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Requirements for Labeled NFS
RFC 7204

Document Type RFC - Informational (April 2014)
Author Thomas Haynes
Last updated 2015-10-14
RFC stream Internet Engineering Task Force (IETF)
Additional resources Mailing list discussion
IESG Responsible AD Martin Stiemerling
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RFC 7204
Internet Engineering Task Force (IETF)                         T. Haynes
Request for Comments: 7204                                        NetApp
Category: Informational                                       April 2014
ISSN: 2070-1721

                      Requirements for Labeled NFS


   This memo outlines high-level requirements for the integration of
   flexible Mandatory Access Control (MAC) functionality into the
   Network File System (NFS) version 4.2 (NFSv4.2).  It describes the
   level of protections that should be provided over protocol components
   and the basic structure of the proposed system.  The intent here is
   not to present the protocol changes but to describe the environment
   in which they reside.

Status of This Memo

   This document is not an Internet Standards Track specification; it is
   published for informational purposes.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Not all documents
   approved by the IESG are a candidate for any level of Internet
   Standard; see Section 2 of RFC 5741.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at

Copyright Notice

   Copyright (c) 2014 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
   ( 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 ....................................................3
   2. Definitions .....................................................3
      2.1. Requirements Language ......................................4
   3. Requirements ....................................................4
      3.1. General ....................................................4
      3.2. Security Services ..........................................5
      3.3. Label Encoding, Label Format Specifiers, and Label
           Checking Authorities .......................................5
      3.4. Labeling ...................................................6
           3.4.1. Client Labeling .....................................6
           3.4.2. Server Labeling .....................................7
      3.5. Policy Enforcement .........................................7
           3.5.1. Client Enforcement ..................................7
           3.5.2. Server Enforcement ..................................8
      3.6. Namespace Access ...........................................8
      3.7. Upgrading Existing Server ..................................9
   4. Modes of Operation ..............................................9
      4.1. Full Mode ..................................................9
      4.2. Limited Server Mode .......................................10
      4.3. Guest Mode ................................................10
   5. Use Cases ......................................................11
      5.1. Full MAC Labeling Support for Remotely Mounted
           File Systems ..............................................11
      5.2. MAC Labeling of Virtual Machine Images Stored on
           the Network ...............................................11
      5.3. Simple Security Label Storage .............................12
      5.4. Diskless Linux ............................................12
      5.5. Multi-Level Security ......................................13
           5.5.1. Full Mode - MAC-Functional Client and Server .......13
           5.5.2. MAC-Functional Client ..............................14
           5.5.3. MAC-Functional Server ..............................15
   6. Security Considerations ........................................15
      6.1. Trust Needed for a Community ..............................15
      6.2. Guest Mode ................................................15
      6.3. MAC-Functional Client Configuration .......................16
   7. References .....................................................16
      7.1. Normative References ......................................16
      7.2. Informative References ....................................16
   Appendix A. Acknowledgments .......................................18

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1.  Introduction

   Mandatory Access Control (MAC) systems (as defined in [RFC4949]) have
   been mainstreamed in modern operating systems such as Linux, FreeBSD,
   and Solaris.  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

   Access control models such as Unix permissions or Access Control
   Lists (ACLs) are commonly referred to as Discretionary Access Control
   (DAC) models.  These systems base their access decisions on user
   identity and resource ownership.  In contrast, MAC models base their
   access control decisions on the label on the subject (usually a
   process) and the object it wishes to access.  These labels may
   contain user identity information but usually contain additional
   information.  In DAC systems, users are free to specify the access
   rules for resources that they own.  MAC models base their security
   decisions on a system-wide policy established by an administrator or
   organization that the users do not have the ability to override.  DAC
   systems offer some protection against unauthorized users running
   malicious software.  However, even an authorized user can execute
   malicious or flawed software with those programs running with the
   full permissions of the user executing it.  Inversely, MAC models can
   confine malicious or flawed software and usually act at a finer
   granularity than their DAC counterparts.

   Besides describing the requirements, this document records the
   functional requirements for the client imposed by the preexisting
   security models on the client.  This document may help those outside
   the NFS community understand those issues.

2.  Definitions

   Foreign Label:  a label in a format other than the format that a MAC
      implementation uses for encoding.

   Label Format Specifier (LFS):  an identifier used by the client to
      establish the syntactic format of the security label and the
      semantic meaning of its components.

   MAC-Aware:  a server that can transmit and store object labels.

   MAC-Functional:  a client or server that is Labeled NFS enabled.
      Such a system can interpret labels and apply policies based on the
      security system.

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   Multi-Level Security (MLS):  a traditional model where objects are
      given a sensitivity level (Unclassified, Secret, Top Secret, etc.)
      and a category set [RH_MLS].

   Object:  a passive resource within the system that we wish to
      protect.  Objects can be entities such as files, directories,
      pipes, sockets, and many other system resources relevant to the
      protection of the system state.

   Policy Identifier (PI):  an optional part of the definition of a
      Label Format Specifier.  The PI allows clients and servers to
      identify specific security policies.

   Subject:  an active entity, usually a process, that is requesting
      access to an object.

2.1.  Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in [RFC2119].

3.  Requirements

   The following initial requirements have been gathered from users and
   developers, and from previous development efforts in this area such
   as the Distributed Trusted Operating System [DTOS] and the NSA's
   experimental NFSv3 enhancements [SENFSv3].

3.1.  General

   A mechanism is required to provide security attribute information to
   NFSv4 clients and servers.  This mechanism has the following

   (1)  Clients MUST be able to convey to the server the client's
        privileges, i.e., the subject, for making the access request.
        The server may provide a mechanism to enforce MAC policy based
        on the requesting client's privileges.

   (2)  Servers MUST be able to store and retrieve the security
        attribute of exported files as requested by the client.

   (3)  Servers MUST provide a mechanism for notifying clients of
        attribute changes of files on the server.

   (4)  Clients and Servers MUST be able to negotiate Label Formats and
        provide a mechanism to translate between them as needed.

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3.2.  Security Services

   Labeled NFS or the underlying system on which the Labeled NFS
   operates MUST provide the following security services for all NFSv4.2

   o  Authentication

   o  Integrity

   o  Privacy

   Mechanisms and algorithms used in the provision of security services
   MUST be configurable so that appropriate levels of protection may be
   flexibly specified per mandatory security policy.

   Strong mutual authentication is required between the server and the
   client for Full Mode operation (Section 4.1).

   MAC security labels and any related security state MUST always be
   protected by these security services when transferred over the
   network, as MUST the binding of labels and state to associated
   objects and subjects.

   Labeled NFS SHOULD support authentication on a context granularity so
   that different contexts running on a client can use different
   cryptographic keys and facilities.

3.3.  Label Encoding, Label Format Specifiers, and Label Checking

   Encoding of MAC labels and attributes passed over the network MUST be
   specified in a complete and unambiguous manner while maintaining the
   flexibility of MAC implementations.  To accomplish this, the labels
   MUST consist of a format-specific component bound with a Label Format
   Specifier (LFS).  The LFS component provides a mechanism for
   identifying the structure and semantics of the opaque component.
   Meanwhile, the opaque component is the security label that will be
   interpreted by the MAC models.

   MAC models base access decisions on security attribute privileges
   bound to subjects and objects, respectively.  With a given MAC model,
   all systems have semantically coherent labeling -- a security label
   MUST always mean exactly the same thing on every system.  While this
   may not be necessary for simple MAC models, it is recommended that
   most Label Formats assigned an LFS incorporate semantically coherent
   labeling into their Label Format.

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   Labeled NFS SHOULD define an initial negotiation scheme with the
   primary aims of simplicity and completeness.  This is to facilitate
   practical deployment of systems without being weighed down by complex
   and overgeneralized global schemes.  Future extensibility SHOULD also
   be taken into consideration.

   Labeled NFS MUST provide a means for servers and clients to identify
   their LFSs for the purposes of authorization, security service
   selection, and security label interpretation.

   Labeled NFS MUST provide a means for servers and clients to identify
   their mode of operation (see Section 4).

   A negotiation scheme SHOULD be provided, allowing systems from
   different Label Formats to agree on how they will interpret or
   translate each other's foreign labels.  Multiple concurrent
   agreements may be current between a server and a client.

   All security labels and related security state transferred across the
   network MUST be tagged with a valid LFS.

   If the LFS supported on a system changes, the system SHOULD
   renegotiate agreements to reflect these changes.

   If a system receives any security label or security state tagged with
   an LFS it does not recognize or cannot interpret, it MUST reject that
   label or state.

   NFSv4.2 includes features that may cause a client to cross an LFS
   boundary when accessing what appears to be a single file system.  If
   LFS negotiation is supported by the client and the server, the server
   SHOULD negotiate a new, concurrent agreement with the client, acting
   on behalf of the externally located source of the files.

3.4.  Labeling

   Implementations MUST validate security labels supplied over the
   network to ensure that they are within a set of labels permitted from
   a specific peer and, if not, reject them.  Note that a system may
   permit a different set of labels to be accepted from each peer.

3.4.1.  Client Labeling

   At the client, labeling semantics for NFS mounted file systems MUST
   remain consistent with those for locally mounted file systems.  In
   particular, user-level labeling operations local to the client MUST
   be enacted locally via existing APIs, to ensure compatibility and
   consistency for applications and libraries.

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   Note that this does not imply any specific mechanism for conveying
   labels over the network.

   When an object is newly created by the client, it will calculate the
   label for the object based on its policy.  Once that is done, it will
   send the request to the server, which has the ability to deny the
   creation of the object with that label based on the server's policy.
   In creating the file, the server MUST ensure that the label is bound
   to the object before the object becomes visible to the rest of the
   system.  This ensures that any access control or further labeling
   decisions are correct for the object.

3.4.2.  Server Labeling

   The server MUST provide the capability for clients to retrieve
   security labels on all exported file system objects where possible.
   This includes cases where only in-core and/or read-only security
   labels are available at the server for any of its exported file

   The server MUST honor the ability for a client to specify the label
   of an object on creation.  If the server is MAC enabled, it may
   choose to reject the label specified by the client, due to
   restrictions in the server policy.  The server SHOULD NOT attempt to
   find a suitable label for an object in the event of different
   labeling rules on its end.  The server is allowed to translate the
   label but MUST NOT change the semantic meaning of the label.

3.5.  Policy Enforcement

   The MAC-Functional client determines if a process request is sent to
   the remote server.  Upon a successful response from the server, it
   must use its own policies on the object's security labels to
   determine if the process can be given access.  The client SHOULD NOT
   need to be cognizant of whether the server is a Limited Server or is
   fully MAC-Functional.

3.5.1.  Client Enforcement

   The client MUST apply its own policy to remotely located objects,
   using security labels for the objects obtained from the server.  It
   MUST be possible to configure the maximum length of time a client may
   cache state regarding remote labels before revalidating that state
   with the server.

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   If the server's policy changes, the client MUST flush all object
   state back to the server.  The server MUST ensure that any flushed
   state received is consistent with current policy before committing it
   to stable storage.

   Any local security state associated with cached or delegated objects
   MUST also be flushed back to the server when any other state of the
   objects is required to be flushed back.

   The implication here is that if the client holds a delegation on an
   object, then it enforces policy to local changes based on the object
   label it got from the server.  When it tries to commit those changes
   to the server, it SHOULD be prepared for the server to reject those
   changes based on the policies of the server.

3.5.2.  Server Enforcement

   A MAC-Functional server MUST enforce its security policy over all
   exported objects, for operations that originate both locally and

   Requests from authenticated clients MUST be processed using security
   labels and credentials supplied by the client as if they originated

   As with labeling, the system MUST also take into account any other
   volatile client security state, such as a change in process security
   context via dynamic transition.  Access decisions SHOULD also be made
   based upon the current client security label accessing the object,
   rather than the security label that opened it, if different.

   The server SHOULD recall delegation of an object if the object's
   security label changes.

3.6.  Namespace Access

   The server SHOULD provide a means to authorize selective access to
   the exported file system namespace based upon client credentials and
   according to security policy.

   This is a common requirement of MLS-enabled systems, which often need
   to present selective views of namespaces based upon the clearances of
   the subjects.

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3.7.  Upgrading Existing Server

   Note that under the MAC model, all objects MUST have labels.
   Therefore, if an existing server is upgraded to include Labeled NFS
   support, then it is the responsibility of the security system to
   define the behavior for existing objects.

4.  Modes of Operation

   In a Labeled NFS client and server interaction, we can describe three
   modes of operation:

   1.  Full

   2.  Limited Server

   3.  Guest

   These modes arise from the level of MAC functionality in the clients
   and servers.  The clients can be non-MAC-Functional and
   MAC-Functional.  The servers can be non-MAC-Functional, MAC-Aware,
   and MAC-Functional.

   A MAC-Functional client MUST be able to determine the level of MAC
   functionality in the server.  Likewise, a MAC-Functional server MUST
   be able to determine whether or not a client is MAC-Functional.  As
   discussed in Section 3.3, the protocol MUST provide for the client
   and server to make those determinations.

4.1.  Full Mode

   The server and the client have mutually recognized MAC functionality
   enabled, and full Labeled NFS functionality is extended over the
   network between both client and server.

   An example of an operation in Full Mode is as follows.  On the
   initial lookup, the client requests access to an object on the
   server.  It sends its process security context over to the server.
   The server checks all relevant policies to determine if that process
   context from that client is allowed to access the resource.  Once
   this has succeeded, the object, with its associated security
   information, is released to the client.  Once the client receives the
   object, it determines if its policies allow the process running on
   the client access to the object.

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   On subsequent operations where the client already has a handle for
   the file, the order of enforcement is reversed.  Since the client
   already has the security context, it may make an access decision
   against its policy first.  This enables the client to avoid sending
   requests to the server that it knows will fail, regardless of the
   server's policy.  If the client passes its policy checks, then it
   sends the request to the server, where the client's process context
   is used to determine if the server will release that resource to the
   client.  If both checks pass, the client is given the resource and
   everything succeeds.

   In the event that the client does not trust the server, it may opt to
   use an alternate labeling mechanism, regardless of the server's
   ability to return security information.

4.2.  Limited Server Mode

   The server is MAC-Aware, and the clients are MAC-Functional.  The
   server can store and transmit labels.  It cannot enforce labels.  The
   server MUST inform clients when an object label changes for a file
   the client has open.

   In this mode, the server may not be aware of the format of any of its
   object labels.  Indeed, it may service several different security
   models at the same time.  A client MUST process foreign labels as
   discussed in Section 3.3.  As with the Guest Mode, this mode's level
   of trust can be degraded if non-MAC-Functional clients have access to
   the server.

4.3.  Guest Mode

   Only one of the server or client is MAC-Functional enabled.

   In the case of the server only being MAC-Functional, the server
   enforces its 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)
   according to security policy.  The level of trust and access extended
   to a client in this mode is configuration-specific.

   In the case of the client only being MAC-Functional, the client MUST
   operate as a standard NFSv4.2 (see [NFSv4_2]) client and SHOULD
   selectively provide processes access to servers based upon the
   security attributes of the local process, and network attributes of
   the server, according to policy.  The client may also override
   default labeling of the remote file system based upon these security
   attributes or other labeling methods such as mount point labeling.

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   In other words, the Guest Mode is standard NFSv4.2 over the wire,
   with the MAC-Functional system mapping the non-MAC-Functional
   system's processes or objects to security labels based on other
   characteristics in order to preserve its MAC guarantees.

5.  Use Cases

   MAC labeling is meant to allow NFSv4.2 to be deployed in site-
   configurable security schemes.  The LFS and opaque data scheme allows
   for flexibility to meet these different implementations.  In this
   section, we provide some examples of how NFSv4.2 could be deployed to
   meet existing needs.  This is not an exhaustive listing.

5.1.  Full MAC Labeling Support for Remotely Mounted File Systems

   In this case, we assume a local networked environment where the
   servers and clients are under common administrative control.  All
   systems in this network have the same MAC implementation and
   semantically identical MAC security labels for objects (i.e., labels
   mean the same thing on different systems, even if the policies on
   each system may differ to some extent).  Clients will be able to
   apply fine-grained MAC policy to objects accessed via NFS mounts and
   thus improve the overall consistency of MAC policy application within
   this environment.

   An example of this case would be where user home directories are
   remotely mounted, and fine-grained MAC policy is implemented to
   protect, for example, private user data from being read by malicious
   web scripts running in the user's browser.  With Labeled NFS, fine-
   grained MAC labeling of the user's files will allow the MAC policy to
   be implemented and provide the desired protection.

5.2.  MAC Labeling of Virtual Machine Images Stored on the Network

   Virtualization is now a commonly implemented feature of modern
   operating systems, and there is a need to ensure that MAC security
   policy is able to protect virtualized resources.  A common
   implementation scheme involves storing virtualized guest file systems
   on a networked server; these file systems are then mounted remotely
   by guests upon instantiation.  In this case, there is a need to
   ensure that the local guest kernel is able to access fine-grained MAC
   labels on the remotely mounted file system so that its MAC security
   policy can be applied.

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5.3.  Simple Security Label Storage

   In this case, a mixed and loosely administered network is assumed,
   where nodes may be running a variety of operating systems with
   different security mechanisms and security policies.  It is desired
   that network file servers be simply capable of storing and retrieving
   MAC security labels for clients that use such labels.  The Labeled
   NFS protocol would be implemented here solely to enable transport of
   MAC security labels across the network.  It should be noted that in
   such an environment, overall security cannot be as strongly enforced
   as when the server is also enforcing and that this scheme is aimed at
   allowing MAC-capable clients to function with its MAC security policy
   enabled rather than perhaps disabling it entirely.

5.4.  Diskless Linux

   A number of popular operating system distributions depend on a
   Mandatory Access Control (MAC) model to implement a kernel-enforced
   security policy.  Typically, such models assign particular roles to
   individual processes, which limit or permit performing certain
   operations on a set of files, directories, sockets, or other objects.
   While the enforcing of the policy is typically a matter for the
   diskless NFS client itself, the file system objects in such models
   will typically carry MAC labels that are used to define policy on
   access.  These policies may, for instance, describe privilege
   transitions that cannot be replicated using standard NFS ACL-based

   For instance, on a SYSV-compatible system (see [SYSV]), if the 'init'
   process spawns a process that attempts to start the 'NetworkManager'
   executable, there may be a policy that sets up a role transition if
   the 'init' process and 'NetworkManager' file labels match a
   particular rule.  Without this role transition, the process may find
   itself having insufficient privileges to perform its primary job of
   configuring network interfaces.

   In setups of this type, a lot of the policy targets (such as sockets
   or privileged system calls) are entirely local to the client.  The
   use of RPCSEC_GSSv3 ([RPC_SEC]) for enforcing compliance at the
   server level is therefore of limited value.  The ability to
   permanently label files and have those labels read back by the client
   is, however, crucial to the ability to enforce that policy.

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5.5.  Multi-Level Security

   In an 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 superset of the compartments on the object.

   The rest of this 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 as
   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.

5.5.1.  Full Mode - MAC-Functional Client and Server

   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 enters the kernel.  Before translating the readdir() system
   call into a request to the NFSv4.2 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 NFSv4.2 call to the server.  For the remote procedure
   call (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 to see 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.

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   In this example, the policy on both the client and server is 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).

5.5.2.  MAC-Functional Client

   In these scenarios, the server is either non-MAC-Aware or MAC-Aware.
   The actions of the client will depend on whether it is configured to
   treat the MAC-Aware server in the same manner as the non-MAC-Aware
   one.  That is, does it utilize the approach presented in Section 4.3,
   or does it allow the MAC-Aware server to return labels?

   With a client that is MAC-Functional and using the example in the
   previous section, the result should be the same.  The one difference
   is that all decisions are made on the client.  MAC-Aware Server

   A process on the client labeled Secret wishes to access a directory
   labeled Top Secret on the server.  This is denied, since Secret does
   not dominate Top Secret.  Note that there will be NFSv4.2 operations
   issued that return an object label for the client to process.

   Note that in this scenario, all of the clients must be
   MAC-Functional.  A single client that does not do its access control
   checks would violate the model.  Non-MAC-Aware Server

   A process on the client labeled Secret wishes to access a directory
   that the client's policies label as Top Secret on the server.  This
   is denied, since Secret does not dominate Top Secret.  Note that
   there will not be NFSv4.2 operations issued.  If the process had a
   Top Secret process label instead of Secret, the client would issue
   NFSv4.2 operations to access the directory on the server.

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5.5.3.  MAC-Functional Server

   With a MAC-Functional server and a client that is not, the client
   behaves as if it were in a normal NFSv4.2 environment.  Since the
   process on the client does not provide a security attribute, the
   server must define a mechanism for labeling all requests from a
   client.  Assume that the server is using the same criteria used in
   the first 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

6.  Security Considerations

6.1.  Trust Needed for a Community

   Labeled NFS is a transport mechanism for labels, a storage
   requirement for labels, and a definition of how to interpret labels.
   It defines the responsibilities of the client and the server in the
   various permutations of being MAC-Functional.  It does not, however,
   dictate in any manner whether assumptions can be made about other
   entities in the relationship.  For example, it does not define
   whether a MAC-Functional client can demand that a MAC-Aware server
   only accept requests from other MAC-Functional clients.  That is a
   policy based on a MAC model, and this document does not impose
   policies on systems.

   As the requirement is a policy, it can be met with the use of a MAC
   model.  Let L be an LFS that implements the Limited Server mode,
   i.e., a MAC-Aware server connected to MAC-Functional clients.  Then
   a new LFS, L', can be created that has the additional policy that
   the MAC-Aware server MUST NOT accept any requests from a
   non-MAC-Functional client.

6.2.  Guest Mode

   When either the client or server is operating in Guest Mode, it is
   important to realize that one side is not enforcing MAC protections.
   Alternate methods are being used to handle the lack of MAC support,
   and care should be taken to identify and mitigate threats from
   possible tampering outside of these methods.

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6.3.  MAC-Functional Client Configuration

   We defined a MAC model as an access control decision made on a system
   in which normal users do not have the ability to override policies
   (see Section 1).  If the process labels are created solely on the
   client, then if a malicious user has sufficient access on that
   client, the Labeled NFS model is compromised.  Note that this is no
   different from:

   o  current implementations in which the server uses policies to
      effectively determine the object label for requests from the
      client, or

   o  local decisions made on the client by the MAC security system.

   Either the server must explicitly trust the client (as in [SENFSv3])
   or the MAC model should enforce that users cannot override policies,
   perhaps via an externally managed source.

   Once the labels leave the client, they can be protected by the
   transport mechanism as described in Section 3.2.

7.  References

7.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

7.2.  Informative References

   [DTOS]     Smalley, S., "The Distributed Trusted Operating System
              (DTOS) Home Page", December 2000, <

   [NFSv4_2]  Haynes, T., "NFS Version 4 Minor Version 2", Work in
              Progress, February 2014.

   [RFC4949]  Shirey, R., "Internet Security Glossary, Version 2", RFC
              4949, August 2007.

   [RH_MLS]   "Multi-Level Security (MLS)", "Deployment, configuration
              and administration of Red Hat Enterprise Linux 5, Edition
              10", Section 49.6, 2014, <

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   [RPC_SEC]  Adamson, W. and N. Williams, "Remote Procedure Call (RPC)
              Security Version 3", Work in Progress, February 2014.

   [SENFSv3]  Carter, J., "Implementing SELinux Support for NFS",

   [SYSV]     AT&T, "System V Interface Definition (SVID)", Third
              Edition, Addison-Wesley, Reading, MA, 1989.

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Appendix A.  Acknowledgments

   David Quigley was the early energy in motivating the entire Labeled
   NFS effort.

   James Morris, Jarrett Lu, and Stephen Smalley all were key
   contributors to both early versions of this document and to many
   conference calls.

   Kathleen Moriarty provided use cases for earlier versions of the

   Dan Walsh provided use cases for Secure Virtualization, Sandboxing,
   and NFS homedir labeling to handle process separation.

   Trond Myklebust provided use cases for secure diskless NFS clients.

   Both Nico Williams and Bryce Nordgren challenged assumptions during
   the review processes.

Author's Address

   Thomas Haynes
   495 East Java Dr.
   Sunnyvale, CA  94089

   Phone: +1 408 215 1519

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