Integrity Measurement for Network File System version 4
draft-ietf-nfsv4-integrity-measurement-07

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Network File System Version 4                                   C. Lever
Internet-Draft                                                    Oracle
Intended status: Standards Track                      September 30, 2019
Expires: April 2, 2020

        Integrity Measurement for Network File System version 4
               draft-ietf-nfsv4-integrity-measurement-07

Abstract

   This document specifies an OPTIONAL extension to NFS version 4 minor
   version 2 that enables Linux Integrity Measurement Architecture
   metadata (IMA) to be conveyed between NFS version 4.2 servers and
   clients.  Integrity measurement authenticates the creator of a file's
   content and helps guarantee the content's integrity end-to-end 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
   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 April 2, 2020.

Copyright Notice

   Copyright (c) 2019 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

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   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  The Linux Integrity Measurement Architecture  . . . . . .   3
       1.1.1.  IMA Metadata  . . . . . . . . . . . . . . . . . . . .   4
       1.1.2.  Creating and Verifying IMA Metadata . . . . . . . . .   4
       1.1.3.  Distributing and Protecting Keying Material . . . . .   5
       1.1.4.  Using IMA to Protect NFS Files  . . . . . . . . . . .   5
     1.2.  An Illustrative Use Case  . . . . . . . . . . . . . . . .   5
   2.  Requirements Language . . . . . . . . . . . . . . . . . . . .   6
   3.  Protocol Extension Considerations . . . . . . . . . . . . . .   6
     3.1.  XDR Extraction  . . . . . . . . . . . . . . . . . . . . .   7
   4.  Managing IMA Metadata on NFS Files  . . . . . . . . . . . . .   7
     4.1.  XDR Definition  . . . . . . . . . . . . . . . . . . . . .   7
       4.1.1.  NFS4ERR_INTEGRITY (Error Code YYYYY)  . . . . . . . .   8
     4.2.  Detecting support for IMA Metadata  . . . . . . . . . . .   8
       4.2.1.  Reporting Server-Side IMA Appraisal Failures  . . . .   9
     4.3.  Storing IMA Metadata  . . . . . . . . . . . . . . . . . .   9
       4.3.1.  Sending IMA Metadata When Creating a New Object . . .  10
       4.3.2.  Authorizing Updates to IMA Metadata . . . . . . . . .  10
     4.4.  Retrieving IMA Metadata . . . . . . . . . . . . . . . . .  11
     4.5.  Using NFS Attribute Fencing (VERIFY/NVERIFY)  . . . . . .  11
   5.  Deployment Examples . . . . . . . . . . . . . . . . . . . . .  12
     5.1.  Terminology . . . . . . . . . . . . . . . . . . . . . . .  12
     5.2.  Instantiating IMA Metadata  . . . . . . . . . . . . . . .  13
     5.3.  Interaction With Legacy Implementations . . . . . . . . .  14
   6.  Implementation Status . . . . . . . . . . . . . . . . . . . .  14
     6.1.  Linux NFS server and client . . . . . . . . . . . . . . .  15
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .  15
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  16
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  16
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .  16
     9.2.  Informative References  . . . . . . . . . . . . . . . . .  17
     9.3.  URIs  . . . . . . . . . . . . . . . . . . . . . . . . . .  17
   Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . .  17
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  18

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

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   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.

1.1.  The Linux Integrity Measurement Architecture

   The Linux Integrity Measurement Architecture (IMA) [SAILER] provides
   assurance that the content of a file is unaltered and authentic to
   what was originally written to that file.  The goal is to detect when
   an attacker, unintentional platform behavior, or local tinkering has
   modified the content of a file, either in transit or at rest.

   This is done by separately storing metadata about a file's content
   and then using that metadata to verify the content before it is used.
   Verification of the content is entirely independent of the file
   system.  File systems, both local and remote, act simply as storage
   for both the content and the metadata, both of which are opaque to
   the storage subsystem.

   An informative description of this mechanism is presented in the
   following subsections to provide context for understanding the NFS
   protocol extension described later in this document.  As the file
   system does not interpret IMA metadata, this description is not
   necessary to implement the extension.

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1.1.1.  IMA Metadata

   First, it is important to understand the distinction between a
   checksum, a hash, and a cryptographically-signed hash.

   o  A checksum, or parity, is designed to detect and possibly correct
      one or two bit errors in a fixed amount of content.

   o  A hash's purpose is to detect both accidental and malicious
      alterations.  Typically a hash is a small fixed size, but can be
      computed over a very large amount of content.

   o  A cryptographically-signed hash is the basis for a digital
      signature.  The signatory of a cryptographically-signed hash gives
      a guarantee that the hash, and therefore the hashed content, has
      not been changed, since the hash was signed.

   A cryptographically-signed hash stored separately from a file's
   content therefore serves as a strong check of file content integrity
   and authenticates the identity of the provider of the file's content.
   The signer is verified at time of content use via a web of trust
   commonly provided by PKI or x.509 certificates [RFC4158].

   The hash is typically computed using either the SHA-1 or SHA-256
   algorithm and is stored as an HMAC [RFC2104].  For the purposes of
   this document, the current document refers to this blob as "IMA
   metadata".

   The precise format of this metadata is determined by policies set by
   the local security administrator; the metadata and its format are
   opaque to the mechanisms that store or transport it (i.e., file
   systems).  The particulars of the PKI and the hash algorithm are set
   by local policy, which is agreed upon out-of-band and recognized by
   all participating IMA subsystems.

1.1.2.  Creating and Verifying IMA Metadata

   In a typical deployment, an authority (such as a software vendor)
   computes the hash of a file after its content has been finalized.
   The hash is then signed and attached to the file.  A web of trust
   typically links the signer to the users of the file's content (such
   as customers of the software vendor).

   Directly before file content is to be used, a security module locally
   re-computes the hash of the file content and stores it in a cache.
   This step is known as "measurement".

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   The next step is referred to as "appraisal".  The security module
   then reads the associated IMA metadata and validates its signature.
   If the signature is invalid or the locally computed hash does not
   match the stored hash, the security module applies an appraisal
   policy.  The file may be flagged in an audit log or access to the
   file may be denied.

   Underlying file and storage systems play no part in measurement or
   appraisal.  They act only as a conduit by which file content and IMA
   metadata move between at-rest storage and the security module on the
   host where that content is to be used.  Both IMA metadata and file
   content are opaque to storage subsystems.

1.1.3.  Distributing and Protecting Keying Material

   A Trusted Platform Module [1] can seal key material used to sign and
   appraise file content.  Unprotected keys are not stored in or
   distributed via file systems.  Distributing and protecting such key
   material is outside the scope of the extension specified in this
   document.

1.1.4.  Using IMA to Protect NFS Files

   The protocol extension in this document enables the storage and use
   of IMA metadata so that measurement and appraisal can occur at point-
   of-use on NFS client and server hosts.  This mechanism is similar to
   NFSv4 Security Labels (specified in [RFC7862] et al).  The purpose of
   the mechanism defined in the current document is to store security-
   related file metadata that is not interpreted by the file system
   itself.

1.2.  An Illustrative Use Case

   To help the reader grasp how IMA on NFS might be used in practice,
   this section contains a decription of an IMA use case.  The purpose
   of using IMA here is to provide a guarantee that a set of users that
   are executing a commercial software product are indeed using the same
   binary executable and libraries that were developed and tested by the
   product's vendor.

   To publish a software product, a vendor might do the following:

   1.  The vendor generates a key pair and publishes the public key.

   2.  The vendor finalizes a version of its software product.

   3.  The vendor generates a hash of each file in the product's
       distribution manifest, and signs each hash with its private key.

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   4.  The vendor publishes the product's files and the signed hashes.

   To install and use the vendor's product, a customer might do the
   following:

   1.  The customer installs the files and the signed hashes in a local
       filesystem.

   2.  When a user executes one of the files, a local security module
       reads the file from disk and computes a hash of its content.
       This is the measurement step, which happens when each file is
       loaded into the system's page cache.

   3.  The security module uses the vendor's public key to verify the
       signature of the file's stored hash, and confirms that the
       locally computed hash matches the stored hash.  This is the
       appraisal step, which happens when each file is about to be
       executed.

   4.  If the locally computed hash is verified, the security module
       allows the operating system to execute the program.  If not, then
       the program fails to execute and an integrity error is logged.

   The purpose of the NFS extension specified in the current document is
   to enable the signed hashes in the above example to be stored by an
   NFS server and retrieved by NFS clients.  Each NFS client could then
   verify that neither the NFS server nor an active network agent had
   altered file content before it was used on the NFS client.

2.  Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "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.

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

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   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 "ima-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'
        < ima-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.

4.  Managing IMA Metadata 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 IMA metadata associated with
   a particular file.

   To enable a single IMA metadata 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 IMA
   metadata MUST NOT exceed 4096 bytes in length.

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   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) 2019 IETF Trust and the person identified
      ///  * as author of the code.  All rights reserved.
      ///  *
      ///  * The author of the code is: C. Lever
      ///  */
      ///
      /// %/*
      /// % * New For Integrity Measurement support
      /// % */
      /// opaque                           ima_data4<4096>;
      ///
      /// const FATTR4_IMA = XXX;          /* to be assigned */
      ///
      /// %/*
      /// % *New value added to enum nfsstat4
      /// % */
      /// const NFS4ERR_INTEGRITY = YYYYY; /* to be assigned */

   <CODE ENDS>

4.1.1.  NFS4ERR_INTEGRITY (Error Code YYYYY)

   The server rejected this request because a data or metadata integrity
   check failed during its execution.

4.2.  Detecting support for IMA Metadata

   An NFS version 4.2 client discovers support for IMA metadata on an
   NFS version 4.2 server by sending an NFS GETATTR operation that
   specifies the FATTR4_SUPPORTED_ATTRS attribute and the FATTR4_IMA
   attribute.  When a server supports IMA metadata, it sets the
   FATTR4_IMA attribute bit in the NFS GETATTR bitmask returned in the
   reply.  Otherwise that bit is clear.

   An NFS version 4.2 server MUST NOT return NFS4ERR_INTEGRITY to a
   client unless that client has queried the server for IMA metadata
   support using the above mechanism.  The server identifies clients
   using their client_id4 for this purpose.

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4.2.1.  Reporting Server-Side IMA Appraisal Failures

   An NFS server that has rigorous integrity checking must somehow
   report integrity-related failures to clients.  Until now, a server
   implementer chose amongst status codes that were available in the
   base NFS version 4.2 protocol, typically NFS4ERR_IO or
   NFS4ERR_ACCESS, even though these code points have generic meanings
   that do not necessarily imply an integrity-related failure.

   Once the above FATTR4_SUPPORTED_ATTRS handshake is done, the server
   has determined that a client can properly recognize the
   NFS4ERR_INTEGRITY status code.  In instances where an NFS request
   fails due to an integrity-related issue, and the server has
   determined that the client recognizes the NFS4ERR_INTEGRITY status
   code, the server MAY return NFS4ERR_INTEGRITY for the following
   operations: ACCESS, COMMIT, CREATE, GETATTR, GETDEVICELIST, LINK,
   LOOKUP, LOOKUPP, NVERIFY, OPEN, OPENATTR, READ, READDIR, READLINK,
   REMOVE, RENAME, SETATTR, VERIFY, WRITE.  The server MUST NOT return
   NFS4ERR_INTEGRITY for any other operation.

   The NFS4ERR_INTEGRITY status code is useful to inform the client (or
   the end user, depending on the client implementation) that access to
   the file's content was not blocked because of a permissions setting
   but rather because an integrity check failed.  This distinction can
   guide the user or client towards a recovery action that is
   appropriate.

4.3.  Storing IMA Metadata

   An NFS version 4.2 client stores IMA metadata by sending an NFS
   SETATTR operation that specifies the FATTR4_IMA attribute and targets
   the file system object associated with the metadata to be stored.
   This attribute completely replaces any previous FATTR4_IMA attribute
   associated with that object.  Modifying an object in any other way
   MUST NOT alter or remove FATTR4_IMA attributes.

   To remove IMA metadata from an object, the client sends a FATTR4_IMA
   attribute whose length is zero.

   When an NFS SETATTR is presented to an NFS version 4.2 server with a
   credential that is not authorized to replace a FATTR4_IMA attribute,
   the server MUST respond with NFS4ERR_ACCESS.

   When an NFS SETATTR is presented to an NFS version 4.2 server with an
   ima_data4 field whose length is larger than 4096 bytes, the server
   MUST respond with NFS4ERR_INVAL.

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   When an NFS SETATTR is presented to an NFS version 4.2 server and the
   target object resides in a file system which supports FATTR4_IMA but
   the object itself does not support the FATTR4_IMA attribute, the
   server MUST respond with NFS4ERR_WRONGTYPE.  For example, if the
   server's file system supports associating IMA metadata with regular
   files but not with sockets or FIFOs, then the result of an attempt to
   associate IMA metadata with a FIFO will be NFS4ERR_WRONGTYPE.

   When an NFS SETATTR is presented to an NFS version 4.2 server but the
   target object resides in a file system which does not support the
   FATTR4_IMA attribute, the server MUST respond with
   NFS4ERR_ATTRNOTSUPP.

   When a client presents an NFS SETATTR that modifies FATTR4_IMA along
   with other attributes and the server responds with an error, the
   client can retry setting each attribute separately to sort out which
   attribute is causing the server to reject the NFS SETATTR operation.

   A detailed description of the NFS SETATTR operation can be found in
   Section 18.30 of [RFC5661].

4.3.1.  Sending IMA Metadata When Creating a New Object

   An alternate way to set an attribute is to provide the attribute
   during an NFS OPEN(CREATE) operation.  Upon creation, an object has
   no content to protect.  If a client presents an FATTR4_IMA attribute
   to an NFS version 4.2 server during NFS OPEN(CREATE), the server MUST
   respond with NFS4ERR_INVAL.

4.3.2.  Authorizing Updates to IMA Metadata

   An NFS server permits a user to replace a file's IMA metadata
   whenever that user is permitted to modify that file's byte content.
   This is consistent with similar mechanisms already used throughout
   the NFS version 4 protocol; for instance, setting an ACL.  If an NFS
   server determines that a user requesting a SETATTR with the
   FATTR4_IMA attribute is not authorized to update the IMA metadata,
   the SETATTR operation MUST return NFS4ERR_ACCESS.

   If an NFS server implementation does not support modification of IMA
   metadata via NFS, the server MUST return NFS4ERR_INVAL to a SETATTR
   request with the FATTR4_IMA attribute, as required by Section 5.5 of
   [RFC5661].

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4.4.  Retrieving IMA Metadata

   An NFS version 4.2 client retrieves IMA metadata by retrieving the
   FATTR4_IMA attribute via an NFS GETATTR operation, specifying the
   file handle of the object associated with the metadata to be
   retrieved.

   The IMA subsystem typically manages its own cache of this metadata to
   maintain reasonable performance.  The NFS client implementation MUST
   always pass retrieval requests for this metadata to the server.  This
   metadata MUST NOT be cached by the NFS client.

   When an NFS GETATTR is presented to an NFS version 4.2 server and the
   target object resides in a file system which supports the FATTR4_IMA
   attribute but the object does not support the FATTR4_IMA attribute,
   the server MUST respond with NFS4ERR_WRONGTYPE.  For example, if the
   server's file system supports associating IMA metadata with regular
   files but not named attributes, then the result of an attempt to
   retrieve IMA metadata on a named attribute will be NFS4ERR_WRONGTYPE.

   When an NFS GETATTR is presented to an NFS version 4.2 server but the
   target object resides in a file system which does not support
   FATTR4_IMA, this does not result in an error and the FATTR4_IMA
   attribute bit is cleared in the server's response.

   Otherwise, if the target object supports FATTR4_IMA and there is no
   IMA metadata is available for the target object, the server returns a
   FATTR4_IMA attribute whose length is zero.

   When a client presents an NFS GETATTR that retrieves FATTR4_IMA along
   with other attributes and the server responds with an error, the
   client can retry by retrieving each attribute separately to sort out
   which attribute is causing the server to reject the NFS GETATTR
   operation.

   A detailed description of the NFS GETATTR operation can be found in
   Section 18.7 of [RFC5661].

4.5.  Using NFS Attribute Fencing (VERIFY/NVERIFY)

   The NFS VERIFY and NVERIFY operations, described in Sections 18.31
   and 18.15 of [RFC5661] respectively, permit a client to add a fence
   in an NFS COMPOUND where, if a provided FATTR4 attribute does or does
   not match, the server can force processing of that COMPOUND to stop.
   The FATTR4_IMA attribute is a valid choice for these operations.

   The server MUST use a simple byte comparison to evaluate whether the
   client-provided FATTR4_IMA matches the FATTR4_IMA attribute

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   associated with the target object.  If the server has a local IMA
   implementation, it MAY prevent the use of the local FATTR4_IMA
   attribute value for the purpose of this comparison (via EVM
   protection).  If the client has indicated support for IMA metadata,
   the server MUST respond with NFS4ERR_INTEGRITY.  Otherwise it MUST
   respond with NFS4ERR_ACCESS.

5.  Deployment Examples

5.1.  Terminology

   Because the protocol extension described in this document is
   OPTIONAL, clients and servers that support it will necessarily
   interact with clients and servers that do not support it.  To aid the
   discussion in this section, we define the following terms:

   Appraiser:  A security module separate from the storage system that
      appraises file content based on a policy and IMA measurement
      results.

   Participating Client:  An NFS version 4.2 client that employs an
      appraiser, supports the OPTIONAL extension described in this
      document, and indicates this support to NFS servers using the
      handshake described in Section 4.2.

   Legacy Client:  Any NFS client that does not support the OPTIONAL
      extension described in this document.

   Participating Server:  An NFS version 4.2 server that supports the
      OPTIONAL extension described in this document, indicates this
      support to clients using the handshake described in Section 4.2,
      and its shared file systems can store IMA metadata.  A
      participating server is not required to implement an appraiser.

   Legacy Server:  Any NFS server that does not support the OPTIONAL
      extension described in this document.

   In addition, there are intermediate modes of operation on
   participating peers:

   Full-function Client:  A participating client that can modify IMA
      metadata via NFS.

   Fetch-only Client:  A participating client that does not support
      modifying IMA metadata on a participating server.

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   Full-function Server:  A participating server that has a local user
      execution environment and supports updating IMA metadata that
      resides on shared local file systems.

   Store-only Server:  A participating server where there is only remote
      access to file content and IMA metadata.

   Lastly, we provide the following possible simple appraisal policies
   that might be applied by an appraiser:

   Strict:  Access is prevented to a file's content if the file has no
      IMA metadata or if the extant IMA metadata 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 IMA metadata or when extant IMA
      metadata fails to verify the file's content.

   Disabled:  IMA metadata is ignored and access to file content is
      never prevented.

5.2.  Instantiating IMA Metadata

   Once a file is written and closed, a specialized tool generates and
   signs the IMA metadata and then writes it to the file system.  The
   tool can be used on a full-function client to sign files on a
   participating server.  Or, the tool can be used on a full-function
   server to sign local files.  The IMA metadata is then visible to
   participating clients and local users on the server (if there are
   any).  Or, an enhanced version of cpio or rsync might copy the
   metadata into place as part of an installation procedure.

   Typically, once IMA metadata is associated with a file, the file's
   content is essentially immutable, even if the file's permissions
   settings permit writing to it.  This is because changing the content
   without updating the associated IMA metadata will make the file's
   content inaccessible, depending on the appraisal policy in effect.

   Updating file content requires access to a signing key in order to
   generate fresh IMA metadata to prevent subsequent IMA appraisal
   failures.  Typically a key like this will be well-protected, and thus
   not available on NFS clients.

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5.3.  Interaction With Legacy Implementations

   Given the example policies and definitions we provided earlier, the
   following statements are true:

   o  A participating client that uses the Disabled policy is equivalent
      to a legacy client, except that a participating server is allowed
      to respond with NFS4ERR_INTEGRITY to a participating client.

   o  A legacy client never prevents access to file content on a
      participating server, but a participating server that has a local
      appraiser may prevent access of a corrupted file to a legacy
      client.

   o  A participating client using the Strict policy never allows access
      to files stored on a legacy server.

   An appraiser on a participating NFS version 4.2 peer needs to be
   prepared to deal gracefully with IMA metadata it does not recognize
   or cannot parse.  Its policy may treat this case as an appraisal
   failure.

   It is not required for an NFS version 4.2 server to implement an
   appraiser.  However, some servers, such as the Linux NFS server, do
   just that, applying local IMA policy to both local and remote file
   accesses.

   If an appraisal failure occurs during a remote access, a
   participating server responds to a legacy client with NFS4ERR_ACCESS.
   The server's local policy decides exactly what a participating client
   sees: Possibilities include an NFS4ERR_INTEGRITY response (and access
   to the file is denied), or access to the file content and IMA
   metadata may be permitted so that the client's own IMA policies can
   be applied.

6.  Implementation Status

   This section records the status of known implementations of the
   protocol defined by this specification at the time of posting of this
   Internet-Draft, and is based on a proposal described in [RFC7942].
   The description of implementations in this section is intended to
   assist the IETF in its decision processes in progressing drafts to
   RFCs.

   Please note that the listing of any individual implementation here
   does not imply endorsement by the IETF.  Furthermore, no effort has
   been spent to verify the information presented here that was supplied
   by IETF contributors.  This is not intended as, and must not be

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   construed to be, a catalog of available implementations or their
   features.  Readers are advised to note that other implementations may
   exist.

6.1.  Linux NFS server and client

   Organization:  The Linux Foundation

   URL:       https://www.kernel.org

   Maturity:  Prototype software based on early versions of this
              document.

   Coverage:  The bulk of this specification is implemented.

   Licensing: GPLv2

   Implementation experience:  No comments from implementors.

7.  Security Considerations

   The design of the NFS extension described in this document assumes
   that all IMA metadata is cryptographically signed to prevent unwanted
   alteration at rest or in transit.

   When IMA metadata for a file exists and the end host has an active
   appraiser, 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 the file's IMA
   metadata.  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 IMA metadata that is cryptographically
   signed, simply by verifying its signature.  Additional protection of
   signed metadata 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 IMA
   metadata at rest, an NFS version 4.2 server can enforce a suitable
   level of privilege before authorizing a local or remote agent to
   alter this information.  See Section 4.3.2 for more detail.

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8.  IANA Considerations

   This document has no IANA actions.

9.  References

9.1.  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>.

   [RFC7942]  Sheffer, Y. and A. Farrel, "Improving Awareness of Running
              Code: The Implementation Status Section", BCP 205,
              RFC 7942, DOI 10.17487/RFC7942, July 2016,
              <https://www.rfc-editor.org/info/rfc7942>.

   [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>.

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9.2.  Informative References

   [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>.

   [RFC4158]  Cooper, M., Dzambasow, Y., Hesse, P., Joseph, S., and R.
              Nicholas, "Internet X.509 Public Key Infrastructure:
              Certification Path Building", RFC 4158,
              DOI 10.17487/RFC4158, September 2005,
              <https://www.rfc-editor.org/info/rfc4158>.

   [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>.

   [SAILER]   Sailer, R., Zhang, X., Jaeger, T., and L. van Doorn,
              "Design and Implementation of a TCG-based Integrity
              Measurement Architecture", Proceedings of the 13th USENIX
              Security Symposium, August 2004.

9.3.  URIs

   [1] 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.

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   Special thanks go to Transport Area Director Magnus Westerlund, NFSV4
   Working Group Chairs Spencer Shepler and Brian Pawlowski, and NFSV4
   Working Group Secretary Thomas Haynes for their support.

Author's Address

   Charles Lever
   Oracle Corporation
   United States of America

   Email: chuck.lever@oracle.com

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