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NFSv4 J. Fields
Internet-Draft A. Gruenbacher
Intended status: Informational Red Hat
Expires: August 25, 2016 February 22, 2016


Allowing inheritable NFSv4 ACLs to override the umask
draft-bfields-nfsv4-umask-01

Abstract

In some environments, inheritable NFSv4 ACLs can be rendered
ineffective by the application of the per-process umask. This is
easily worked around by transmitting the umask and create mode
separately to allow servers to make more intelligent decisions about
the new mode on a file.

Status of This Memo

This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.

Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
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and may be updated, replaced, or obsoleted by other documents at any
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material or to cite them other than as "work in progress."

This Internet-Draft will expire on August 25, 2016.

Copyright Notice

Copyright (c) 2016 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
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the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.



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Table of Contents

1. Conventions Used in This Document . . . . . . . . . . . . . . 2
2. Problem Statement . . . . . . . . . . . . . . . . . . . . . . 2
3. mode_umask Attribute . . . . . . . . . . . . . . . . . . . . 3
4. Security Considerations . . . . . . . . . . . . . . . . . . . 4
5. Normative References . . . . . . . . . . . . . . . . . . . . 4
Appendix A. Acknowledgments . . . . . . . . . . . . . . . . . . 5
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 5

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). In the absence of inheritable permissions,
the umask specifies which permissions must be turned off when
creating new file system objects. With "POSIX" Access Control Lists
[POSIX-1003.1e], in the presence of inheritable permissions, the
umask must be ignored. Other Access Control List implementations on
Unix-like systems may ignore the umask in a similar way.

The NFSv4 protocol currently does not include the umask concept;
applying the umask is left to clients. Unfortunately, clients have
no way of atomically checking for inheritable permissions and
applying the umask only when necessary. Instead, they err on the
safe side and always apply the umask. Thus the mode the server
receives in an OPEN already has the umask applied.

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 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".)

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. Thus the named users and groups
in an inherited ACL end up being mostly ignored.




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This leads to file permissions which are more restrictive than they
should be in common cases; permission inheritance over NFSv4 is
broken.

To address this problem, a new attribute is proposed which allows the
server to apply the umask only when there are no inheritable
permissions.

3. mode_umask Attribute

struct mode_umask4 {
mode4 mu_mode;
mode4 mu_umask;
};

+------------+----+-------------+-----+------------+
| Name | Id | Data Type | Acc | Defined in |
+------------+----+-------------+-----+------------+
| mode_umask | 81 | mode_umask4 | W | Section 3 |
+------------+----+-------------+-----+------------+

Table 1

The NFSv4.2 mode_umask attribute is based on the open mode and umask
that 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); the server SHOULD reject it for
other operations that take fattr4 arguments.

The server MUST ignore any mode attribute set in the same operation
as mode_umask.

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



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to use for creating the object becomes (mu_mode & ~mu_umask)
instead.

4. Security Considerations

The proposed attribute allows to shift the decision when to apply the
umask to the server. Becuse 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
proposal specifies that servers SHOULD ignore the umask if there are
inheritable permissions, allowing servers to ignore this
recommendation in cases when that should be preferable.

The practice of ignoring the umask when there are inheritable
permissions in the form of a "POSIX" default ACL is common practice;
there are no known security concerns. The "POSIX" default ACL
mechanism and the mechanism of inheriting permissions in NFSv4 is
equivalent for this purpose.

5. Normative References

[LEGAL] IETF Trust, "Legal Provisions Relating to IETF Documents",
November 2008, IETF-Trust-License-Policy.pdf>.

[POSIX-1003.1e]
Portable Applications Standards Committee of the IEEE
Compute Society, "POSIX 1003.1e Withdrawn Draft 17",
October 1997.

[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.

[RFC5661] Shepler, S., Ed., Eisler, M., Ed., and D. Noveck, Ed.,
"Network File System (NFS) Version 4 Minor Version 1
Protocol", RFC 5661, January 2010.

[RFC5662] Shepler, S., Ed., Eisler, M., Ed., and D. Noveck, Ed.,
"Network File System (NFS) Version 4 Minor Version 1
External Data Representation Standard (XDR) Description",
RFC 5662, January 2010.

[RFC7530] Haynes, T. and D. Noveck, "Network File System (NFS)
version 4 Protocol", RFC 7530, March 2015.




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[SUSv4] The Open Group, "Single UNIX Specification Version 4",
2013.

Appendix A. Acknowledgments

Thanks to Dave Noveck and Trond Myklebust for review.

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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