Network Working Group D. Quigley
Internet-Draft NSA
Intended status: Standards Track J. Morris
Expires: July 26, 2009 Red Hat, Inc.
January 22, 2009
MAC Security Label Support for NFSv4
draft-quigley-nfsv4-sec-label-00.txt
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Abstract
This Internet-Draft describes additions to NFSv4 minor version one to
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support Mandatory Access Control systems. The current draft
describes the mechanism for transporting a MAC security label using
the NFSv4.1 protocol and the semantics required for that label. In
addition to this it describes an example system of using this label
in a fully MAC aware environment.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terms and Definitions . . . . . . . . . . . . . . . . . . . . 3
3. MAC Security Attribute . . . . . . . . . . . . . . . . . . . . 4
3.1. Interpreting security_attribute . . . . . . . . . . . . . 5
3.2. Delegations . . . . . . . . . . . . . . . . . . . . . . . 5
3.3. Permission Checking . . . . . . . . . . . . . . . . . . . 5
3.4. Object Creation . . . . . . . . . . . . . . . . . . . . . 5
4. MAC Security NFS Modes of Operation . . . . . . . . . . . . . 6
4.1. Full Mode . . . . . . . . . . . . . . . . . . . . . . . . 6
4.1.1. Initial Labeling and Translation . . . . . . . . . . . 6
4.1.2. Policy Enforcement . . . . . . . . . . . . . . . . . . 7
4.2. Smart Client Mode . . . . . . . . . . . . . . . . . . . . 7
4.2.1. Initial Labeling and Translation . . . . . . . . . . . 7
4.2.2. Policy Enforcement . . . . . . . . . . . . . . . . . . 8
4.3. Smart Server Mode . . . . . . . . . . . . . . . . . . . . 8
4.3.1. Initial Labeling and Translation . . . . . . . . . . . 8
4.3.2. Policy Enforcement . . . . . . . . . . . . . . . . . . 9
5. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
5.1. Multi-Level Security . . . . . . . . . . . . . . . . . . . 9
5.1.1. Full Mode . . . . . . . . . . . . . . . . . . . . . . 9
5.1.2. Smart Client Mode . . . . . . . . . . . . . . . . . . 10
5.1.3. Smart Server Mode . . . . . . . . . . . . . . . . . . 10
6. Security Considerations . . . . . . . . . . . . . . . . . . . 11
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 11
8. Normative References . . . . . . . . . . . . . . . . . . . . . 11
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 11
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1. Introduction
Mandatory Access Control (MAC) systems have been mainstreamed in
modern operating systems such as Linux (R), FreeBSD (R), Solaris
(TM), and Windows Vista (R). MAC systems bind security attributes to
subjects (processes) and objects within a system. These attributes
are used with other information in the system to make access control
decisions.
People desire to use NFSv4 with these systems. A mechanism is
required to provide security attribute information to NFSv4 clients
and servers. This mechanism has the following requirements:
o Clients must be able to convey to the server the security
attribute of the subject making the access request. The server
may provide a mechanism to enforce MAC policy based on the
requesting subject's security attribute.
o Server must be able to store and retrieve the security attribute
of exported files as requested by the client.
o Server must provide a mechanism for notifying clients of attribute
changes of files on the server.
o Clients and Servers must be able to negotiate Domains of
Interpretation (DOI) and provide a mechanism to translate between
them as needed.
These four requirements are key to the system with only requirements
two and three requiring changes to NFSv4.1. The ability to convey
the security attribute of the subject as described in requirement one
falls upon the RPC layer to implement. Requirement four allows
communication between different MAC implementations. The management
of DOIs and the translation between them does not require any support
from NFS on a protocol level and is out of the scope of this
document.
2. Terms and 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 described in [RFC2119].
Domain of Interpretation: A Domain of Interpretation (DOI) represents
an administrative security boundary, where all systems within the DOI
have semantically coherent labeling. That is, a security attribute
must always mean exactly the same thing anywhere within the DOI.
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Object: An object is a passive resource within the system that we
wish to be protected. Objects can be entities such as files,
directories, pipes, sockets, and many other system resources relevant
to the protection of the system state.
Subject: A subject is an active entity usually a process which is
requesting access to an object.
3. MAC Security Attribute
MAC models base access decisions on security attributes bound to
subjects and objects. This information can range from a user
identity for an identity based MAC model, sensitivity levels for
Multi-level security, or a type for Type Enforcement. These models
base their decisions on different criteria but the semantics of the
security attribute remain the same. The semantics required by the
security attributes are listed below:
o Must provide flexibility with respect to MAC model.
o Must provide the ability to atomically set security information
upon object creation
o Must provide the ability to enforce access control decisions both
on the client and the server
o Must not expose an object to either the client or server name
space before its security information has been bound to it.
NFSv4 provides several options for implementing the security
attribute. The first option is to implement the security attribute
as a named attribute. Named attributes provide flexibility since
they are treated as an opaque field but lack a way to atomically set
the attribute on creation. In addition, named attributes themselves
are file system objects which need to be assigned a security
attribute. This raises the question of how to assign security
attributes to the file and directories used to hold the security
attribute for the file in question. The inability to atomically
assign the security attribute on file creation and the necessity to
assign security attributes to its subcomponents makes named
attributes unacceptable as a method for storing security attributes.
The second option is to implement the security attribute as a
recommended attribute as defined in section 5.2 of RFC 3530.
Recommended attributes have a fixed format and semantics, which
conflicts with the flexible nature of the security attribute. To
resolve this the security attribute consists of two components. The
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first component is a DOI to allow for interoperability between MAC
mechanisms. The second component is an opaque field which the actual
security attribute data. To allow for various MAC models NFSv4
should be used solely as a transport mechanism for the security
attribute. It is the responsibility of the endpoints to consume the
security attribute and make access decisions based on their
respective models. In addition, creation of objects through OPEN and
CREATE allows for the security attribute to be specified upon
creation. By providing an atomic create and set operation for the
security attribute it is possible to enforce the second and fourth
requirements. To this end the attribute number 76 is requested for
the security_attribute recommended attribute.
3.1. Interpreting security_attribute
The security_attribute field contains two components the first
component being a Domain of Interpretation(DOI). A DOI serves to
provide the receiving end with the information necessary to translate
the security attribute into a form that is usable by the endpoint.
The translation used to interpret the security attribute is not
specified as part of the protocol since it can depend on various
factors. The second component is an opaque section which contains
the data of the attribute. This component is dependent on the MAC
model to interpret and enforce.
3.2. Delegations
In the event that a security attribute is changed while a client
holds a delegation on the file, the client should follow the existing
protocol with respect to attribute changes. It should flush all
changes back to the server and relinquish the delegation.
3.3. Permission Checking
It is not feasible to enumerate all possible MAC models and even
levels of protection within a subset of these models. This means
that the NFSv4 client and servers can not be expected to directly
make access control decisions based on the security attribute.
Instead NFSv4 should defer permission checking on this attribute to
the host system. These checks are performed in addition to existing
DAC and ACL checks outlined in the NFSv4 protocol. Section 4 gives a
specific example of how the security attribute is handled under a
particular MAC model.
3.4. Object Creation
When creating files in NFSv4 the OPEN and CREATE operations are used.
One of the parameters to these operations is an fattr4 structure
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containing the attributes the file is to be created with. This
allows NFSv4 to atomically set the security attribute of files upon
creation. When a client is MAC aware it must always provide the
initial security attribute upon file creation. In the event that the
server is the only MAC aware entity in the system it should ignore
the security attribute specified by the client and instead make the
determination itself. A more in depth explanation can be found in
Section 4.
4. MAC Security NFS Modes of Operation
A system using Labeled NFS may operate in three modes. The first
mode provides the most protection and is called "full" mode. In this
mode both the client and server implement a MAC model allowing each
end to make an access control decision. The remaining two modes are
variations on each other and are called "smart client" and "smart
server" modes. In these modes one end of the connection is not
implementing a MAC model and because of this these operating modes
offer less protection than full mode.
4.1. Full Mode
Full mode environments consist of MAC aware NFSv4 servers and clients
and may be composed of mixed MAC models and policies. The system
requires that both the client and server have an opportunity to
perform an access control check based on all relevant information
within the network. The file object security attribute is provided
using the mechanism described in Section 3. The security attribute
of the subject making the request is transported at the RPC layer
using the mechanism described in XXX: Insert RFC for RPC layer label
draft.
4.1.1. Initial Labeling and Translation
The ability to create a file is an action that a MAC model may wish
to mediate. The client is given the responsibility to determine the
initial security attribute to be placed on a file. This allows the
client to make a decision as to the acceptable security attributes to
create a file with before sending the request to the server. Once
the server receives the creation request from the client it may
choose to evaluate if the security attribute is acceptable.
Security attributes on the client and server may vary based on MAC
model and policy. To handle this the security attribute field has a
DOI component. This component is a mechanism for the host to
identify the meaning of the opaque portion of the security attribute.
A full mode environment may contain hosts operating in several
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different DOIs. In this case a mechanism for translating the opaque
portion of the security attribute is needed. The actual translation
function will vary based on MAC model and policy and is out of the
scope of this document. If a translation is unavailable for a given
DOI then the request SHOULD be denied. Another recource is to allow
the host to provide a fallback mapping for unknown security
attributes.
4.1.2. Policy Enforcement
In full mode access control decisions are made by both the clients
and servers. When a client makes a request it takes the security
attribute from the requesting process and makes an access control
decision based on that attribute and the security attribute of the
object it is trying to access. If the client denies that access an
RPC call to the server is never made. If however the access is
allowed the client will make a call to the NFS server.
When the server receives the request from the client it extracts the
security attribute conveyed in the RPC request. The server then uses
this security attribute and the attribute of the object the client is
trying to access to make an access control decision. If the server's
policy allows this access it will fulfill the client's request,
otherwise it will return NFS4ERR_ACCESS
Implementations MAY validate security attributes supplied over the
network to ensure that they are within a set of attributes permitted
from a specific peer, and if not, reject them. Note that a system
may permit a different set of attributes to be accepted from each
peer. An example of this can be seen in Section 5.1.1.
4.2. Smart Client Mode
Smart client environments consist of NFSv4 servers are not MAC aware
but NFSv4 clients that are. All of the clients in this environment
are assumed to be implementing the same MAC model and will be running
the same policy. The system requires that all clients in the
environment be responsible for access control checks. Due to the
amount of trust placed in the clients this mode is only to be used in
a trusted environment.
4.2.1. Initial Labeling and Translation
Just like in full mode the client is responsible for determining the
initial label upon object creation. The server in smart client mode
does not implement a MAC model, however, it may provide the ability
to restrict the creation and labeling of object with certain labels
based on different criteria as described in Section 4.1.2.
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In a smart client environment all of the clients operate in the same
DOI. This removes the need for the clients to maintain a set of DOI
translations. In order to support multiple DOIs in the same
environment it was considered that the DOI of the creating process
should be stored with the security attribute on the server. This was
abandoned due to the number of required DOI translations. In
practice all clients in this type of environment operate in the same
DOI.
4.2.2. Policy Enforcement
In smart client mode access control decisions are made by the
clients. When a client accesses an object it obtains the security
attribute of the object from the server and combines it with the
security attribute of the process making the request to make an
access control decision. This check is in addition to the DAC checks
provided by NFSv4 so this may fail based on the DAC criteria even if
the MAC policy grants access. As the policy check is located on the
client an access control denial should take the form that is native
to the platform.
4.3. Smart Server Mode
Smart server environments consist of NFSv4 servers that are MAC aware
and one or more MAC unaware clients. The server is the only entity
enforcing policy, and may selectively provide standard NFS services
to clients based on their authentication credentials and/or
associated network attributes (e.g. IP address, network interface).
The level of trust and access extended to a client in this mode is
configuration-specific.
4.3.1. Initial Labeling and Translation
In smart server mode all labeling and access control decisions are
performed by the NFSv4 server. In this environment the NFSv4 clients
are not MAC aware so they can not provide input into the access
control decision. This requires the server to determine the initial
labeling of objects. Normally the subject to use in this calculation
would originate from the client. Instead the NFSv4 server may choose
to assign the subject security attribute based on their
authentication credentials and/or associated network attributes (e.g.
IP address, network interface).
In smart server mode security attributes are contained solely within
the NFSv4 server. This means that all security attributes used in
the system remain within a single DOI. Since security attributes
will not cross DOIs there is no need to provide any translation
functionality above that which is needed internally by the MAC model.
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4.3.2. Policy Enforcement
All access control decisions in smart server mode are made by the
server. The server will assign the subject a security attribute
based on some criteria (e.g. IP address, network interface). Using
the newly calculated security attribute and the security attribute of
the object being requested the MAC model makes the access control
check and returns NFS4ERR_ACCESS on a denial and NFS4_OK on success.
This check is done transparently to the client so if the MAC
permission check fails the client may be unaware of the reason for
the permission failure. When operating in this mode administrators
attempting to debug permission failures should be aware to check the
MAC policy running on the server in addition to the DAC settings.
5. Examples
The section below outlines an example of the Labeled NFS operation
modes using a traditional Multi Level Security(MLS) model where
objects are given a sensitivity level (Unclassified, Secret, Top
Secret etc..) and a category set. This is just one example of a
possible MAC model and is not required by labeled NFS
implementations.
5.1. Multi-Level Security
In a Multi-level security (MLS) system objects are generally assigned
a sensitivity level, and a set of compartments. The sensitivity
levels within the system are given an order ranging from lowest to
highest classification level. Read access to an object is allowed
when the sensitivity level of the subject "dominates" the object it
wants to access. This means that the sensitivity level of the
subject is higher than that of the object it wishes to access and
that its set of compartments is a super-set of the compartments on
the object.
The rest of the section will just use sensitivity levels. In general
the example is a client that wishes to list the contents of a
directory. The system defines the sensitivity levels
Unclassified(U), Secret(S), and Top Secret(TS). The directory to be
searched is labeled Top Secret which means access to read the
directory will only be granted if the subject making the request is
also labeled Top Secret.
5.1.1. Full Mode
In the first part of this example a process on the client is running
at the Secret level. The process issues a readdir system call which
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enters the kernel. Before translating the readdir system call into a
request to the NFSv4 server the host operating system will consult
the MAC module to see if the operation is allowed. Since the process
is operating at Secret and the directory to be accessed is labeled
Top Secret the MAC module will deny the request and an error code is
returned to user space.
Consider a second case where instead of running at Secret the process
is running at Top Secret. In this case the sensitivity of the
process is equal to or greater than that of the directory so the MAC
module will allow the request. Now the readdir is translated into
the necessary NFSv4 call to the server. For the RPC request the
client is using the proper credential to assert to the server that
the process is running at Top Secret.
When the server receives the request it extracts the security label
from the RPC session and retrieves the label on the directory. The
server then checks with its MAC module if a Top Secret process is
allowed to read the contents of the Top Secret directory. Since this
is allowed by the policy then the server will return the appropriate
information back to the client.
In this example the policy on the client and server were both the
same. In the event that they were running different policies a
translation of the labels might be needed. In this case it could be
possible for a check to pass on the client and fail on the server.
The server may consider additional information when making its policy
decisions. For example the server could determine that a certain
subnet is only cleared for data up to Secret classification. If that
constraint was in place for the example above the client would still
succeed, but the server would fail since the client is asserting a
label that it is not able to use (Top Secret on a Secret network).
5.1.2. Smart Client Mode
In smart client mode the example is identical to the first part of a
full mode operation. A process on the client labeled Secret wishes
to access a Top Secret directory. As in the full mode example this
is denied since Secret does not dominate Top Secret. If the process
were operating at Top Secret it would pass the local access control
check and the NFSv4 operation would proceed as in a normal NFSv4
environment.
5.1.3. Smart Server Mode
In a smart server mode the client behaves as if it were in a normal
NFSv4 environment. Since the process on the client does not provide
a security attribute the server must define a mechanism for labeling
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all requests from a client. Assume that the server is using the same
criteria used in the full mode example. The server sees the request
as coming from a subnet that is a Secret network. The server
determines that all clients on that subnet will have their requests
labeled with Secret. Since the directory on the server is labeled
Top Secret and Secret does not dominate Top Secret the server would
fail the request with NFS4ERR_ACCESS.
6. Security Considerations
Depending on the level of protection the MAC system offers there may
be a requirement to tightly bind the security attribute to the data.
It must be taken into consideration that when used in a pNFS
environment is it possible that the security attribute and file data
will be stored on separate servers.
7. IANA Considerations
It is requested that IANA creates a registry of Domain of
Interpretation numbers to be consumed by a Domain of Interpretation
authority which will provide the mappings.
XXX: This section should probably be replaced with the section from
the CALIPSO document on DOIs. If we agree on that format we could
draw up an RFC for it and say that they should be managed by IANA as
specified in rfc****.
8. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", March 1997.
[RFC2434] Narten, T. and H. Alvestrand, "Guidelines for Writing an
IANA Considerations Section in RFCs", BCP 26, RFC 2434,
October 1998.
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Authors' Addresses
David P. Quigley
National Security Agency
9800 Savage Rd.
Suite 6534
Ft. Meade, MD 20755-6534
Email: dpquigl@tycho.nsa.gov
James Morris
Red Hat, Inc.
Email: jmorris@namei.org
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