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Network Working Group                              Marius Aamodt Eriksen
Internet Draft                                           J. Bruce Fields
Document: draft-ietf-nfsv4-acl-mapping-02.txt               October 2004



               Mapping Between NFSv4 and Posix Draft ACLs



Status of this Memo



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   closed, in accordance with RFC 3668.

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   "Copyright (C) The Internet Society (2002-2004).  All Rights
   Reserved."

Abstract


   NFS version 4 [rfc3530] (NFSv4) specifies a flavor of Access Control
   Lists (ACLs) resembling Windows NT ACLs.  A number of operating sys-
   tems use a different flavor of ACL based on a withdrawn POSIX draft.
   NFSv4 clients and servers on such operating systems may wish to map



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   between NFSv4 ACLs and their native ACLs.  To this end, we describe a
   mapping from POSIX draft ACLs to a subset of NFSv4 ACLs.

















































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Table of Contents


   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . 4
   2.  NFSv4 ACLs . . . . . . . . . . . . . . . . . . . . . . . . . 4
   3.  POSIX ACLs . . . . . . . . . . . . . . . . . . . . . . . . . 5
   4.  Mapping Posix ACLs to NFSv4 ACLs . . . . . . . . . . . . . . 6
   5.  Using the Mapping in NFSv4 Implementations . . . . . . . . . 8
   6.  Security Considerations  . . . . . . . . . . . . . . . . .  10
   7.  Bibliography . . . . . . . . . . . . . . . . . . . . . . .  11
   8.  Author's Address . . . . . . . . . . . . . . . . . . . . .  12
   9.  Copyright  . . . . . . . . . . . . . . . . . . . . . . . .  12







































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1.  Introduction



   Access Control Lists (ACLs) are used to specify fine-grained access
   rights to file system objects.  An ACL consists of a number of Access
   Control Entries (ACEs), each specifying some level of access for an
   entity.  The entity may be a user, a group, or a special entity (such
   as "everyone").  The level of access is described using an access
   mask, which is a bitmask with each bit corresponding to a type of
   access (such as "read" or "append").

   In the following sections we describe two ACL models: NFSv4 ACLs, and
   ACLs based on a withdrawn POSIX draft, which we will refer to as
   "POSIX ACLs".  Since NFSv4 ACLs are much finer-grained than POSIX
   ACLs, it is not possible in general to map an arbitrary NFSv4 ACL to
   a POSIX ACL with the same semantics.  It does, however, turn out to
   be possible to map any POSIX ACL to a NFSv4 ACL that has nearly iden-
   tical semantics.  We will describe such a mapping, and discuss how it
   might be used in NFSv4 client and server implementations.


2.  NFSv4 ACLs



   An NFSv4 ACL is an ordered sequence of ACEs, each having an entity, a
   type, and an access mask.  The entity may be the name of a user or
   group, or may also be one of a small set of special entities.  Among
   the special entities are "OWNER" (the current owner of the file),
   "GROUP" (the group associated with the file), and "EVERYONE".

   The access mask includes bits for access types that are more fine-
   grained than the traditional "read", "write", and "execute" permis-
   sions used in UNIX mode bits.

   The type may be ALLOW or DENY.  (AUDIT or ALARM are also allowed, but
   they are not relevant to our discussion).

   The NFSv4 ACL permission-checking algorithm is straightforward.
   Given an ACL and a requestor asking for a set of permissions speci-
   fied by an access mask:

   1) Walk through the list of ACEs from the ACL in order.

   2) Ignore any ACE for with an entity not matching requestor.





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   3) Process all ACEs until all the bits in the requested access mask
      have been ALLOWed by an ALLOW ace with that bit set.   Once a par-
      ticular bit has been ALLOWed by an ACE, it is no longer considered
      in further processing.

   4) If a bit in the requested access mask is DENYed (while that bit is
      still under consideration), the request is denied.

   5) If all bits have been ALLOWed, the access is granted.  Otherwise
      behavior is undefined.


   There are also a number of flags that can be applied to an NFSv4 ACE.
   Three flags that we will need to use in the following discussion
   apply to ACEs in a directory ACL.  They are: ACE4_DIREC-
   TORY_INHERIT_ACE, which indicates that the ACE should be added to new
   subdirectories of the directory; ACE4_FILE_INHERIT_ACE, which does
   the same for new files; and ACE4_INHERIT_ONLY, which indicates that
   the ACE should be ignored when determining access to the directory
   itself.

   We refer the reader to [rfc3530] for further details.


3.  POSIX ACLs


   A number of operating systems, including Linux and FreeBSD, implement
   ACLs based on the withdrawn POSIX 1003.1e/1003.2c Draft Standard 17
   [posixacl].  We will refer to such ACLs as "POSIX ACLs".

   POSIX ACLs use access masks with only the traditional "read",
   "write", and "execute" bits.  Each ACE in a POSIX ACL is one of five
   types: ACL_USER_OBJ, ACL_USER, ACL_GROUP_OBJ, ACL_GROUP, ACL_MASK,
   and ACL_OTHER.  Each ACL_USER ACE has a uid associated with it, and
   each ACL_GROUP ACE has a gid associated with it.  Every POSIX ACL
   must have exactly one ACL_USER_OBJ, ACL_GROUP, and ACL_OTHER ACE, and
   at most one ACL_MASK ace.  The ACL_MASK ace is required if the ACL
   has any ACL_USER or ACL_GROUP aces.  There may not be two ACL_USER
   aces with the same uid, and there may not be two ACL_GROUP aces with
   the same gid.

   Given a POSIX ACL and a requestor asking for access, permission is
   determined as follows:


   1) If the requestor is the file owner, then allow or deny access
      depending on whether the ACL_USER_OBJ ACE allows or denies it.



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      Otherwise,

   2) if the requestor's uid matches the uid of one of the ACL_USER
      ACE's, then allow or deny access depending on whether the
      ACL_USER_OBJ ACE allows or denies it.  Otherwise,

   3) Consider the set of all ACL_GROUP ACE's whose gid the requestor is
      a member of.  Add to that set the ACL_GROUP_OBJ ACE, if the
      requestor is also a member of that group.  Allow access if one of
      the ACE's in the resulting set allows access.  If the set of
      matching ACEs is nonempty, and none allow access, then deny
      access.  Otherwise, if none of these ACEs match,

   4) if the requester's access mask is allowed by the ACL_OTHER ACE,
      then grant access.  Otherwise, deny access.


   Steps (2) and (3) have an additional criteria; in addition to check-
   ing whether the requested access mask is allowed by the access mask
   in the ACE, the requested bits also have to be in the access mask of
   the special ACE with the ACL_MASK entity.  This allows file owners to
   specify a maximum level of access allowed by any other user or group
   that has any access to the file system object.

   In addition to a regular POSIX ACL, a directory in the file system
   may also have associated with it a default ACL.  This default ACL
   does not affect permissions to the directory itself.  Instead, it
   governs the ACL a file system object will be assigned initially when
   it is created as a child of the particular directory.


4.  Mapping Posix ACLs to NFSv4 ACLs


   Given the differences between POSIX and NFSv4 ACLs, any conversion
   between the two is difficult.  However, POSIX ACLs are a subset of
   NFSv4 ACLs, and any POSIX ACL can be emulated with an NFSv4 ACL using
   the following mapping.

   First, the uid's and gid's on the ACL_USER and ACL_GROUP ACEs must be
   translated into NFSv4 names--a system-dependent process, which, on
   UNIX for example, may be done by lookups to /etc/passwd.  Also, the
   special ACL_USER_OBJ, ACL_GROUP_OBJ, and ACL_OTHER ACEs must be
   translated to NFSv4 ACEs with the special entities "OWNER", "GROUP",
   and "EVERYONE", respectively.

   The ACE access mask is translated as follows.  The read bit of the
   POSIX access mask is translated to the logical OR of the



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   ACE4_READ_DATA and ACE4_READ_NAMED_ATTRS NFSv4 access mask fields.
   The write bit of the POSIX access mask is translated to the logical
   OR of the ACE4_WRITE_DATA, ACE4_WRITE_NAMED_ATTRS and
   ACE4_APPEND_DATA NFSv4 access mask fields.  The execute bit of the
   POSIX access mask is translated into the ACE4_EXECUTE  and
   ACE4_READ_DATA NFSv4 acess mask fields.  Note that NFSv4 defines
   ACE4_READ_DATA, ACE4_WRITE_DATA, and ACE4_APPEND_DATA to be equal to
   ACE4_LIST_DIRECTORY, ACE4_ADD_FILE, and ACE4_ADD_SUBDIRECTORY,
   respectively, so this translation makes sense for directories as
   well.  However, on directories the ACE4_DELETE_CHILD field must be
   included in the translation of the POSIX write bit.

   In addition to the above, the OWNER entity must always be given
   ACE4_WRITE_ACL and ACE4_WRITE_ATTRIBUTES, and all entities must be
   given ACE4_READ_ACL, ACE4_READ_ATTRIBUTES, and ACE4_SYNCHRONIZE.  The
   ACE4_DELETE bit should be neither allowed nor denied by any ACE.

   The ACE flag field also has a simple translation.  If the file system
   object is a directory, and the POSIX ACE belongs to a default ACL,
   the ACE4_INHERIT_ONLY_ACE, ACE4_DIRECTORY_INHERIT, and
   ACE4_FILE_INHERIT flags are set in the NFSv4 ACE.  If the entity in
   the POSIX ACE refers to a group, the "ACE4_IDENTIFIER_GROUP" flag is
   set in the NFSv4 ACE.

   The POSIX ACL_USER_OBJ ACE is also always given the permission bits
   "ACE4_READ_ACL" and "ACE4_WRITE_ACL."

   Completing the mapping reduces to being able to emulate an ACL_MASK
   and compensate for some differences in the permission-checking algo-
   rithms of the two ACL implementations.

   The difference in permission-checking algorithms is handled as fol-
   lows:

   Every user ACE in the POSIX ACL maps into 2 NFSv4 ACEs; one ALLOW ACE
   which is translated as specified by the above scheme, then a comple-
   menting DENY ACE which is also translated as specified by the above
   scheme, with the exception that the access mask is inverted.  Note
   that the ACL_USER_OBJ ACE is placed first in this list.

   Every group ACE in the POSIX ACL produces a similar pair, but instead
   of being in sequence, all of the ALLOW ACEs are all in sequence, fol-
   lowed by all the DENY ACEs.  The ACL_GROUP_OBJ ACE is placed first in
   both lists.

   Lastly, the POSIX ACL_OTHER ACE is translated into a pair of ACEs as
   in the user ACE case.




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   With this done, the NFSv4 permission-checking algorithm applied to
   the resulting NFSv4 ACL will produce the same result as the POSIX
   permission-checking algorithm did on the original POSIX ACL.

   To handle the special POSIX entity ACL_MASK, we slightly modify the
   above translation:

   With the exception of the "OWNER" and "EVERYONE" ACEs, another ACE is
   prepended to the ACE.  The prepended ACE is a DENY ACE with the same
   entity as the following ALLOW ACE, but with a permission mask set to
   the complement of the POSIX ACL_MASK.

   This method allows us to preserve the real permission bits of each
   ACE should the ACL_MASK be changed.


5.  Using the Mapping in NFSv4 Implementations



   Note that the algorithm described in the previous section not only
   provides a way to map any POSIX ACL to be mapped to an NFSv4 ACL with
   similar semantics, but also provides the reverse mapping in the case
   where the NFSv4 ACL is precisely in the format of an ACL produced by
   the algorithm above.

   The algorithm can therefore be used to implement a subset of the
   NFSv4 ACL model.  This may be useful to NFSv4 clients and servers
   with preexisting system interfaces that support POSIX ACLs and that
   cannot be modified to support NFSv4 ACLs.

   A server, for example, that wishes to export via NFSv4 a filesystem
   that supports only POSIX ACLs, may use this mapping to answer client
   requests for existing ACLs by translating POSIX ACLs on its filesys-
   tem to NFSv4 ACLs to send to the client.  However, when a client
   attempts to set an ACL, the server faces a problem.  If the given ACL
   happens to be in precisely the format of an ACL produced by this map-
   ping (as would happen if, for example, the client was performing the
   same translation), then the server can map it to a POSIX ACL to store
   on the filesystem.  But for any other NFSv4 ACL, the server should
   return an error to avoid any chance of inaccurately representing the
   client's intention.

   The language of [rfc3530] allows a server some flexibility in han-
   dling ACLs that it cannot enforce completely accurately, as long as
   it adheres to "the guiding principle... that the server must not
   accept ACLs that appear to make [a file] more secure than it really
   is."



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   It may therefore be possible for a server to accept a wider range of
   NFSv4 ACLs, as long as it can ensure that in every case the resulting
   POSIX ACL denies at least all access that the original NFSv4 ACL
   denied.  The results of such a mapping may, however, be somewhat
   unexpected, and it is preferable simply to refuse all NFSv4 ACLs that
   do not map accurately, and provide clients with software to help gen-
   erate POSIX-mappable NFSv4 ACLs if necessary.

   Similarly, a client that uses NFSv4 ACLS to implement user interfaces
   that only deal in POSIX ACLs may handle user requests to set ACLs
   easily enough, but should return errors when the user requests ACLs
   that, on consulting the server, turn out to not be mappable to POSIX
   ACLs.






































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6.  Security Considerations



   Any automatic mapping from one ACL model to another must provide
   guarantees that the mapping preserves semantics, or risk misleading
   users about the permissions set on filesystem objects.  For this rea-
   son, we recommend performing such mapping only when it can be done
   accurately, and returning errors in all other cases.










































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7.  Bibliography



   [rfc3530]
   Shepler, S. et. al., "NFS version 4 Protocol", April 2003.

   http://www.ietf.org/rfc/rfc3530.txt


   [posixacl]
   IEEE, "IEEE Draft P1003.1e", October 1997 (last draft).

   http://wt.xpilot.org/publications/posix.1e/download.html





































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8.  Author's Address


   Address comments related to this memorandum to:

      marius@umich.edu bfields@umich.edu

   Marius Aamodt Eriksen
   J. Bruce Fields
   University of Michigan / CITI
   535 West William
   Ann Arbor, Michigan

   E-mail: marius@umich.edu
   E-mail: bfields@umich.edu

9.  Copyright


   Copyright (C) The Internet Society (2004). This document is subject
   to the rights, licenses and restrictions contained in BCP 78, and
   except as set forth therein, the authors retain all their rights.

   This document and the information contained herein are provided on an
   "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
   OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET
   ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED,
   INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFOR-
   MATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES
   OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.





















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