFSv4 was included for a number of reasons
which still hold true today, despite the fact that the original
mistake, to make no reference to the security consequences of doing
so, is now being corrected. Such provision is necessary for the
following reasons, that go beyond the need to temporarily accommodate
implementations following the older specifications, for a number of
reasons:
* When considered, as NFS was to intended to be, as consistent with
local access as possible, AUTH_SYS was the natural way of
providing authentication, just as it had been done for local
files.
While use of AUTH_SYS exposes user passwords to the client
operating system, the fact that user are unable or unwilling to
use different passwords for different files in a multi-server
namespace means this issue will be present even when AUTH_SYS is
not used.
* [Elaboration Needed (Item #55a)]: In many important environments
including cloud environments, important implementation constraints
has made use of Kerberos impractical.
[Verification Needed (Item #55a)]: In such environments, client
credentials are maintained by the cloud customer while the cloud
provider manages network access.
18.4.6.2. Attacks on Name/Userid Mapping Facilities
NFSv4 provides for the identification users and groups in two ways
(i.e. by means of strings of the form name@domain or strings
containing numeric uid/gid values) while file systems used on NFSv4
servers typically use 32-bit uids and gids.
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As a result, NFSv4 server implementations are required to have some
means of translating between the name@domain form and the numeric
form used internally. While the specifics of this translation are
not specified as part of the NFSv4 protocols, is required for server
implementations to work, and, if it not done securely and attackers
have the ability to interfere with this translation, it gives them
the ability to interfere with authorization as follows:
* When authentication occurs using user names, as occurs when
RPCSEC_GSS, a mistranslation might allow the numeric value used in
authorization to allow access to a file the authenticated user
would not be allowed to access.
* When any authentication occurs on the client and the uid is
presented to the server using AUTH_SYS a mistranslation to the
string form could result in confusion and uncertainty about the
users allowed to access the file.
18.4.7. Disruption and Denial-of-Service Attacks
18.4.7.1. Attacks Based on the Disruption of Client-Server Shared State
When data is known to both the client and server, a rogue client can
interfere with the correct interaction between client and server, by
modifying that shared data, including locking state and session
information.
For this reason, it is recommended that client-peer authentication be
in effect, because, if it were not, a different client could easily
modify data that the current client depend on, disrupting ones
interaction with the server.
It is still possible, if one's client is somehow compromised, as
described in Section 18.4.3, for various forms of mischief to occur:
* Locks required for effective mutual exclusion can be released,
causing application failures.
* Mandatory share locks can be obtained preventing those with valid
access from opening file that they are supposed to have access to.
* Session slot sequence numbers may be rendered invalid if requests
are issued on existing sessions. As a result, the client that
issued a request would receive unexpected sequence errors.
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18.4.7.2. Attacks Based on Forcing the Misuse of Server Resources
It is also possible for attacks to be mounted, in the absence of the
ability to obtain or modify confidential data, with the sole goal of
the attack being to make spurious requests, with no expectation that
the request will be authorized but with the goal of causing resources
that would otherwise be used to service valid requests to be
unavailable due to the burden of dealing with numerous invalid
requests.
The design of the NFSv4 protocols requires that clients establishing
new connections make initial requests which establishes a shared
context referred to by subsequent requests which might request
substantive actions (e.g. client and session ids). This structure
helps mitigate the effect of such denial-of-service attacks as
described below.
* The server can limit the resources devoted to connections not yet
fully identified without unduly restricted connections which have
identified themselves.
* The recommendation that client peers authenticate themselves,
allows unknown clients to be dispensed with at an early stage
negating their ability to make requests which could require file
system action to obtain information needed to make authorization
decisions (e.g. ACLs or other authorization-related) file
attributes.
19. IANA Considerations
[Author Aside]: All unannotated paragraphs in this section are to be
considered part of Consensus Item #32c.
Because of the shift from implementing security-related services only
in connection with RPCSEC_GSS to one in which connection-oriented
security has a prominent role, a number if new values need to be
assigned.
These include new authstat values to guide selection of a connection
types acceptable to both client and server, presented in Section 19.1
and new pseudo-flavors to be used in the process of security
negotiation, presented in Section 19.2.
19.1. New Authstat Values
[Author Aside]: All unannotated paragraphs in this section are to be
considered part of Consensus Item #32d.
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The following new authstat values are necessary to enable a server to
indicate that the server's policy does not allows requests to be made
on the current connection because of security issues associated with
connection type used. In the event they are received, the client
needs to establish a new connection.
* The value XP_CRYPT indicates that the server will not support
access using unencrypted connections while the current connection
is not encrypted.
* The value XP_CPAUTH indicates that the server will not support
access using connections for which the client peer has not
authenticated itself as part of connection while the current
connection has not been set up in that way.
19.2. New Authentication Pseudo-Flavors
[Author Aside]: All unannotated paragraphs in this section are to be
considered part of Consensus Item #32e.
The new pseudo-flavors described in this section are to be made
available to allow their return as part of the response to the
SECINFO and SECINFO_NONAME operations. How these operations are to
used to negotiate the use of appropriate auth flavors and associated
security-relevant connection characteristics is discussed in
Section 16.
The following pseudo-flavors are to be defined:
* PFL_CRYPT is returned to indicate that subsequent secinfo entries
are to considered limited to use on connections for which
transport-level encryption is provided.
* PFL_CRYPTMPA is returned to indicate that subsequent secinfo
entries are to be considered limited to use on connections on
which mutual peer authentication has been provided at connection
setup.
* PFL_ANY is returned to indicate that subsequent secinfo entries
are not to be considered limited to any particular type of
connection.
20. References
20.1. Normative References
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[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[RFC2203] Eisler, M., Chiu, A., and L. Ling, "RPCSEC_GSS Protocol
Specification", RFC 2203, DOI 10.17487/RFC2203, September
1997, <https://www.rfc-editor.org/info/rfc2203>.
[RFC2623] Eisler, M., "NFS Version 2 and Version 3 Security Issues
and the NFS Protocol's Use of RPCSEC_GSS and Kerberos V5",
RFC 2623, DOI 10.17487/RFC2623, June 1999,
<https://www.rfc-editor.org/info/rfc2623>.
[RFC4121] Zhu, L., Jaganathan, K., and S. Hartman, "The Kerberos
Version 5 Generic Security Service Application Program
Interface (GSS-API) Mechanism: Version 2", RFC 4121,
DOI 10.17487/RFC4121, July 2005,
<https://www.rfc-editor.org/info/rfc4121>.
[RFC5531] Thurlow, R., "RPC: Remote Procedure Call Protocol
Specification Version 2", RFC 5531, DOI 10.17487/RFC5531,
May 2009, <https://www.rfc-editor.org/info/rfc5531>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
[RFC7530] Haynes, T., Ed. and D. Noveck, Ed., "Network File System
(NFS) Version 4 Protocol", RFC 7530, DOI 10.17487/RFC7530,
March 2015, <https://www.rfc-editor.org/info/rfc7530>.
[RFC8178] Noveck, D., "Rules for NFSv4 Extensions and Minor
Versions", RFC 8178, DOI 10.17487/RFC8178, July 2017,
<https://www.rfc-editor.org/info/rfc8178>.
[RFC8881] Noveck, D., Ed. and C. Lever, "Network File System (NFS)
Version 4 Minor Version 1 Protocol", RFC 8881,
DOI 10.17487/RFC8881, August 2020,
<https://www.rfc-editor.org/info/rfc8881>.
[RFC7862] Haynes, T., "Network File System (NFS) Version 4 Minor
Version 2 Protocol", RFC 7862, DOI 10.17487/RFC7862,
November 2016, <https://www.rfc-editor.org/info/rfc7862>.
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[RFC9289] Myklebust, T. and C. Lever, Ed., "Towards Remote Procedure
Call Encryption by Default", RFC 9289,
DOI 10.17487/RFC9289, September 2022,
<https://www.rfc-editor.org/info/rfc9289>.
[nist-209] NIST, "SP 800-209 Security Guidelines for Storage
Infrastructure".
20.2. Informative References
[RFC8257] Bensley, S., Thaler, D., Balasubramanian, P., Eggert, L.,
and G. Judd, "Data Center TCP (DCTCP): TCP Congestion
Control for Data Centers", RFC 8257, DOI 10.17487/RFC8257,
October 2017, <https://www.rfc-editor.org/info/rfc8257>.
[RFC3010] Shepler, S., Callaghan, B., Robinson, D., Thurlow, R.,
Beame, C., Eisler, M., and D. Noveck, "NFS version 4
Protocol", RFC 3010, DOI 10.17487/RFC3010, December 2000,
<https://www.rfc-editor.org/info/rfc3010>.
[RFC3530] Shepler, S., Callaghan, B., Robinson, D., Thurlow, R.,
Beame, C., Eisler, M., and D. Noveck, "Network File System
(NFS) version 4 Protocol", RFC 3530, DOI 10.17487/RFC3530,
April 2003, <https://www.rfc-editor.org/info/rfc3530>.
[RFC5661] Shepler, S., Ed., Eisler, M., Ed., and D. Noveck, Ed.,
"Network File System (NFS) Version 4 Minor Version 1
Protocol", RFC 5661, DOI 10.17487/RFC5661, January 2010,
<https://www.rfc-editor.org/info/rfc5661>.
[RFC1813] Callaghan, B., Pawlowski, B., and P. Staubach, "NFS
Version 3 Protocol Specification", RFC 1813,
DOI 10.17487/RFC1813, June 1995,
<https://www.rfc-editor.org/info/rfc1813>.
[I-D.ietf-nfsv4-internationalization]
Noveck, D., "Internationalization for the NFSv4
Protocols", Work in Progress, Internet-Draft, draft-ietf-
nfsv4-internationalization-11, 22 August 2024,
<https://datatracker.ietf.org/doc/html/draft-ietf-nfsv4-
internationalization-11>.
[I-D.dnoveck-nfsv4-acls]
Noveck, D., "ACLs within the NFSv4 Protocols", Work in
Progress, Internet-Draft, draft-dnoveck-nfsv4-acls-05, 20
August 2024, <https://datatracker.ietf.org/doc/html/draft-
dnoveck-nfsv4-acls-05>.
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[errata] IESG, "IESG Processing of RFC Errata for the IETF Stream",
July 2008,
<https://www.ietf.org/about/groups/iesg/statements/
processing-rfc-errata/>.
Appendix A. Changes Being Made
This appendix summarizes the significant changes between the
treatment of security in previous minor version specification
documents (i.e. RFCs 3630, 7530, 5661 and 8881) and the new
treatment that is intended to apply to NFSv4 as a whole. This
includes changes made in both this document and in the companion
document relating to ACLs [I-D.dnoveck-nfsv4-acls].
This review is expected to be helpful to implementers familiar with
previous specifications but also has an important role in guiding the
search for working group consensus as to these changes and in
identifying potential compatibility issues.
A.1. Motivating Security Changes
A.1.1. Fundamental Security Changes
A number of changes reflect the basic motivation for a new treatment
of NFSv4 security. These include the ability to obtain privacy and
integrity services from RPC on a per-connection basis rather than as
a service ancillary to a specific auth flavor.
This need to properly accommodate this new focus motivated a major
reorganization of the treatment of security together with further
emphasis on the security of data in flight, described in detail in
Section 15. In addition, the security negotiation framework for
NFSv4 has been enhanced to support the combined negotiation of
authentication-related services and connection characteristics, as
described in Section 16.
Despite these major changes, there are not expected to be troublesome
compatibility issues between peers supporting provision of security
services on a per-connection basis and those without such support.
Clients who are adapted to the new security facilities can use them
when the server supports them, but will sometimes choose to use the
older facilities when interacting with older servers. Servers who
support the new facilities have the option of rejecting client
connections from older clients but are encouraged to adopt policies
rejecting such connections. Over time, it is expected that use of
the older facilities will become less common as the newer facilities
become more commonly implemented and the importance of security
becomes more generally recognized
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Another such change was in the treatment of AUTH_SYS, previously
described, inaccurately, as an "OPTIONAL means of authentication"
with the unfortunate use of the RFC2119 keyword obscuring the
negative consequences of the typical use of AUTH_SYS (in the clear,
without client-peer authentication) for security by enabling the
execution of unauthenticated requests.
The new treatment avoids the inappropriate use of term
"authentication" for all activities triggered by the use of RPC auth
flavors and clearly distinguishes those flavors providing
authentication from those providing identification only or neither
identification nor authentication. For details, see Section 14.
As part of the necessary shift from a narrative focused on justifying
security choices already made to evaluating the adequacy of existing
security facilities, there has been a need to discuss NFSv4 security
gaps, which might be addressed later. See Section 17.1 for a
discussion of possible security-related extensions.
There are twelve consensus items (described in Appendix B) that are
involved in effecting this class of changes. Although it is
generally understood that the working group needs to take advantage
of RPC-with-TLS, change how use of AUTH_SYS is presented, and include
a credible threat analysis, there needs to be further discussion of
these items as working group members are likely to disagree about the
relative importance of:
* accommodating implementations based on previous specifications.
* providing a description of the expected path toward secure
implementations
* maintaining and improving implementation performance
For each of the following items, there may need to be extensive
discussion to arrive at a consensus text.
* Consensus Item #32 concerns extension of the process of SECINFO
negotiation to accommodate the use of facilities such as rpc-with-
tls, in addition to the current approach which directs only the
choice of a suitable auth flavor.
In the current document, pseudo-flavors are added to allow the
specification of connection characteristics, although it would be
possible to avoid use and select connection characteristics by a
process of trial and error.
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* Consensus Item #52 concerns the handling of superuser semantics
within NFSv4.
There is a long tradition, within NFS, of treating superuser
semantics, involving the elimination of file authorization checks,
as part of protocol semantics. The security problems that this
creates have been dealt with via "root-squashing", outside the
protocol specification. Given that a threat analysis would
essentially make such suppression of superuser semantics
unavoidable, the current document deals with such semantics
outside of the NFSv4 protocol.
* Consensus Item #36 concerns how to deal with the troublesome
legacy of use of AUTH_SYS without client authentication and the
common transmission of requests and responses without encryption.
The specification now makes it clear that this has the potential
to cause harm, while needing to deal with the fact that it is a
long time before this could be forbidden or recommended against
without allowances being made for current implementations. The
ways that this issue needs to be dealt with require further
discussion.
* Consensus Item #38 concerns temporary accommodations to deal with
previous implementations which are unsafe.
This is a consequence of existing implementations based on current
RFCs. The wording will require extensive discussion.
* Consensus Item #39 defines the new safer approach to AUTH_SYS.
There needs to be discussion about how this safety should be
characterized, given that you are trusting the client OS.
* Consensus Item #40 concerns the scope for the threat analysis in
the Security Considerations section.
Will need to reach a consensus on this as there appears to be no
way we can make allowances in which physical isolation makes a
significant difference, since this essentially ignores insider
threats.
* Consensus Item #41 discusses the major new security
recommendations regarding protection of data in flight and client
peer authentication. Also, covers the circumstances under which
such recommendations can be bypassed.
Will have to address concerns about performance effects.
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* Consensus Item #47 concerns dubious paragraph regarding AUTH_NONE
in SECINFO response which should be deleted if there are no
compatibility issues that make that impossible.
Will have to ascertain whether this is in fact supported by any
server or relied on by any client.
* Consensus Item #35 discusses possible work on future security
needs.
Need to discuss whether there is too much or too little forward-
looking material.
* Consensus Item #53 concerns the possible use of multi-factor
authentication.
* Consensus Item #54 discusses prevention of code on a compromised
client from hijacking the client machine's peer authentication.
This relates to getting clarity about the safety of the client
peer authentication available in rpc-with-tls.
* Consensus Item #55 discusses issues related to potential use of
Kerberos in cloud environments.
We will have to be sure we have a strong case here, since there
might be a feeling within the IESG that AUTH_SYS is per se
unacceptable.
A.1.2. ACL-Related Changes
In addition to the above, there has been a respecification of ACL-
related features appearing in the companion document
[I-D.dnoveck-nfsv4-acls] and summarized in Section 6.3.
Relevant consensus items relating to this respecification are
discussed in the companion document.
A.2. Need for Clarifying Changes
The need to make the major changes discussed in Appendix A.1 has
meant that much text dealing with security has needed to be
significantly revised or rewritten. As a result of that process,
many issues involving unclear, inconsistent, or otherwise
inappropriate text were uncovered and now need to be dealt with.
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While the author believes that changes in this situation are
necessary, the fact that we are changing a document adopted by
consensus requires the working group to be clear about the
acceptability of the changes. This applies to both the troublesome
issues discussed in Section 3.4 and to the other changes included
below.
The primary source of these not-clearly specified elements is the
disparate goals of many participants in the specification process:
* For many participants, the ACL definition needed to include a
large part of Windows-oriented semantics, including elements
clearly distinct from the Unix semantics supported by earlier
versions of NFS.
* For other participants, such semantic extensions were of little
interest.
While the author believes such changes are necessary, the fact that
we are changing a document adopted by consensus requires the working
group to be clear about the acceptability of the changes. This
applies to both the troublesome issues discussed in Section 3.4 and
to the other changes included below.
Because of the need for re-organization of subjects relating to
authorization, the ordering of the list follows the text of the
current version which may differ considerably from that in earlier
versions of the I-D.
The subjects related to authorization needed to be reorganized
because of the following complexities:
* There are three kinds of authorization to deal with.
* The most important kind of authorization, file access
authorization, is controlled by mode attributes and a set of ACL-
related attributes, creating multiple authorization models and a
need to coordinate them.
* ACLs, besides their prominent role with regard to file access
authorization, provide control of alarm and audit facilities.
As a result, these matters are dealt with in the eight top-level
sections within this document and the associated ACL document from 5
through 13. Of these, three sections, Sections 6, 9 and 13, have
become brief overviews which introduce matters dealt with in more
detail in the companion ACL document [I-D.dnoveck-nfsv4-acls].
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* Section 5,a new top-level section, describes all the
authorization-related attributes. Although this section cover all
of the attributes, details relating to the ACL-related ones appear
in the companion ACL document.
* Section 6, a new top-level section, provide an introduction to the
structure of ACLs with the corresponding detail appearing in the
companion ACL document [I-D.dnoveck-nfsv4-acls]. Later their uses
are described later sections 8 through 10 and in Section 13 of
which the last two serve as introductions to matters dealt with in
the companion ACL document.
* In Section 7, there is a discussion of authorization in general.
This includes a discussion of user-based file authentication,
NFSv4's approach to mandatory access control, and a discussion of
locking state authorization, each of which is described in later
top-level sections.
* File access authorization is described primarily in Section 8.
Because this authorization can be controlled by either the mode
attribute or various ACL-related attributes, there is an
introduction to these authorization- determining attributes in
Section 8.1.
Because file access authorization can be controlled by the POSIX-
related authorization attributed decribed in Section 5.3 and by
ACL-related attributes described in Section 5.4, there is an
introduction to how these relate in Section 8.2 followed by
discussion of the semantics of the two kinds of supported ACLs in
Section 8.4.
Issues related to the existence of two separate file access are
dealt with in Sections 9, dealing with the similarities between
these two, and 10 dealing with their co-existence and potential
conflict.
* Sections 11 through 13 deal with, labelled NFS authorization,
state modification authorization, and the uses of ACLs outside
authorization.
Beyond the reorganization described above, there are numerous matters
to be dealt with, reflecting the previous inability to clearly
specify two sets of authorization behaviors and their potential
interactions.
* Also in Section 8.2, there is added clarity in the discussion of
support for multiple authorization approaches by eliminating use
of the subjective term "reasonable semantics".
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In connection with this clarification, we have switched from
describing the needed co-ordination between modes and acls as
"goals" to describing them as "requirements" to give clients some
basis for expecting interoperability in handling these issues.
As a result of this shift to a prescriptive framework applying to
all minor versions it becomes possible to treat all minor versions
together. In existing RFCs and some earlier versions of this
document, it had been assumed that NFSv4.0 was free to ignore the
relevant prescriptions first put forth in RFC 5661 and only needed
to address the "goals" for this co-ordination.
A.3. Addressing the Need for Clarifying Changes
Unlike the issues discussed in Appendix A.1, for which the path to
make the necessary changes is laid out in that same section, the
changes discussed in Appendix A.2, are discussed here, because
resolving them is a more involved matter.
The difficulty derives principally from the fact that there are
existing clients and server with very different approaches to these
matters, making it difficult or impossible to reach a consensus that
will make some existing clients non-compliant.
In such a situation, we need to allow a set of possibilities, as has
been done for internationalization of file names in [RFC7530] for
NFSv4.0 and in [I-D.ietf-nfsv4-internationalization], for all minor
versions.
As in the case of the internationalization of file names, we will
have to accommodate a range of possible server implementations, but
the task will, in this case, involve a lot more work. To see why, we
will first explore the situation for internationalization of file
names and later see how the situation for these issues poses greater
difficulties and how they might be addressed. Some of these
difficulties match other internationalization issues, such as the
handling of internationalized domain names and the proper treatment
of case-insensitive handling of file names that dealt with
incorrectly or not at all in [RFC7530].
* The treatment of internationalization of file names in [RFC7530]
and [RFC5661] was so divorced from the needs of implementations
that it was necessary to be ignore it.
On the other hand, the treatment of internationalization of file
names in the obsoleted document [RFC3010] was available as a base
and had been implemented by many clients and servers.
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* The vast majority of clients and servers implemented the same
approach to internationalization, even though it was not limited
to the use of UTF-8.
Despite the general aversion to this approach within the IESG, it
was accepted, most likely because we were describing NFSv4
internationalization on "as-implemented" basis.
* Overall, we wound up allowing a total of three sorts of server
behaviors, two of which were UTF-8-aware.
One UTF-8-aware option that had been implemented, simply
considered canonically equivalent names as designating the same
file with changing the name to a different canonically equivalent
name. Although this approach diverged from IESG expectations
regarding normalization, it was accepted.
Another UTF-8-aware option was allowed, despite the absence of any
implementations. It allowed servers to normalize file names while
forbidding them from rejecting unnormalized names. This was
allowed because it would cause no difficulties for clients and
because forbidding it might have caused problems for IESG
acceptance.
* We were able, where necessary, to give the client the ability to
determine which form of internationalization supported provided by
the server for any particular file system, using an OPTIONAL
attribute made available in NFSv4.1.
The situation with regard to the clarifying changes needed in this
document is different for the reasons listed below.
* Rather than three valid choices for acceptable server behavior,
with authorization we have a vast number.
Given that there are nine ACE mask bits, each of which might not
be supported, there are at least 512 possible valid behaviors,
even if none of the dubious instances of SHOULD creates additional
valid behavioral patterns.
Added to that are multiple (at least two) ways of mapping ACLs to
modes and at least three different behaviors in modifying ACLs to
reflect changes in mode.
As a result, existing RFCs allow at least three thousand different
server behaviors.
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* There is little information available about the particular
behaviors manifested by existing server-fs combinations.
It may be difficult to get this information because the
implementations were done by people no longer participating in the
working or who were never involved in the working group
* There are no existing attributes that might be used to
communicates server/fs choices to the client.
This creates a difficult situation. To fully resolve it will require
substantial work as follows:
* Determining whether each of the dubious instances of "SHOULD" is
in fact relied on by existing servers or might be needed by future
servers.
* Determining which ACE mask bits are supported by all existing
servers or not supported by any existing server to further limit
the set of OPTIONAL features that clients need to be made aware
of.
There might also be cases where some set of mask bits are always
either supported or unsupported together.
* Determining the set of mappings from acls to modes that are
actually and making each such OPTIONAL as opposed to the current
situation in which an "intentional" "SHOULD" is used despite the
mismatch between this use and [RFC2119].
* Making a similar determination of the actions actually applied to
acl when the mode is changed.
Once we have the above information, it will be clear how we could
create address these issues inn either of the ways discussed below:
* Have a way of determining whether support of NFSv4 ACL semantics,
core UNIX ACL semantics, or some hybrid of the two is provided by
any particular file system, such as that discussed in consensus
item #61.
This approach would address the motive for the current under-
specification but would not address possible negative consequences
arising from that under-specification.
A further difficulty with it is that it does not address the
possibility of hybrids of the two models.
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* providing an appropriate extension as described in Section 6.3 to
provide clients with an adequate description of expected server
behavior. This document will be limited to providing client
implementation guidance about dealing with the uncertainty in
cases in which new attributes is not available.
Appendix B. Issues for which Consensus Needs to be Ascertained
This section, together with a corresponding Appendix in the ACL
document, helps to keep track of specific changes which the author
has made or intends to make to deal with issues found in RFCs 7530
and 8881.
The changes listed exclude those that only apply to the ACL document
but does include cases in which there might need co-ordination
between the two documents. The changes listed here exclude those
which are clearly editorial but include some that the author believes
are editorial but for which the issues are sufficiently complicated
that working group consensus on the issue is probably necessary.
These changes are presented in the table below, organized into a set
of "Consensus Items" identified by the numeric code appearing in
annotations in the proposed document text. For each such item, a
type code is assigned with separate sets of code define for pending
items and for those which are no longer pending.
This document and the companion ACL document use a shared set of
numeric ids to identify Consensus Items, Some of these were
originally defined in this document and subsequently moved while
others were created as part of the ACL document. In order to
maintain a common list without requiring excessive document
coordination the follow practices and consequences should be noted.
* Before the documents were split the numbers from one to sixty-two
were assigned to Consensus Items while some of these were moved to
the ACL document.
Item numbers in this range might appear in either document,
although each number will appear in only a single document.
* New issues will receive a number based on the document in which
the issue appears:
Item numbers sixty-two through ninety-nine will be assigned to
issues in this document while those above one-hundred will be used
in the ACL document.
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* As part of the documents being split some items that apply to both
were split, with the portion appearing in this document retaining
the old number a new number (over one-hundred) being assigned to
the ACL-related portion.
In such cases, the related document items in the two document will
refer to one another.
The following type codes are defined for pending consensus items:
* "NM" denotes a change which is new material that is not purely
editorial and thus requires Working Group consensus for eventual
publication.
* "BE" denotes a change which the author believes is editorial but
for which the change is sufficiently complex that the judgment is
best confirmed by the Working Group.
* "BC" denotes a change which is a substantive change that the
author believes is correct. This does not exclude the possibility
of compatibility issues becoming an issue but is used to indicate
that the author believes any such issues are unlikely to prevent
its eventual acceptance.
* "CI" denotes a change for which the potential for compatibility
issues is a major concern with the expected result that working
group discussion of change will focus on clarifying our knowledge
of how existing clients and server deal with the issue and how
they might be affected by the change or the change modified to
accommodate them.
* "NS" denotes a change which represents the author's best effort to
resolve a difficulty but for which the author is not yet confident
that it will be adopted in its present form, principally because
of the possibility of troublesome compatibility issues.
* "NE" denotes change based on an existing issue in the spec but for
which the replacement text is incomplete and needs further
elaboration.
* "WI" denotes a potential change based on an existing issue in the
spec but for which replacement text is not yet available because
further working group input is necessary before drafting. It is
expected that replacement text will be available in a later draft
once that discussion is done.
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* "LD" denotes a potential change based on an existing issue in the
spec but for which replacement text is not yet available due to
the press of time. It is expected that replacement text will be
available in a later draft.
* "EV" denote a potential change which is tentative or incomplete
because further details need to be provide or because the author
is unsure that he has a correct explanation of the issue. It is
expected that replacement text will be available in a later draft.
The following codes are defined for consensus items which are no
longer pending.
* "RT" designates a former item which has been retired, because it
has been merged with another one or otherwise organized out of
existence.
Such items no longer are referred to the document source although
the item id is never reassigned. They are no longer counted among
the set of total items.
* "CA" designates a former item for which consensus has been
achieved in the judgment of the author, although not by any
official process.
Items reaching this state are effected in the document source
including the deletion of annotations and the elimination of
obsoleted previous treatments.
Items in this state are still counted among the total of item but
are no longer considered pending
* "CV" designates a former item for which consensus has been
achieved and officially verified.
Items in this state are not counted among the item totals. They
may be kept in the table but only to indicate that the item id is
still reserved.
* "DR" designates a former item which has been dropped, because it
appears that working group acceptance of it, even with some
modification, is unlikely.
Such items no longer are referred to the document source although
the item id is never reassigned. They are no longer counted among
the set of total items.
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When asterisk is appended to a state of "NM", "BC" or "BE" it that
there has been adequate working group discussion leading one to
reasonably expect it will be adopted, without major change, in a
subsequent document revision.
Such general acceptance is not equivalent to a formal working group
consensus and it not expected to result in major changes to the draft
document,
On the other hand, once there is a working group consensus with
regard to a particular issue, the document will be modified to remove
associated annotations, with the previously conditional text
appearing just as other document text does. The issue will remain in
this table as a non-pending item. It will be mentioned in Appendices
A.1 or A.2 to summarize the changes that have been made.
It is to be expected that these designations will change as
discussion proceeds and new document versions are published. It is
hoped that most such shifts will be upward in the above list or
result in the deletion of a pending item, by reaching a consensus to
accept or reject it. This would enable, once all items are dealt
with, an eventual request for publication as an RFC, with this
appendix having been deleted.
[Editor Aside]: The following Consensus Items have been moved, in
their entirety, to the new acl spec: #3, #4, #5, #8, #9, #7, #10,
#11, #12, #13, #14, #15, #16, #17, #26, #27, #28, #29, #30, #31, #50,
#51, #6 1. They have retained their numeric existing numeric ids.
[Editor Aside]: The following Consensus Items had to be split between
this document and the new acl spec, with a new id assigned for the
portion in the acl spec: #6, #25, #56, #58. The new ids assigned are
in the range between #62 and #65.
[Editor aside]: New items added to this spec after the split will be
assigned id's between #66 and #99. New item s added to the ACL spec
will be assigned ids in the range between #100 and #199.
+====+======+==================+====================================+
| # | Type | ...References... | Substance |
+====+======+==================+====================================+
| 1 | CA | Assumed OK. | New approach distinguishing |
| | | | authentication and |
| | | | identification. |
+----+------+------------------+------------------------------------+
| 2 | CA | Assumed OK. | Outline of new negotiation |
| | | | framework. |
+----+------+------------------+------------------------------------+
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| 6 | CI | #6a in S 5.3.1 | New/revised description of the |
| | | | role of the "sticky bit" for |
| | | | directories, both with respect to |
| | | | ACL handling and mode handling. |
| | | | |
| | | | Needs to be considered together |
| | | | with Item #62 in the ACL |
| | | | document. |
+----+------+------------------+------------------------------------+
| 18 | RT | | Originally concerned with need |
| | | | for support of owner, |
| | | | owner_group. |
| | | | |
| | | | Has now been combined with item |
| | | | #57. |
+----+------+------------------+------------------------------------+
| 19 | CI | #19a in S 8.2 | Revised discussion of |
| | | | coordination of mode and the ACL- |
| | | | related attributes. |
+----+------+------------------+------------------------------------+
| 20 | CI | #20 in S 5.3.1 | More closely align ACL-based and |
| | | | mode-based semantics with regard |
| | | | to SVTX. |
+----+------+------------------+------------------------------------+
| 21 | BC | #21a in S 4.1 | Introduce the concept of reverse- |
| | | | slope modes and deal properly |
| | | #21b in S 5.3.1 | with them. The decision as to |
| | | | the proper handling is addressed |
| | | | as Consensus Item #28. |
+----+------+------------------+------------------------------------+
| 22 | BC | #22a in S 9.2 | Revise treatment of divergences |
| | | | between ACL/mode authorization |
| | | | and server behavior, dividing the |
| | | | treatment between cases in which |
| | | | something authorized is still not |
| | | | allowed (OK), and those in which |
| | | | something not authorized is |
| | | | allowed (highly problematic from |
| | | | a security point of view). |
+----+------+------------------+------------------------------------+
| 23 | BC | #23a in S 9.1 | Revise discussion of client |
| | | | interpretation of ACLs and the |
| | | #23b in S 9.3 | use of ACCESS instead |
+----+------+------------------+------------------------------------+
| 24 | BE | #24a in S 9.3 | Delete bogus reference. |
+----+------+------------------+------------------------------------+
| 25 | CI | #25a in S 3.3 | Revised description of co- |
| | | | ordination of acl and mode |
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| | | | attributes to apply to NFSv4 as a |
| | | | whole. While this includes many |
| | | | aspects of the shift to be more |
| | | | specific about the co-ordination |
| | | | requirements including addressing |
| | | | apparently unmotivated uses of |
| | | | the terms "SHOULD" and "SHOULD |
| | | | NOT", it excludes some arguably |
| | | | related matters dealt with as |
| | | | Consensus Items #26 and #27. |
| | | | |
| | | | Needs to be considered together |
| | | | with Item #63 in the ACL |
| | | | document. |
+----+------+------------------+------------------------------------+
| 32 | BC | #32a in S 16 | Expanded negotiation framework to |
| | | | accommodate multiple transport |
| | | #32b in S 16.2 | types and security services |
| | | | provided on a per-connection |
| | | #32c in S 19 | basis, i.e. encryption and peer |
| | | | authentication. |
| | | #32d in S 19.1 | |
| | | | |
| | | #32e in S 19.2 | |
+----+------+------------------+------------------------------------+
| 33 | RT | Material | Reorganization of description of |
| | | formerly here | SECINFO op to apply to all minor |
| | | moved to #32. | versions. (Dropped) |
+----+------+------------------+------------------------------------+
| 34 | RT | Superseded by | Revision to NFSv4.0 SECINFO |
| | | simpler | implementation section (Dropped). |
| | | treatment. | |
+----+------+------------------+------------------------------------+
| 35 | NM | #35a in S 17.1 | Now has preliminary work on |
| | | | future security needs. |
+----+------+------------------+------------------------------------+
| 36 | CI | #36a in S 18.1.3 | Threat analysis only dealing with |
| | | | RECOMMENDED behavior regarding |
| | | | use of per-connection security |
| | | | facilities and handling of |
| | | | AUTH_SYS. |
+----+------+------------------+------------------------------------+
| 37 | CI | #37a in S 18.1.3 | Threat analysis only dealing with |
| | | | RECOMMENDED behavior with regard |
| | | | to acl support including ACL/mode |
| | | | coordination. |
+----+------+------------------+------------------------------------+
| 38 | CI | #38a in S 18.1.4 | Address the need to temporarily |
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| | | | allow unsafe behavior mistakenly |
| | | | allowed by previous |
| | | | specifications |
+----+------+------------------+------------------------------------+
| 39 | CI | #39a in S 18.1.5 | Define new approach to AUTH_SYS. |
+----+------+------------------+------------------------------------+
| 40 | CI | #40a in S 18.2.1 | Discussion of scope for security |
| | | | considerations and the |
| | | #40a in S 18.2.2 | corresponding threat analysis. |
+----+------+------------------+------------------------------------+
| 41 | CI | #41a in S 9.2 | Discuss major new security |
| | | | recommendations regarding |
| | | #41b in S 18.3.1 | protection of data in flight and |
| | | | client peer authentication. |
| | | #41c in S 18.3.2 | Also, covers the circumstances |
| | | | under which such recommendations |
| | | #41d in S 18.3.4 | can be bypassed. |
| | | | |
+----+------+------------------+------------------------------------+
| 42 | RT | #42a in S 18.4.5 | Former placeholders for threat |
+----+------+------------------+ analysis subsections have now |
| 43 | RT | #43a in S | been superseded by new proposed |
| | | 18.4.6.1 | subsections. |
+----+------+------------------+ |
| 44 | RT | #44a in S | |
| | | 18.4.6.2 | |
+----+------+------------------+ |
| 45 | RT | #45a in S | |
| | | 18.4.7.1 | |
+----+------+------------------+ |
| 46 | RT | #46a in S | |
| | | 18.4.7.2 | |
+----+------+------------------+------------------------------------+
| 47 | CI | gone for now. | Dubious paragraph regarding |
| | | | AUTH_NONE is SECINFO response |
| | | | which should be deleted if there |
| | | | are no compatibility issues that |
| | | | make that impossible. |
+----+------+------------------+------------------------------------+
| 48 | RT | Superseded by | Missing pieces of secinfo |
| | | simpler | processing algorithm that didn't |
| | | treatment. | get done in -02. |
+----+------+------------------+------------------------------------+
| 49 | RT | Superseded by | Secinfo processing algorithm that |
| | | simpler | was expected to be finished in |
| | | treatment. | -04. |
+----+------+------------------+------------------------------------+
| 52 | NS | #52b in S 9.2 | Eliminate superuser semantics as |
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| | | | it had been, as valid by |
| | | #52c in S 18.3.3 | implication and essentially |
| | | | ignoring the new format for user |
| | | | ids. Also, deal with the |
| | | | security consequences of its |
| | | | inclusion appropriately. |
+----+------+------------------+------------------------------------+
| 53 | EV | #53a in S 18.4.2 | Discussion of possible adaptation |
| | | | of RPCSEC_GSS/Kerberos to multi- |
| | | | factor authentication. |
+----+------+------------------+------------------------------------+
| 54 | EV | #54a in S | Discussion of prevention of code |
| | | 18.4.6.1 | on a compromised client from |
| | | | hijacking the client machine's |
| | | | peer authentication. |
+----+------+------------------+------------------------------------+
| 55 | EV | #55a in S | Discussion of issues with |
| | | 18.4.6.1 | potential use of Kerberos in |
| | | | cloud environments |
+----+------+------------------+------------------------------------+
| 56 | NI | #56a in S 4.1 | Discussion of issues related to |
| | | | the handling of allowed variants |
| | | | of the NFSv4 ACL model, including |
| | | | subsets based on the Unix ACL |
| | | | model. |
| | | | |
| | | | Needs to be considered together |
| | | | with Item #64 in the ACL |
| | | | document. |
+----+------+------------------+------------------------------------+
| 57 | BC | #57a in S 1.1.3 | Designation of mode, owner, and |
| | | | owner_group as REQUIRED |
| | | #57b in S 4.1 | attributes. |
| | | | |
| | | #57c in S 5 | Eliminates the undue complexity |
| | | | and gratuitous interoperability |
| | | #57d in S 5.1 | problems providing implementation |
| | | | that do not provide POSIX |
| | | #57e in S 5.3 | authorization semantics, |
| | | | including those with no |
| | | #57f in S 8.1 | authorization support at all. |
| | | | |
| | | #57g in S 8.2 | |
+----+------+------------------+------------------------------------+
| 58 | NM | #58a in S 5 | Designation of the acl, dacl, |
| | | | sacl, and sec_label attributes as |
| | | #58e in S 5.5.1 | Experimental, rather than |
| | | | OPTIONAL. |
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| | | | |
| | | | Note that this is separate from |
| | | | the possibility of sufficiently |
| | | | clarifying the description of the |
| | | | acl, dacl, and sacl attributes to |
| | | | make the Experimental designation |
| | | | unnecessary, which will be |
| | | | covered as Item #XX. |
| | | | |
| | | | Needs to be considered together |
| | | | with Item #65 in the ACL |
| | | | document. |
+----+------+------------------+------------------------------------+
| 59 | NM | #59a in S 1.1.3 | Major clarification of the |
| | | | handling of id strings, including |
| | | #59b in S 5.1 | removal of denigration of the |
| | | | numeric form. |
+----+------+------------------+------------------------------------+
| 60 | BC | #60a in S 5.1 | Addressing properly issue of |
| | | | special users such as root and |
| | | | nobody in the context of id |
| | | | strings. |
+----+------+------------------+------------------------------------+
| 66 | NE | #66a in S 5.3.5 | Discussion needed of |
| | | | authorization semantics for |
| | | #66b in S 5.3.6 | access to named attributes. |
| | | | |
| | | | Needs to be coordinated with |
| | | | handling of Item #100, addressed |
| | | | in the companion ACL document. |
+----+------+------------------+------------------------------------+
| 67 | BC | #67a in S 1.1.2 | Changes to adapt to new ACL |
| | | | architecture. These include: |
| | | #67b in S 1.1.3 | |
| | | | * Treating the core UNIX ACL |
| | | #67c in S 1.3.2 | model as the REQUIRED with the |
| | | | rest of the NFSv4 ACL model as |
| | | #67d in S 8.1 | a set of OPTIONAL extensions. |
| | | | |
| | | #67e in S 8.2 | * An architecture in which |
| | | | multiple ACL models can be |
| | | #67f in S 8.4 | supported. |
| | | | |
| | | #67g in S 9 | |
| | | | |
| | | #67h in S 10 | |
+----+------+------------------+------------------------------------+
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Table 2
The following table summarizes the issues in each particular pending
state.
+=======+=====+===========================================+
| Type | Cnt | Issues |
+=======+=====+===========================================+
| BC | 7 | 21, 22, 23, 32, 57, 60, 67 |
+-------+-----+-------------------------------------------+
| BE | 1 | 24 |
+-------+-----+-------------------------------------------+
| CI | 11 | 6, 19, 20, 25, 36, 37, 38, 39, 40, 41, 47 |
+-------+-----+-------------------------------------------+
| EV | 3 | 53, 54, 55 |
+-------+-----+-------------------------------------------+
| NE | 1 | 66 |
+-------+-----+-------------------------------------------+
| NI | 1 | 56 |
+-------+-----+-------------------------------------------+
| NM | 3 | 35, 58, 59 |
+-------+-----+-------------------------------------------+
| NS | 1 | 52 |
+-------+-----+-------------------------------------------+
| *All* | 28 | Grand total for above table. |
+-------+-----+-------------------------------------------+
Table 3
The following table summarizes the issues in each particular non-
pending state..
+=======+=====+====================================+
| Type | Cnt | Issues |
+=======+=====+====================================+
| CA | 2 | 1, 2 |
+-------+-----+------------------------------------+
| RT | 9 | 18, 33, 34, 42, 43, 44, 45, 46, 48 |
+-------+-----+------------------------------------+
| *All* | 11 | Grand total for above table. |
+-------+-----+------------------------------------+
Table 4
Acknowledgments
The author wishes to thank Tom Haynes for his helpful suggestion to
deal with security for all NFSv4 minor versions in the same document.
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The author wishes to thank Chris Inacio for pointing out the
difficulties created by the then-existing handling of name mapping.
The author wishes to thank Brian Pawlowski for his helpful insights
into the history of NFsv4 security handling.
The author wishes to draw people's attention to Nico Williams' remark
that NFSv4 security was not so bad, except that there was no
provision for authentication of the client peer. This perceptive
remark, which now seems like common sense, did not seem so when made,
but it has served as a beacon for those working on putting NFSv4
security on a firmer footing. We appreciate Nico's perceptive
guidance.
The author wishes to thank Bruce Fields for his helpful comments
regarding ACL support which had a major role in the evolution of this
document.
The author wishes to acknowledge the important role of the authors of
RPC-with-TLS, Chuck Lever and Trond Myklebust, in moving the NFS
security agenda forward and thank them for all their efforts to
improve NFS security.
The author wishes to thank Chuck Lever for his many helpful comments
about nfsv4 security issues, his explanation of many unclear points,
and much important guidance he provided that is reflected in this
document, including his work to streamline the security negotiation
process by the definition of new pseudo-flavors.
The author wishes to thank Rick Macklem for his help in resolving
NFSv4 security issues. These include clarifying possible server
policies regarding RPC-with-TLS, helping to clarify "owner-override
semantics" and bringing to the Working Group's attention the
possibility of deriving limited principal identification from client
peer authentication while still staying within the boundaries of RPC-
with-TLS.
Author's Address
David Noveck (editor)
NetApp
201 Jones Road, Suite 16
Waltham, MA 02451
United States of America
Phone: +1-781-572-8038
Email: davenoveck@gmail.com
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