[Docs] [txt|pdf|xml|html] [Tracker] [Email] [Nits]



Network File System Version 4                                   C. Lever
Internet-Draft                                                    Oracle
Intended status: Standards Track                           April 9, 2018
Expires: October 11, 2018


      Linux-related Extensions to NFS version 4.2 Security Labels
              draft-cel-nfsv4-linux-seclabel-xtensions-00

Abstract

   NFS version 4.2 introduces an optional feature known as NFSv4
   Security Labels.  This document extends NFSv4 Security Labels to
   support Linux file capabilities and the Linux Integrity Measurement
   Architecture.

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 October 11, 2018.

Copyright Notice

   Copyright (c) 2018 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (https://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.




Lever                   Expires October 11, 2018                [Page 1]


Internet-Draft          Linux Seclabel Extensions             April 2018


Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Linux Integrity Measurement Architecture  . . . . . . . .   2
     1.2.  Linux File Capabilities . . . . . . . . . . . . . . . . .   3
     1.3.  NFSv4 Security Labels . . . . . . . . . . . . . . . . . .   4
   2.  Requirements Language . . . . . . . . . . . . . . . . . . . .   4
   3.  Handling File Capabilities on NFS Mounts  . . . . . . . . . .   4
   4.  Handling IMA Metadata on NFS Mounts . . . . . . . . . . . . .   5
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .   6
     5.1.  Protection of File Capability Metadata  . . . . . . . . .   6
     5.2.  Protection of IMA Metadata  . . . . . . . . . . . . . . .   7
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   7
   7.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   7
     7.1.  Normative References  . . . . . . . . . . . . . . . . . .   7
     7.2.  Informative References  . . . . . . . . . . . . . . . . .   8
   Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . .   8
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .   8

1.  Introduction

   In the current document, we define a way to manage both IMA metadata
   and Linux file capabilities on NFS files using existing NFSv4
   Security Label protocol elements.  Before specifying new protocol,
   let's contextualize and define the terminology used in the body of
   this document.

1.1.  Linux Integrity Measurement Architecture

   The Linux Integrity Measurement Architecture (hereafter, IMA)
   provides assurance that the content of a file is unaltered and
   authentic to what was originally written to that file.  In addition,
   an Extended Verification Module (hereafter, EVM) can assure that a
   file's attribute information remains unaltered.

   The goal is to detect when a remote attacker, a local attacker, or
   unintentional software behavior has modified the content or
   attributes of a file.  This is done by cryptographically signing HMAC
   hashes of a file's content and attribute metadata, and then storing
   the signed hashes separately.  These hashes are typically updated
   whenever the file is legitimately modified.

   Integrity verification is not performed by applications or by file
   systems.  Applications are not exposed to the operation of integrity
   verification.  File systems are responsible only for persistent
   storage of file content and signed hashes.  When a file is first
   read, content and hashes are passed to IMA modules for measurement




Lever                   Expires October 11, 2018                [Page 2]


Internet-Draft          Linux Seclabel Extensions             April 2018


   and appraisal.  Application access is denied if the hashes cannot be
   verified.

   Some files may be immutable, in which case their integrity metadata
   is signed by an RSA public key signature [RFC8017].  These files can
   only be accessed in read-only mode, or deleted by a privileged
   process.

   Key material used to sign and verify file content and attribute
   metadata must be protected.  A Trusted Platform Module [TPM-SUM] can
   be used to seal the key material.  This use case is typical for
   providing a read-only operating system image that is
   cryptographically verified; for example, in a cloud environment or on
   mobile devices.

   The goals and use cases of the Linux Integrity Measurement
   Architecture (IMA) are presented in further detail in [IMA-WP].

1.2.  Linux File Capabilities

   The Linux kernel divides privileges traditionally associated with the
   superuser into distinct subprivileges, referred to as "capabilities".
   These capabilies include but are not limited to

   o  The ability to override file read and write permission checks

   o  The ability to signal processes without permission checks

   o  The ability to bind to a privileged socket port or perform other
      privileged network administration tasks

   o  The ability to perform a range of system administrative tasks such
      as rebooting or setting the system clock

   Being broken out from the monolithic superuser privilege, individual
   capabilities can be enabled and disabled independently of one
   another.  Individual capabilities are associated with a process and
   are inherited during execve(2), similar to the traditional superuser
   privilege.

   File capabilities enable capabilities to be associated with a file
   containing an executable object, like a setuid permission bit.  File
   capability sets are combined with the capability sets of a parent
   process to determine the capabilities of a child process after an
   execve(2).

   This enables an otherwise unaware application to be given a
   particular and specific privilege -- say, the privilege to bind to a



Lever                   Expires October 11, 2018                [Page 3]


Internet-Draft          Linux Seclabel Extensions             April 2018


   privileged port -- without granting it the privilege to reboot the
   system or kill processes that do not share its owner UID, thus
   improving overall system security.

   The Linux capability implementation is based on the withdrawn
   POSIX.1e draft standard (see [POSIX1e]).  An overview of the facility
   can be found in the Linux capabilities(7) man page (citation needed).

1.3.  NFSv4 Security Labels

   Section 9 of the NFS version 4.2 specification [RFC7862] defines an
   optional feature, known as NFSv4 Security Labels, that permits per-
   file extended security labels to be conveyed between NFSv4 clients
   and servers.

   The intention of these labels is to enable the conveyance of MAC
   security labels, but in special cases, non-MAC security information
   may be handled.  There is no prohibitory language in [RFC7204],
   [RFC7569], or [RFC7862] which excludes non-MAC security metadata from
   being conveyed via an NFSv4 Security Label, for instance.

2.  Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in BCP 14 [RFC2119]
   [RFC8174] when, and only when, they appear in all capitals, as shown
   here.

3.  Handling File Capabilities on NFS Mounts

   Linux file capabilities are typically manipulated in a native binary
   format.  However, a capability set can be converted to a human-
   readable text format.  See the Linux cap_from_text(3) man page
   (citation needed).

   An NFSv4 client stores the capability set associated with an NFS file
   by first converting the capability set to a text format and then
   conveying it to an NFSv4 server via a SETATTR operation, using the
   LFS number assigned for this purpose (see Section 6).  This
   capability set completely replaces the previous capability set for
   that file.

   Likewise, an NFSv4 client retrieves the capability set associated
   with a file by first retrieving the text form of the capability set,
   as stored in an NFSv4 Security Label, via a GETATTR operation, using
   the LFS number assigned for this purpose, and then converting the




Lever                   Expires October 11, 2018                [Page 4]


Internet-Draft          Linux Seclabel Extensions             April 2018


   retrieved text form into the client's native capability set
   representation.

   There is no MAC policy associated with the file capabilities LFS
   number.  The contents of the lfs_pi field MUST be ignored by clients
   and servers when this LFS number is used.  See Section 12.2.4 of
   [RFC7862] for further details about how clients and servers form
   these GETATTR and SETATTR requests.

   In order to enable file capabilities to be retrieved or updated in a
   single RPC, the text format representation of a capability set MUST
   NOT exceed 8192 bytes in length.

   This document does not specify how an NFSv4 server handles file
   capability metadata.  Server implementation choices include:

   o  If there are accessors of shared files that are local to an NFSv4
      server, the server may choose to store file capabilities in a
      native format and convert them to a text form when an NFSv4 client
      retrieves them.

   o  Alternately, an NFSv4 server may itself not support file
      capabilities at all, in which case the server can store capability
      sets in text form without conversion to a native format.

4.  Handling IMA Metadata on NFS Mounts

   On Linux, there are two parts to a file's IMA metadata:

   o  A cryptographically signed HMAC hash of the file's byte stream,
      stored in the file's security.ima extended attribute

   o  A cryptographically signed HMAC hash of the file's attributes,
      stored in the file's security.evm extended attribute

   An NFSv4 client stores the signed hash of an NFS file's byte stream
   or attributes by conveying the hash to an NFSv4 server via a SETATTR
   operation, using the LFS number appropriate for the hash (see
   Section 6).  This signed hash completely replaces the previous hash.
   This document does not specify a policy for authorizing changes to
   IMA or EVM hashes.

   Likewise, an NFSv4 client retrieves the signed hash of an NFS file's
   byte stream or attributes by retrieving the appropriate NFSv4
   Security Label via a GETATTR operation using the LFS number assigned
   for this purpose.  An NFSv4 server MUST NOT prevent an NFSv4 client
   from accessing a file based on IMA verification failures on the
   server.



Lever                   Expires October 11, 2018                [Page 5]


Internet-Draft          Linux Seclabel Extensions             April 2018


   There is no MAC policy associated with the IMA or EVM LFS numbers.
   The contents of the lfs_pi field MUST be ignored by clients and
   servers when these LFS numbers are used.  See Section 12.2.4 of
   [RFC7862] for further details about how clients and servers form
   these GETATTR and SETATTR requests.

   In order to enable IMA metadata to be retrieved or updated in a
   single RPC, a signed hash MUST NOT exceed 4096 bytes in length.

   This document does not specify how an NFSv4 server handles IMA
   metadata.  If there are local accessors of shared files on an NFSv4
   server, the server may choose to store IMA metadata in a native
   format that can be handled by the server's local integrity modules.

   A note about performance: IMA measurement and appraisal is always
   performed on the entirety of a file's byte stream.  In other words, a
   file's entire byte stream must be read over to an NFSv4 client in
   order for its IMA module to verify its integrity.  It is recognized
   that this requirement can have a significant performance impact for
   large files.  An NFSv4 client may employ other mechanisms, not
   specified here, to reduce this performance impact.  For example,
   instead of signing a hash of the file's byte stream, a Merkle tree
   can be constructed that allows clients to verify the integrity of
   smaller portions of a large file, and that tree can be signed instead
   of signing the file content.

5.  Security Considerations

   An NFSv4 server is required to enforce a suitable level of privilege
   before allowing a local or remote agent to alter NFSv4 Security
   Labels.  Consult Section 9.6 of [RFC7862] for further details.

   This document does not specify a policy for authorizing changes to
   IMA or EVM metadata or file capabilities.

5.1.  Protection of File Capability Metadata

   File capabilities are conveyed as text strings.  To prevent a man-in-
   the-middle from escalating the capability set of a file in transit,
   these strings must be protected in transit using a GSS service that
   provides integrity verification [RFC7861].  A server response that
   indicates that a file has no associated file capabilities must be
   similarly cryptographically protected.  While at rest on durable
   storage or in a cache, capability metadata requires the same
   protections as other file attribute metadata.






Lever                   Expires October 11, 2018                [Page 6]


Internet-Draft          Linux Seclabel Extensions             April 2018


5.2.  Protection of IMA Metadata

   IMA metadata is cryptographically signed.  Receivers can detect
   unintentional and malicious alteration of this metadata simply by
   verifying the signature.  Therefore additional protection using GSS
   [RFC7861] or other security mechanisms is not mandatory.

6.  IANA Considerations

   This section provides guidance to the Internet Assigned Numbers
   Authority (IANA) regarding the addition of new entries in the
   "Security Label Format Selection Registry" in accordance with
   Section 5.2 of [RFC7569].


   +---------------+---------------------+--------+--------------------+
   | Label Format  | Description         | Status | Reference          |
   | Specifier     |                     |        |                    |
   +---------------+---------------------+--------+--------------------+
   | 1             | LINUX-IMA           | active | [RFC-TBD]          |
   | 2             | LINUX-EVM           | active | [RFC-TBD]          |
   | 3             | LINUX-FCAP          | active | [RFC-TBD]          |
   +---------------+---------------------+--------+--------------------+

        New entries in the Security Label Format Selection Registry

                                  Table 1


7.  References

7.1.  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>.

   [RFC7569]  Quigley, D., Lu, J., and T. Haynes, "Registry
              Specification for Mandatory Access Control (MAC) Security
              Label Formats", RFC 7569, DOI 10.17487/RFC7569, July 2015,
              <https://www.rfc-editor.org/info/rfc7569>.

   [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>.





Lever                   Expires October 11, 2018                [Page 7]


Internet-Draft          Linux Seclabel Extensions             April 2018


   [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>.

7.2.  Informative References

   [IMA-WP]   Safford, D., "An Overview of The Linux Integrity
              Subsystem", <http://downloads.sf.net/project/linux-ima/
              linux-ima/Integrity_overview.pdf>.

   [POSIX1e]  POSIX, "POSIX.1e Specification",
              <http://wt.tuxomania.net/publications/posix.1e/>.

   [RFC7204]  Haynes, T., "Requirements for Labeled NFS", RFC 7204,
              DOI 10.17487/RFC7204, April 2014,
              <https://www.rfc-editor.org/info/rfc7204>.

   [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>.

   [RFC8017]  Moriarty, K., Ed., Kaliski, B., Jonsson, J., and A. Rusch,
              "PKCS #1: RSA Cryptography Specifications Version 2.2",
              RFC 8017, DOI 10.17487/RFC8017, November 2016,
              <https://www.rfc-editor.org/info/rfc8017>.

   [TPM-SUM]  Trusted Computing Group, "Trusted Platform Module (TPM)
              Summary", April 2008, <https://trustedcomputinggroup.org/
              wp-content/uploads/
              Trusted-Platform-Module-Summary_04292008.pdf>.

Acknowledgments

   The author thanks Trond Myklebust for suggesting these extentions to
   NFSv4 Security Labels.  Special thanks go to Transport Area Director
   Spencer Dawkins, NFSV4 Working Group Chair Spencer Shepler, and NFSV4
   Working Group Secretary Thomas Haynes for their support.

Author's Address

   Charles Lever
   Oracle Corporation
   1015 Granger Avenue
   Ann Arbor, MI  48104
   United States of America

   Phone: +1 248 816 6463
   Email: chuck.lever@oracle.com



Lever                   Expires October 11, 2018                [Page 8]


Html markup produced by rfcmarkup 1.126, available from https://tools.ietf.org/tools/rfcmarkup/