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Allowing inheritable NFSv4 ACLs to override the umask

The information below is for an old version of the document.
Document Type
This is an older version of an Internet-Draft that was ultimately published as RFC 8275.
Authors J. Bruce Fields , Andreas Gruenbacher
Last updated 2016-04-10
RFC stream Internet Engineering Task Force (IETF)
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IESG IESG state Became RFC 8275 (Proposed Standard)
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NFSv4                                                          J. Fields
Internet-Draft                                            A. Gruenbacher
Intended status: Informational                                   Red Hat
Expires: October 10, 2016                                 April 08, 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 of 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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   Internet-Drafts are draft documents valid for a maximum of six months
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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 October 10, 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
   ( 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
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   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.  mode_umask Attribute  . . . . . . . . . . . . . . . . . . . .   3
   4.  Security Considerations . . . . . . . . . . . . . . . . . . .   4
   5.  Normative References  . . . . . . . . . . . . . . . . . . . .   4
   Appendix A.  Acknowledgments  . . . . . . . . . . . . . . . . . .   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 that
   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 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.  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 attribute is proposed which allows the
   server to apply the umask only when there are no inheritable

3.  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 3  |

                                  Table 1

   The NFSv4.2 mode_umask attribute is based on the open mode and umask
   that 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); the server SHOULD reject it for
   other operations that take fattr4 arguments.

   The server MUST ignore any mode attribute set in the same operation
   as mode_umask.

   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

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      to use for creating the object becomes (mu_mode & ~mu_umask)

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, <

              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.

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

Appendix A.  Acknowledgments

   Thanks to Dave Noveck and Trond Myklebust for review.

Authors' Addresses

   J. Bruce Fields
   Red Hat, Inc.


   Andreas Gruenbacher
   Red Hat, Inc.


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