Allowing Inheritable NFSv4 Access Control Entries to Override the Umask
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
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
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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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