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Internet Engineering Task Force (IETF)                         J. FieldsRequest for Comments: 8275                                A. GruenbacherCategory: Standards Track                                        Red HatISSN: 2070-1721                                            December 2017Allowing Inheritable NFSv4 Access Control Entries to Override the UmaskAbstract   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 inSection 2 of RFC 7841.   Information about the current status of this document, any errata,   and how to provide feedback on it may be obtained athttps://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 toBCP 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 2017Table of Contents1.  Problem Statement . . . . . . . . . . . . . . . . . . . . . .22.  Conventions Used in This Document . . . . . . . . . . . . . .33.  Protocol Extension Considerations . . . . . . . . . . . . . .34.  XDR Extraction  . . . . . . . . . . . . . . . . . . . . . . .35.  The mode_umask Attribute  . . . . . . . . . . . . . . . . . .46.  Security Considerations . . . . . . . . . . . . . . . . . . .57.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .58.  References  . . . . . . . . . . . . . . . . . . . . . . . . .58.1.  Normative References  . . . . . . . . . . . . . . . . . .58.2.  Informative References  . . . . . . . . . . . . . . . . .6   Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . .7   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .71.  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]

RFC 8275                       NFSv4 Umask                 December 2017   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 inBCP 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.xFields & Gruenbacher         Standards Track                    [Page 3]

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.Fields & Gruenbacher         Standards Track                    [Page 4]

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.  References8.1.  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 inRFC2119 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.Fields & Gruenbacher         Standards Track                    [Page 6]

RFC 8275                       NFSv4 Umask                 December 2017Acknowledgments   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.comFields & Gruenbacher         Standards Track                    [Page 7]