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Versions: 00 01                                                         
NFSv4                                                          J. Fields
Internet-Draft                                            A. Gruenbacher
Intended status: Informational                                   Red Hat
Expires: August 06, 2016                               February 03, 2016

         Allowing inheritable NFSv4 ACLs to override the umask


   In some environments, inheritable NFSv4 ACLs can be rendered
   ineffective by the application of the per-process umask.  This is
   easily worked around by transmitting the umask and create mode
   separately to allow servers to make more intelligent decisions about
   the new mode on a file.

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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   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 August 06, 2016.

Copyright Notice

   Copyright (c) 2016 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
   (http://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
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   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.  Conventions Used in This Document . . . . . . . . . . . . . .   2
   2.  Problem Statement . . . . . . . . . . . . . . . . . . . . . .   2
   3.  umask Attribute . . . . . . . . . . . . . . . . . . . . . . .   3
   4.  Security Considerations . . . . . . . . . . . . . . . . . . .   4
   5.  Normative References  . . . . . . . . . . . . . . . . . . . .   4
   Appendix A.  RFC Editor Notes . . . . . . . . . . . . . . . . . .   5
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   5

1.  Conventions Used in This Document

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   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).  In the absence of inheritable permissions,
   the umask specifies which permissions must be turned off when
   creating new file system objects.  With "POSIX" Access Control Lists
   [POSIX-1003.1e], in the presence of inheritable permissions, the
   umask must be ignored.  Other Access Control List implementations on
   Unix-like systems may ignore the umask in a similar way.

   The NFSv4 protocol currently does not include the umask concept;
   applying the umask is left to clients.  Unfortunately, clients have
   no way of atomically checking for inheritable permissions and
   applying the umask only when necessary.  Instead, they err on the
   safe side and always apply the umask.  Thus the mode the server
   receives in an OPEN already has the umask applied.

   When applying the mode, section of [RFC7530] recommends
   servers "SHOULD" restrict permissions granted to any user or group
   named in the 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".)

   On may 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.  Thus the named users and groups
   in an inherited ACL end up being mostly ignored.

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   This leads to file permissions which are more restrictive than they
   should be in common cases; permission inheritance over NFSv4 is

   To address this problem, a new umask attribute is proposed which
   allows the server to apply the umask only when there are no
   inheritable permissions.

3.  umask Attribute

               | Name  | Id | Data Type | Acc | Defined in |
               | umask | 81 | mode4     | W   | Section 3  |

                                  Table 1

   The NFSv4.2 umask attribute is based on the UNIX file mode creation
   mask.  Only the nine low-order mode4 permission bits are defined.  A
   server MUST return NFS4ERR_INVAL if bits other than those nine are

   The umask attribute is only meaningful for operations that create
   objects (CREATE and OPEN); the server SHOULD reject it for other
   operations that take fattr4 arguments.

   The umask may only be set when the mode is also set.  If the server
   receives a CREATE or OPEN with a fattr4 argument including the umask
   attribute but not (in that same fattr4 argument) the mode attribute,
   the server MUST fail the operation with NFS4ERR_INVAL.

   When both the server and client support the umask attribute, a client
   that creates a file SHOULD NOT apply the umask to the mode attribute
   set at creation time.  Instead, it should set the mode attribute to
   the unmodified mode provided by the user, and the umask attribute to
   the umask of the requesting process.

   The server then uses the umask as follows:

   o  On a server that supports ACL attributes, if an object inherits
      any ACEs from its parent directory, the umask SHOULD be ignored.

   o  Otherwise, the 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
      to use for creating the object becomes (mode & ~umask) instead.

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4.  Security Considerations

   The proposed attribute allows to shift the decision when to apply the
   umask to the server.  Becuse 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
   proposal specifies that servers SHOULD ignore the umask if there are
   inheritable permissions, allowing servers to ignore this
   recommendation in cases when that should be preferable.

   The practice of ignoring the umask when there are inheritable
   permissions in the form of a "POSIX" default ACL is common practice;
   there are no known security concerns.  The "POSIX" default ACL
   mechanism and the mechanism of inheriting permissions in NFSv4 is
   equivalent for this purpose.

5.  Normative References

   [LEGAL]    IETF Trust, "Legal Provisions Relating to IETF Documents",
              November 2008, <http://trustee.ietf.org/docs/IETF-Trust-

              Portable Applications Standards Committee of the IEEE
              Compute Society, "POSIX 1003.1e Withdrawn Draft 17",
              October 1997.

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

   [RFC4506]  Eisler, M., "XDR: External Data Representation Standard",
              STD 67, RFC 4506, May 2006.

   [RFC5661]  Shepler, S., Ed., Eisler, M., Ed., and D. Noveck, Ed.,
              "Network File System (NFS) Version 4 Minor Version 1
              Protocol", RFC 5661, January 2010.

   [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, January 2010.

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

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

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Appendix A.  RFC Editor Notes

   [RFC Editor: please remove this section prior to publishing this
   document as an RFC]

   [RFC Editor: prior to publishing this document as an RFC, please
   replace all occurrences of RFCTBD10 with RFCxxxx where xxxx is the
   RFC number of this document]

Authors' Addresses

   J. Bruce Fields
   Red Hat, Inc.

   Email: bfields@redhat.com

   Andreas Gruenbacher
   Red Hat, Inc.

   Email: agruenba@redhat.com

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