Allowing Inheritable NFSv4 Access Control Entries to Override the Umask
RFC 8275

Document Type RFC - Proposed Standard (December 2017; Errata)
Last updated 2017-12-05
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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

Abstract

   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
   https://www.rfc-editor.org/info/rfc8275.

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
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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.

Fields & Gruenbacher         Standards Track                    [Page 1]
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 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 "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
   [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.

Fields & Gruenbacher         Standards Track                    [Page 2]
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