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Versions: (draft-bfields-nfsv4-umask) 00 01 02 03 04

NFSv4                                                          J. Fields
Internet-Draft                                            A. Gruenbacher
Intended status: Standards Track                                 Red Hat
Expires: January 21, 2018                                  July 20, 2017


Allowing Inheritable NFSv4 Access Control Entries to Override the Umask
                       draft-ietf-nfsv4-umask-04

Abstract

   In many environments, inheritable NFSv4 Access Control Entries (ACEs)
   can be rendered ineffective by the application of the per-process
   umask.  This can be addressed by transmitting the umask and create
   mode as separate pieces of data, allowing the server to make more
   intelligent decisions about the permissions to set on new files.
   This document proposes a protocol extension which accomplishes that.

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
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   This Internet-Draft will expire on January 21, 2018.

Copyright Notice

   Copyright (c) 2017 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
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   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.  Conventions Used in This Document . . . . . . . . . . . . . .   2
   2.  Problem Statement . . . . . . . . . . . . . . . . . . . . . .   2
   3.  Protocol Extension Considerations . . . . . . . . . . . . . .   3
   4.  mode_umask Attribute  . . . . . . . . . . . . . . . . . . . .   3
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .   4
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   5
   7.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   5
     7.1.  Normative References  . . . . . . . . . . . . . . . . . .   5
     7.2.  Informative References  . . . . . . . . . . . . . . . . .   5
   Appendix A.  Acknowledgments  . . . . . . . . . . . . . . . . . .   5
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   5

1.  Conventions Used in This Document

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in [RFC2119].

2.  Problem Statement

   On Unix-like systems, each process is associated with a file mode
   creation mask (umask), which specifies which permissions must be
   turned off when creating new file system objects.

   When applying the mode, section 6.4.1.1 of [RFC7530] recommends that
   servers SHOULD restrict permissions granted to any user or group
   named in the Access Control List (ACL) to be no more than the
   permissions granted by the MODE4_RGRP, MODE4_WGRP, and MODE4_XGRP
   bits.  Servers aiming to provide clients with Unix-like chmod
   behavior may also be motivated by the same requirements in [SUSv4].
   (See the discussion of additional and alternate access control
   mechanisms in section "4.4 File Permissions" of that document.)

   On many existing installations, all ordinary users by default use the
   same effective group ID.  To prevent granting all users full access
   to each other's files, such installations usually default to a umask
   with very restrictive permissions.  As a result, inherited ACL
   entries (inheritable ACEs) describing the permissions to be granted
   to named users and groups are often ignored.  This makes inheritable
   ACEs useless in some common cases.






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   Linux solves this problem on local filesystems by ignoring the umask
   in the case the parent of the newly-created file has inheritable
   ACEs; see [LinuxACL].

   The same solution should work for NFS.  However, the NFSv4 protocol
   does not currently give the client a way to transmit the umask of the
   process opening a file.  And clients have no way of atomically
   checking for inheritable permissions and applying the umask only when
   necessary.  As a result, the server receives an OPEN with a mode
   attribute that already has the umask applied.

   This document solves the problem by defining a new attribute which
   allows the client to transmit umask and the mode specified at file
   creation separately, allowing the client to ignore the umask in the
   presence of inheritable ACEs.  At least in the Linux case, this
   allows NFSv4 to provide the same semantics available using local
   access.

3.  Protocol Extension Considerations

   This document presents an extension to minor version 2 of the NFSv4
   protocol as described in [nfsv4-versioning].  It describes a new
   OPTIONAL feature.  NFSv4.2 servers and clients implemented without
   knowledge of this extension will continue to interoperate with
   clients and servers that are aware of the extension (whether they
   support it or not).

   Note that [RFC7862] does not define NFSv4.2 as non-extensible, so
   that it is considered by [nfsv4-versioning] to be an extensible minor
   version.  As a result, upon publication of this document as a
   Proposed Standard, the extension described herein will effectively be
   part of NFSv4.2, even though this document does not update [RFC7862]
   or [RFC7863].

4.  mode_umask Attribute

         struct mode_umask4 {
           mode4  mu_mode;
           mode4  mu_umask;
         };

           +------------+----+-------------+-----+------------+
           | Name       | Id | Data Type   | Acc | Defined in |
           +------------+----+-------------+-----+------------+
           | mode_umask | 81 | mode_umask4 | W   | Section 4  |
           +------------+----+-------------+-----+------------+

                                  Table 1



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   The NFSv4.2 mode_umask attribute is based on the umask and on the
   mode bits specified at open time, which together determine the mode
   of a newly created UNIX file.  Only the nine low-order mode4 bits of
   mu_umask are defined.  A server MUST return NFS4ERR_INVAL if bits
   other than those nine are set.

   The mode_umask attribute is only meaningful for operations that
   create objects (CREATE and OPEN); in other operations that take
   fattr4 arguments, the server MUST reject it with NFS4ERR_INVAL.

   The server MUST return NFS4ERR_INVAL if the client attempts to set
   both mode and mode_umask in the same operation.

   When the server supports the mode_umask attribute, a client creating
   a file should use mode_umask in place of mode, with mu_mode set to
   the unmodified mode provided by the user, and mu_umask set to the
   umask of the requesting process.

   The server then uses mode_umask as follows:

   o  On a server that supports ACL attributes, if an object inherits
      any ACEs from its parent directory, mu_mode SHOULD be used, and
      mu_umask ignored.

   o  Otherwise, mu_umask MUST be used to limit the mode: all bits in
      the mode MUST be turned off which are set in the umask; the mode
      assigned to the new object becomes (mu_mode & ~mu_umask) instead.

5.  Security Considerations

   The mode_umask attribute shifts to the server the decision about when
   to apply the umask.  Because the server MUST apply the umask if there
   are no inheritable permissions, the traditional semantics are
   preserved in the absence of a permission inheritance mechanism.  The
   only relaxation of permissions comes in the case servers follow the
   recommendation that they ignore the umask in the presence of
   inheritable permissions.

   The practice of ignoring the umask when there are inheritable
   permissions in the form of a "POSIX" default ACL is of long standing
   and has not given rise to security issues.  The "POSIX" default ACL
   mechanism and the mechanism for permission inheritance in NFSv4 are
   equivalent from a security perspective.








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6.  IANA Considerations

   This document does not require any actions by IANA.

7.  References

7.1.  Normative References

   [nfsv4-versioning]
              Noveck, D., "Rules for NFSv4 Extensions and Minor
              Versions", draft-ietf-nfsv4-versioning-08 (work in
              progress), December 2016.

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

   [RFC7530]  Haynes, T. and D. Noveck, "Network File System (NFS)
              version 4 Protocol", RFC 7530, March 2015.

   [RFC7862]  Haynes, T., "Network File System (NFS) Version 4 Minor
              Version 2 Protocol", RFC 7862, November 2016.

   [RFC7863]  Haynes, T., "Network File System (NFS) Version 4 Minor
              Version 2 External Data Representation Standard (XDR)
              Description", RFC 7863, November 2016.

7.2.  Informative References

   [LinuxACL]
              Gruenbacher, A., "ACL(5) - Access Control Lists", Linux
              man pages ACL(5), March 2002, <http://kernel.org/doc/man-
              pages/online/pages/man5/acl.5.html>.

   [SUSv4]    The Open Group, "Single UNIX Specification Version 4",
              2013.

Appendix A.  Acknowledgments

   Thanks to Dave Noveck and Trond Myklebust for review.

Authors' Addresses

   J. Bruce Fields
   Red Hat, Inc.

   Email: bfields@redhat.com





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   Andreas Gruenbacher
   Red Hat, Inc.

   Email: agruenba@redhat.com















































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