RFC 8275

Internet Engineering Task Force (IETF)                         J. Fields
Request for Comments: 8275                                A. Gruenbacher
Category: Standards Track                                        Red Hat
ISSN: 2070-1721                                            December 2017

Allowing Inheritable NFSv4 Access Control Entries to Override the Umask


   In many environments, inheritable NFSv4 Access Control Entries (ACEs)
   can be rendered ineffective by the application of the per-process
   file mode creation mask (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 to accomplish that.

Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 7841.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at

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
   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.

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RFC 8275                       NFSv4 Umask                 December 2017

Table of Contents

   1.  Problem Statement . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Conventions Used in This Document . . . . . . . . . . . . . .   3
   3.  Protocol Extension Considerations . . . . . . . . . . . . . .   3
   4.  XDR Extraction  . . . . . . . . . . . . . . . . . . . . . . .   3
   5.  The mode_umask Attribute  . . . . . . . . . . . . . . . . . .   4
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .   5
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   5
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   5
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .   5
     8.2.  Informative References  . . . . . . . . . . . . . . . . .   6
   Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . .   7
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   7

1.  Problem Statement

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

   When applying the mode, Section 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 "File Permissions", Section 4.4 of [SUSv4].)

   On many existing installations, all ordinary users use the same
   effective group ID by default.  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.

   Linux solves this problem on local file systems by ignoring the umask
   whenever a newly created file inherits ACEs from its parent; see

   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.

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RFC 8275                       NFSv4 Umask                 December 2017

   This document solves the problem by defining a new attribute that
   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

2.  Conventions Used in This Document

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "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 presents an extension to minor version 2 of the NFSv4
   protocol as described in [RFC8178].  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 or not they support

   Note that [RFC7862] does not define NFSv4.2 as non-extensible, so
   [RFC8178] treats it as an extensible minor version.  This Standards
   Track RFC extends NFSv4.2 but does not update [RFC7862] or [RFC7863].

4.  XDR Extraction

   The additional lines of External Data Representation (XDR) [RFC4506]
   description embedded in this document can be extracted by feeding
   this document into the following shell script:


   grep '^ *///' $* | sed 's?^ */// ??' | sed 's?^ *///$??'


   That is, if the above script is stored in a file called "extract.sh",
   and this document is in a file called "umask.txt", then the reader
   can do:

   sh extract.sh < umask.txt > umask.x

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RFC 8275                       NFSv4 Umask                 December 2017

   The effect of the script is to remove leading white space from each
   line, plus a sentinel sequence of "///".

   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.

5.  The mode_umask Attribute


   ///   struct mode_umask4 {
   ///     mode4  mu_mode;
   ///     mode4  mu_umask;
   ///   };
   ///   %/*
   ///   % * New For UMASK
   ///   % */
   ///   const FATTR4_MODE_UMASK         = 81;


           | Name       | Id | Data Type   | Acc | Defined in |
           | mode_umask | 81 | mode_umask4 | W   | Section 5  |

                                  Table 1

   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.

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RFC 8275                       NFSv4 Umask                 December 2017

   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 that are set in the unmask MUST be turned off; the mode
      assigned to the new object becomes (mu_mode & ~mu_umask) instead.

6.  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 in which 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.

7.  IANA Considerations

   This document does not require any IANA actions.

8.  References

8.1.  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,

   [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>.

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RFC 8275                       NFSv4 Umask                 December 2017

   [RFC7530]  Haynes, T., Ed. and D. Noveck, Ed., "Network File System
              (NFS) Version 4 Protocol", RFC 7530, DOI 10.17487/RFC7530,
              March 2015, <https://www.rfc-editor.org/info/rfc7530>.

   [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>.

   [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,

8.2.  Informative References

   [LinuxACL] Gruenbacher, A., "ACL(5) - Access Control Lists", Linux
              man pages online, ACL(5), March 2002,

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

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RFC 8275                       NFSv4 Umask                 December 2017


   Thanks to Trond Myklebust and Dave Noveck for their review and the
   suggestion to define this as a (mode, umask) pair rather than just
   umask.  Thanks to Warren Kumari, Adam Roach, Spencer Dawkins, Mike
   Kupfer, and Thomas Haynes for their review and to Thomas Haynes for
   help with XDR.

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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