Internet DRAFT - draft-ietf-nfsv4-umask
draft-ietf-nfsv4-umask
NFSv4 J. Fields
Internet-Draft A. Gruenbacher
Intended status: Standards Track Red Hat
Expires: March 3, 2018 August 30, 2017
Allowing Inheritable NFSv4 Access Control Entries to Override the Umask
draft-ietf-nfsv4-umask-05
Abstract
In many environments, inheritable NFSv4 Access Control Entries (ACEs)
can be rendered ineffective by the application of the per-process
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 which accomplishes that.
Status of This Memo
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This Internet-Draft will expire on March 3, 2018.
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the Trust Legal Provisions and are provided without warranty as
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Table of Contents
1. Conventions Used in This Document . . . . . . . . . . . . . . 2
2. Problem Statement . . . . . . . . . . . . . . . . . . . . . . 2
3. Protocol Extension Considerations . . . . . . . . . . . . . . 3
4. XDR Extraction . . . . . . . . . . . . . . . . . . . . . . . 3
5. 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
Appendix A. Acknowledgments . . . . . . . . . . . . . . . . . . 6
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 6
1. Conventions Used in This Document
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
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), which 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 section "4.4 File Permissions" of that document.)
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. 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.
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Linux solves this problem on local filesystems by ignoring the umask
in the case the parent of the newly-created file has inheritable
ACEs; 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.
This document solves the problem by defining a new attribute which
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.
3. Protocol Extension Considerations
This document presents an extension to minor version 2 of the NFSv4
protocol as described in [nfsv4-versioning]. 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 they
support it or not).
Note that [RFC7862] does not define NFSv4.2 as non-extensible, so
that it is considered by [nfsv4-versioning] to be an extensible minor
version. As a result, upon publication of this document as a
Proposed Standard, the extension described herein will effectively be
part of NFSv4.2, even though this document 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>
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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
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 appropiate 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. 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.
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The server MUST return NFS4ERR_INVAL if the client attempts to set
both mode and mode_umask in the same operation.
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
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 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 actions by IANA.
8. References
8.1. Normative References
[nfsv4-versioning]
Noveck, D., "Rules for NFSv4 Extensions and Minor
Versions", draft-ietf-nfsv4-versioning-08 (work in
progress), December 2016.
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[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.
[RFC7530] Haynes, T. and D. Noveck, "Network File System (NFS)
version 4 Protocol", RFC 7530, March 2015.
[RFC7862] Haynes, T., "Network File System (NFS) Version 4 Minor
Version 2 Protocol", RFC 7862, November 2016.
[RFC7863] Haynes, T., "Network File System (NFS) Version 4 Minor
Version 2 External Data Representation Standard (XDR)
Description", RFC 7863, November 2016.
8.2. Informative References
[LinuxACL]
Gruenbacher, A., "ACL(5) - Access Control Lists", Linux
man pages 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.
Appendix A. Acknowledgments
Thanks to Trond Myklebust and Dave Noveck for the suggestion to
define this as a (mode, umask) pair rather than just umask. Thanks
for review to them and to Warren Kumari, Adam Roach, Spencer Dawkins,
Mike Kupfer, and Thomas Haynes for review, and to Thomas Haynes for
XDR help.
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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