RFC8275: Allowing Inheritable NFSv4 Access Control Entries to Override the Umask
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Related keywords:
(NFSv4)
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
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.
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.
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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
access.
2. Conventions Used in This Document
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"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
it).
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:
<CODE BEGINS>
#!/bin/sh
grep '^ *///' $* | sed 's?^ */// ??' | sed 's?^ *///$??'
<CODE ENDS>
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
<CODE BEGINS>
/// struct mode_umask4 {
/// mode4 mu_mode;
/// mode4 mu_umask;
/// };
///
/// %/*
/// % * New For UMASK
/// % */
/// const FATTR4_MODE_UMASK = 81;
<CODE ENDS>
+------------+----+-------------+-----+------------+
| 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,
<https://www.rfc-editor.org/info/rfc2119>.
[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>.
Fields & Gruenbacher Standards Track [Page 5]
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,
<https://www.rfc-editor.org/info/rfc8178>.
8.2. Informative References
[LinuxACL] Gruenbacher, A., "ACL(5) - Access Control Lists", Linux
man pages online, ACL(5), March 2002,
<http://kernel.org/doc/man-pages/online/pages/man5/
acl.5.html>.
[SUSv4] The Open Group, "Single UNIX Specification, Version 4",
2013.
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RFC 8275 NFSv4 Umask December 2017
Acknowledgments
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
Fields & Gruenbacher Standards Track [Page 7]