Network File System Version 4 C. Lever
Internet-Draft Oracle
Intended status: Standards Track October 8, 2018
Expires: April 11, 2019
File Content Provenance for Network File System version 4
draft-ietf-nfsv4-integrity-measurement-02
Abstract
This document specifies an OPTIONAL extension to NFS version 4 minor
version 2 that enables file provenance information to be conveyed
between NFS version 4.2 servers and clients. File provenance
information authenticates the creator of a file's content and helps
guarantee the content's integrity from creation to use.
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
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material or to cite them other than as "work in progress."
This Internet-Draft will expire on April 11, 2019.
Copyright Notice
Copyright (c) 2018 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(https://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Requirements Language . . . . . . . . . . . . . . . . . . . . 3
3. Protocol Extension Considerations . . . . . . . . . . . . . . 4
4. Managing File Provenance Information on NFS Files . . . . . . 5
5. Operation . . . . . . . . . . . . . . . . . . . . . . . . . . 7
6. Security Considerations . . . . . . . . . . . . . . . . . . . 10
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 11
8. References . . . . . . . . . . . . . . . . . . . . . . . . . 11
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 12
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 12
1. Introduction
The security of software distribution systems is complex and
challenging, especially as software distribution has become
increasingly decentralized. An end administrator needs to trust that
she is running executables just as they are supplied by a software
vendor; in other words, that they have not been modified by malicious
actors, contracted system administration services, or broken hardware
or software. Software vendors want a guarantee that customer-
installed executables that fall under support contracts have
similarly not been modified.
There already exist mechanisms that protect file data during certain
portions of a file's life cycle:
o Whole file system checksumming can verify so-called Golden Master
installation media before it is used to install the software it
contains.
o File or block integrity mechanisms can protect data at rest on
storage servers.
o For a distributed file system such as NFS, transport layer
security or a GSS integrity service (as described in [RFC7861])
can protect data while it traverses a network between a storage
server and a client.
A more extensive mechanism is needed to guarantee that no
modification of a particular file has occurred since it was created,
perhaps even after several generations of copies have been made of
the file's content.
This guarantee can be accomplished by separately preserving a keyed
hash, such as an HMAC [RFC2104], of a file's content. The checksum
and its signature are verified as the file's content is read into
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memory immediately before it is used. If verification fails, access
to the file's content is prevented. The hash is updated and re-
signed only when the file is legitimately modified.
1.1. Architecture Summary
A keyed hash authenticates the identity of the last modifier of a
file's content and serves as a strong check of the content's
integrity. For the purposes of this document, we refer to this
metadata using the generic term "file provenance information".
File provenance information is generated and signed by a "provenance
authority", and then placed in the file system using special tools.
A security module separate from the file system stack specifies the
format of the file provenance information and enforces a policy for
utilizing it to determine when a protected file's content is safe to
use on the local system. For the purposes of this document, we refer
to this module as a "provenance assessor", and the policy it uses as
the "provenance assessment policy".
NFS acts as a conduit by which file provenance information and file
content move between storage on an NFS server and the provenance
assessor where that content is to be accessed. NFS peers accessing a
set of shared files must all agree on the at-rest file provenance
information format. The format is specified by the provenance
assessor and is therefore not described in this document.
A Trusted Platform Module [TPM-SUM] can seal the key material used to
sign and verify file content. Distributing and protecting such key
material is outside the scope of the OPTIONAL extension specified in
this document.
The Linux Integrity Measurement Architecture (IMA) is an example of a
mechanism that provides a full provenance assessment service
[IMA-WP]. The protocol extension in this document enables the
storage and use of file provenance information to protect files
stored on an NFS server.
2. Requirements Language
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 described in BCP 14 [RFC2119]
[RFC8174] when, and only when, they appear in all capitals, as shown
here.
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3. Protocol Extension Considerations
This document specifies an OPTIONAL extension to NFS version 4 minor
version 2 [RFC7862], hereafter referred to as NFS version 4.2. NFS
version 4.2 servers and clients implemented without knowledge of this
extension will continue to interoperate with NFS version 4.2 clients
and servers that are aware of the extension, whether or not they
support it.
Because [RFC7862] does not define NFS version 4.2 as non-extensible,
[RFC8178] treats it as an extensible minor version. Therefore this
Standards Track RFC extends NFS version 4.2 but does not update
[RFC7862] or [RFC7863].
3.1. XDR Extraction
Section 4.1 contains a description of an extension to the NFS version
4.2 protocol, expressed in the External Data Representation (XDR)
language [RFC4506]. This description is provided in a way that makes
it simple to extract into ready-to-compile form. The reader can
apply the following sed script to this document to produce a machine-
readable XDR description of the extension.
<CODE BEGINS>
sed -n -e 's:^ */// ::p' -e 's:^ *///$::p'
<CODE ENDS>
That is, if this document is in a file called "provenance-
extension.txt" then the reader can do the following to extract an XDR
description file:
<CODE BEGINS>
sed -n -e 's:^ */// ::p' -e 's:^ *///$::p'
< provenance-extension.txt > ima.x
<CODE ENDS>
Once that extraction is done, these added lines need to be inserted
into an appropriate base XDR of the generated XDR from [RFC7863]
together with XDR from any additional extensions to be recognized by
the implementation. This will result in a ready-to-compile XDR file.
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4. Managing File Provenance Information on NFS Files
4.1. XDR Definition
This section defines a new data type to encapsulate and a new
OPTIONAL attribute to access and update file provenance information
associated with a particular file.
To enable a single file provenance information payload to be
retrieved or updated via a single RPC, and to constrain the transport
resources required for the operations defined in this section, the
length of file provenance information MUST NOT exceed 4096 bytes in
length.
When an NFS version 4.2 server does not recognize, or does recognize
but does not support, this new attribute, the server responds in
accordance with the requirements specified in Section 4.3 of
[RFC8178].
<CODE BEGINS>
/// /*
/// * Copyright (c) 2018 IETF Trust and the person identified
/// * as author of the code. All rights reserved.
/// *
/// * The author of the code is: C. Lever
/// */
///
/// const FILEPROV4_MAXSIZE = 4096;
/// typedef opaque file_prov4<FILEPROV4_MAXSIZE>;
///
/// %/*
/// % * New For File Provenance Information
/// % */
/// const FATTR4_FILE_PROVENANCE = XXX; /* to be assigned */
///
/// typedef file_prov4 fattr4_file_provenance;
<CODE ENDS>
4.2. Storing File Provenance Information
An NFS version 4.2 client stores file provenance information by
sending a SETATTR operation that specifies the FATTR4_FILE_PROVENANCE
attribute, targeting the file object associated with the file
provenance information to be stored. This attribute completely
replaces any previous one.
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To remove this attribute from a file, the client sends a
FATTR4_FILE_PROVENANCE attribute whose length is zero. [ cel: Does
writing to a file have any effect on IMA metadata? ]
When a SETATTR is presented to an NFS version 4.2 server with a
credential that is not authorized to replace the
FATTR4_FILE_PROVENANCE attribute, the server MUST respond with
NFS4ERR_ACCESS.
When a SETATTR is presented to an NFS version 4.2 server with a
fattr4_file_provenance field whose length is larger than
FILEPROV4_MAXSIZE, the server MUST respond with NFS4ERR_INVAL.
When a SETATTR is presented to an NFS version 4.2 server and the
target object resides in a file system which supports
FATTR4_FILE_PROVENANCE but the object does not support this
attribute, the server MUST respond with NFS4ERR_WRONGTYPE.
When a SETATTR is presented to an NFS version 4.2 server but the
target object resides in a file system which does not support
FATTR4_FILE_PROVENANCE, the server MUST respond with
NFS4ERR_ATTRNOTSUPP.
A detailed description of the SETATTR operation can be found in
Section 18.30 of [RFC5661].
4.3. Retrieving File Provenance Information
An NFS version 4.2 client retrieves file provenance information by
retrieving the FATTR4_FILE_PROVENANCE attribute via a GETATTR
operation, specifying the file handle of the file object associated
with the information to be retrieved. This information may have been
computed and signed previously on this client or by some other agent.
When a GETATTR is presented to an NFS version 4.2 server and the
target object resides in a file system which supports
FATTR4_FILE_PROVENANCE but the object does not support this
attribute, the server MUST respond with NFS4ERR_WRONGTYPE.
When a GETATTR is presented to an NFS version 4.2 server but the
target object resides in a file system which does not support
FATTR4_FILE_PROVENANCE, this does not result in an error and the
FATTR4_FILE_PROVENANCE attribute bit is clear in the server's
response.
Otherwise, if the target object supports FATTR4_FILE_PROVENANCE and
there is no file provenance information is available for the target
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object, the server returns a FATTR4_FILE_PROVENANCE attribute whose
length is zero.
Provenance assessors operate after file content has been delivered
but immediately before that content is to be used. To enable
provenance assessors on NFS clients to verify file provenance
information, NFS version 4.2 servers do not prevent access to file
content if they have a local provenance assessor and it indicates
that provenance verification has failed.
A detailed description of the GETATTR operation can be found in
Section 18.7 of [RFC5661].
5. Operation
5.1. Terminology
To aid the discussion in this section, we define a few handy terms:
o A "participating client" is an NFS version 4.2 client that
supports the OPTIONAL extension described in this document and
employs a provenance assessor.
o A "non-participating client" is an NFS version 4.2 client that
does not support the OPTIONAL extension described in this document
or does not employ a provenance assessor.
o A "participating server" is an NFS version 4.2 server that
supports the OPTIONAL extension described in this document and its
shared filesystems can store file provenance information.
o A "non-participating server" is an NFS version 4.2 server that
does not support the OPTIONAL extension described in this document
or its shared filesystems are not capable of storing file
provenance information.
In addition, there are intermediate modes of operation on
participating peers:
o A "full-function client" is a participating client that supports
updating remote file provenance information.
o A "fetch-only client" is a participating client that does not
support modifying file provenance information on a participating
server.
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o A "full-function server" is a participating server that supports
updating file provenance information that resides on local shared
file systems.
o A "store-only server" is a participating server where there is
only remote access to file provenance information.
5.2. Instantiating File Provenance Information
Once a file is written, file provenance information is generated and
signed by an appropriate provenance authority. Using the OPTIONAL
extension specified in this document, the information can be
associated with a file on either a full-function server or client
using a tool with appropriate privileges that writes the provenance
information to the shared file system. When using a store-only
server, only a full-function client can place file provenance
information in the shared file system.
Typically, once file provenance information is associated with a
file, the file's content is essentially immutable, even if the file
has write permissions. This is because changing the content without
updating the associated file provenance information will make the
content inaccessible, depending on the provenance assessment policy
in effect. Thus updating the file content usually requires
generating fresh file provenance information.
5.2.1. Authorizing Updates to File Provenance Information
A participating server should ensure that modifications to file
provenance information are done only by appropriately authorized
agents.
[ cel: TBD. Regular users are probably not able to modify a local
security.ima xattr. What kind of authority should be required to
modify FPI remotely? ]
5.3. Interaction With Non-Participating Implementations
Because the protocol extension described herein is OPTIONAL, clients
and servers that support it must necessarily interact with clients
and servers that do not support it. To set the stage for a
discussion of interactions that might occur, consider the following
possible simple provenance assessment policies that might be adopted
by a provenance assessor (actual polices are left to provenance
assessors):
Strict
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Access is prevented to a file's content if the file has no
provenance information or if the provenance information fails to
verify the file content. Otherwise access to the file's content
is not prevented.
Audit
Access to a file's content is never prevented. Warnings are
reported when a file has no provenance information or when
existing provenance information fails to verify the file's
content.
Disabled
Access to file content is never prevented and provenance
information is ignored.
Given the above example policies and the definitions we provided
earlier for participating and non-participating implementations, the
following statements are true:
o A participating client that uses the Disabled policy is equivalent
to a non-participating client.
o A non-participating client never prevents access to file content
on a participating server.
o A participating client using the Strict policy never allows access
to files stored on a non-participating server.
A provenance assessor on an NFS version 4.2 peer needs to be prepared
to deal with file provenance information it does not recognize or
cannot parse. Typically its policy treats this case as a provenance
verification failure.
Note that an NFS version 4.2 server may use a provenance assessor to
prevent access by local users to protected files. To enable NFS
version 4.2 clients to do their own assessment, an NFS version 4.2
server should never prevent remote access to clients if local
provenance assessment fails.
5.4. Performance Cost of Using File Provenance Information
A provenance assessor typically checksums the entirety of a file's
content. When a file's content is first accessed, after it changes,
or if any portion of a file is evicted from an NFS version 4.2
client's cache, the client must retrieve any missing content before
its local provenance assessor can compute a fresh checksum to verify
the file's content.
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Thus provenance assessment can incur a significant performance impact
for large files, files that change frequently, or files where only a
portion of the content is used on that client (e.g., software
libraries). A provenance assessor can employ mechanisms not
specified here to reduce this impact.
For example, instead of signing a hash of the file's byte stream, a
Merkle tree can be constructed that allows assessors to verify the
integrity of smaller portions of a large file [MERKLE]. The root
hash of that tree, being of sufficiently limited size, can be signed
and stored as file provenance information. The Merkle tree, which is
stored elsewhere, can be used to verify portions of the file's
content without the need to read the whole file.
6. Security Considerations
The design of the OPTIONAL extension described in this document
assumes that all file provenance information is keyed or otherwise
cryptographically signed by a provenance authority to prevent
unwanted alteration at rest or in transit.
When file provenance information for a file exists, the content of a
file is protected from creation to use. Receivers can reliably
detect unintentional or malicious alteration of file content by
verifying its content using file provenance information. Additional
protection of file content while at rest or in transit on an
untrusted network is unnecessary.
Likewise, receivers can also reliably detect unintentional or
malicious alteration of file provenance information that is
cryptographically signed, simply by verifying its signature.
Additional protection of signed file provenance information while at
rest or in transit on an untrusted network is unnecessary.
Like other mechanisms that protect data integrity during transit, A
malicious agent or a network malfunction can create a denial-of-
service condition by repeatedly triggering integrity verification
failures on NFS version 4.2 clients.
To prevent a malicious denial-of-service attempt by altering file
provenance information at rest, an NFS version 4.2 server should
enforce a suitable level of privilege before authorizing a local or
remote agent to alter this information. See Section 5.2.1 for more
detail.
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7. IANA Considerations
This document does not require any actions by IANA.
8. References
8.1. Normative References
[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>.
[RFC4506] Eisler, M., Ed., "XDR: External Data Representation
Standard", STD 67, RFC 4506, DOI 10.17487/RFC4506, May
2006, <https://www.rfc-editor.org/info/rfc4506>.
[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>.
[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>.
[RFC7863] Haynes, T., "Network File System (NFS) Version 4 Minor
Version 2 External Data Representation Standard (XDR)
Description", RFC 7863, DOI 10.17487/RFC7863, November
2016, <https://www.rfc-editor.org/info/rfc7863>.
[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>.
[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>.
8.2. Informative References
[IMA-WP] Safford, D., "An Overview of The Linux Integrity
Subsystem", <http://downloads.sf.net/project/linux-ima/
linux-ima/Integrity_overview.pdf>.
[MERKLE] Merkle, R., ""A Digital Signature Based on a Conventional
Encryption Function" Advances in Cryptology - CRYPTO '87",
DOI 10.1007/3-540-48184-2_32, 1988.
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[RFC2104] Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed-
Hashing for Message Authentication", RFC 2104,
DOI 10.17487/RFC2104, February 1997,
<https://www.rfc-editor.org/info/rfc2104>.
[RFC5662] Shepler, S., Ed., Eisler, M., Ed., and D. Noveck, Ed.,
"Network File System (NFS) Version 4 Minor Version 1
External Data Representation Standard (XDR) Description",
RFC 5662, DOI 10.17487/RFC5662, January 2010,
<https://www.rfc-editor.org/info/rfc5662>.
[RFC7861] Adamson, A. and N. Williams, "Remote Procedure Call (RPC)
Security Version 3", RFC 7861, DOI 10.17487/RFC7861,
November 2016, <https://www.rfc-editor.org/info/rfc7861>.
[TPM-SUM] Trusted Computing Group, "Trusted Platform Module (TPM)
Summary", April 2008, <https://trustedcomputinggroup.org/
wp-content/uploads/
Trusted-Platform-Module-Summary_04292008.pdf>.
Acknowledgments
The author wishes to thank Mimi Zohar and James Morris for their
early review of the concepts in this document, Wim Coekaerts for his
encouragement of this work, and Dave Noveck for his work on NFS
version 4 extensibility.
The author wishes to acknowledge review comments from Dave Noveck,
Craig Everhart, and Bruce Fields which helped to make this a better
document.
The XDR extraction conventions were first described by the authors of
the NFS version 4.1 XDR specification [RFC5662]. Herbert van den
Bergh suggested the replacement sed script used in this document.
Special thanks go to Transport Area Director Spencer Dawkins, NFSV4
Working Group Chairs Spencer Shepler and Brian Pawlowski, and NFSV4
Working Group Secretary Thomas Haynes for their support.
Author's Address
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Charles Lever
Oracle Corporation
1015 Granger Avenue
Ann Arbor, MI 48104
United States of America
Phone: +1 248 816 6463
Email: chuck.lever@oracle.com
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