wdiff draft-ietf-nfsv4-acl-mapping-02.txt draft-ietf-nfsv4-acl-mapping-03.txt

Network Working Group Marius Aamodt Eriksen
Internet Draft J. Bruce Fields
Document: draft-ietf-nfsv4-acl-mapping-02.txt October 2004 draft-ietf-nfsv4-acl-mapping-03.txt February 2005

Mapping Between NFSv4 and Posix Draft ACLs

Status of this Memo

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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tems use a different flavor of ACL based on a withdrawn POSIX draft.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

Mapping NFSv4 ACLs October 2004 February 2005

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

Mapping NFSv4 ACLs October 2004 February 2005

Table of Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 4
2. NFSv4 ACLs . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. POSIX ACLs . . . . . . . . . . . . . . . . . . . . . . . . . 5
4. Mapping Posix POSIX ACLs to NFSv4 ACLs . . . . . . . . . . . . . . 6
5. Using the Mapping in NFSv4 Implementations . . . . . . . . . 8 9
6. Security Considerations . . . . . . . . . . . . . . . . . 10 11
7. Bibliography . . . . . . . . . . . . . . . . . . . . . . . 11 12
8. Author's Address . . . . . . . . . . . . . . . . . . . . . 12 13
9. Copyright . . . . . . . . . . . . . . . . . . . . . . . . 12 13

Mapping NFSv4 ACLs October 2004 February 2005

1. Introduction

Access Control Lists (ACLs) are used to specify fine-grained access
rights to file system objects. An ACL consists of is a number list 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 user) and some
level of access (such as "read" or "append"). for that entity.

In the following sections we describe two ACL models: NFSv4 ACLs, and
ACLs based on a withdrawn POSIX draft, which we draft. We will refer to the latter
as "POSIX ACLs". Since NFSv4 ACLs are much finer-grained more fine-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 However, it is possible to map
any POSIX ACL to a NFSv4 ACL that has with nearly iden-
tical identical semantics. We
will describe such a mapping, and discuss how it
might be used its use in NFSv4 client clients
and server implementations. servers.

2. NFSv4 ACLs

An NFSv4 ACL is an ordered sequence of ACEs, each having an entity, a
type, some flags, 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"
"OWNER@" (the current owner of the file),
"GROUP" "GROUP@" (the group associated associ-
ated 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. "EVERYONE@".

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 mask has 14 separate bits, including bits to control read,
write, execute, append, ACL in order.

2) Ignore any ACE modification, file owner modification,
etc.; consult [rfc3530] for with an entity not matching requestor.

Mapping NFSv4 ACLs October 2004

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 full list.

Of the requested access mask flags, four are relevant here. The ACE4_IDENTIFIER_GROUP flag
is DENYed (while used to indicate that bit is
still under consideration), the request entity name is denied.

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

There are also a number name 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 group. The
other three concern inheritance: ACE4_DIRECTORY_INHERIT_ACE indicates
that the ACE should be added to new subdirectories of the directory; ACE4_FILE_INHERIT_ACE, which
ACE4_FILE_INHERIT_ACE does the same for new files; and ACE4_INHERIT_ONLY, which
ACE4_INHERIT_ONLY indicates that the ACE should be ignored when
determining access to the directory itself.

We refer the

The NFSv4 ACL permission-checking algorithm is straightforward.
Assume a a requester asks for access, as specified by a single bit in

Mapping NFSv4 ACLs February 2005

the access bitmask. We allow the access if the first ACE in the ACL
that matches the requester and that has that bit set is an ALLOW ACE,
and we deny the access if the first such ACE is a DENY ACE. If no
matching ACE has the bit in question set, behaviour is undefined. If
an access mask consisting of more than one bit is requested, it suc-
ceeds if and only if each bit in the mask is allowed.

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

3. POSIX ACLs

A number of operating systems, including Linux and FreeBSD, systems 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. ACE. The ACL_MASK ace ACE is required if the ACL
has any ACL_USER or ACL_GROUP aces. ACEs. There may not be two ACL_USER
aces
ACEs with the same uid, and there may not be two ACL_GROUP aces ACEs with
the same gid.

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

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

Mapping NFSv4 ACLs October 2004
Otherwise,

2) if the requestor's requester's uid matches the uid of one of the ACL_USER
ACE's,
ACEs, 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 ACEs whose gid the requestor requester is
a member of. Add to that set the ACL_GROUP_OBJ ACE, if the
requestor
requester is also a member of that the file's group. Allow access if one of
the ACE's
any ACE in the resulting set allows access. If the set of
matching match-
ing ACEs is nonempty, and none allow access, then deny access.
Otherwise, if none the set of these matching ACEs match, is empty,

Mapping NFSv4 ACLs February 2005

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

Steps

The above description omits one detail: in 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, (3), the
requested bits also have to must be in the access mask of granted both by the special matching ACE with and by the
ACL_MASK entity. This allows file owners to
specify a ACE. The ACL_MASK ACE thus limits the maximum level of access allowed permissions
which may be granted by any other user ACL_USER or group
that has any access to the file system object.

In addition to a regular POSIX ACL, a directory in ACL_GROUP ACE, or by the
ACL_GROUP_OBJ ACE.

Each file system may also have associated with it a default ACL. This default single POSIX ACL
does not affect permissions associated with it, used to
determine access to that file. Directories, however, may have two
ACLs: one, the directory itself. Instead, it
governs "access ACL", used to determine access to the direc-
tory, and one, the "default ACL", used only as the ACL a file system object will to be assigned initially when
it is inher-
ited by newly created as a child of objects in the particular directory.

4. Mapping Posix 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

We now describe an algorithm which maps any POSIX ACL can be emulated with to an NFSv4 ACL using
with the following mapping. same semantics.

First, translate 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 names. This is an implementation-dependent process.
It might be done, for example, may be done by lookups to /etc/passwd. consulting a directory service or a
password file. 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", "OWNER@", "GROUP@", and "EVERYONE", "EVERYONE@", respectively.

The

Next, map each POSIX ACE access (excepting any mask is translated ACE) in the given POSIX
ACL to an NFSv4 ALLOW ACE with an entity determined as follows. The read bit of above, and
with a bitmask determined from the permission bits on the POSIX access mask ACE
as follows:

1) If the read bit is translated to set in the logical OR of POSIX ACE, then set ACE4_READ_DATA.

2) If the

Mapping NFSv4 ACLs October 2004

ACE4_READ_DATA and ACE4_READ_NAMED_ATTRS NFSv4 access mask fields.
The write bit of is set in the POSIX access mask is translated to ACE, then set ACE4_WRITE_DATA
and ACE4_APPEND_DATA. If the logical
OR of object carrying the ACL is a direc-
tory, set ACE4_DELETE_CHILD as well.

3) If the ACE4_WRITE_DATA, ACE4_WRITE_NAMED_ATTRS and
ACE4_APPEND_DATA NFSv4 access mask fields. The execute bit of is set in the POSIX access mask ACE, then set ACE4_EXECUTE.

4) Set ACE4_READ_ACL, ACE4_READ_ATTRIBUTES, and ACE4_SYNCHRONIZE
unconditionally.

5) If the ACE is translated into for the ACE4_EXECUTE special "OWNER@" entity, set ACE4_WRITE_ACL
and
ACE4_READ_DATA ACE4_WRITE_ATTRIBUTES.

Mapping NFSv4 acess mask fields. Note that ACLs February 2005

6) Clear all other bits in the 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 bitmask.

In addition, we set the ACE4_DELETE_CHILD field must be
included GROUP flag in each ACE which corresponds to a
named group (but not in the translation GROUP@ ACE, or any of the POSIX write bit.

In addition to other special
entity ACEs). At this point, we've replaced the above, POSIX ACL by an
NFSv4 ACL with 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 same number of ACEs (ignoring any ACE.

The ACE flag field also has a simple translation. If the file system
object is a directory, and mask ACE). To
emulate the POSIX ACE belongs ACL permission-checking algorithm, we need to a default ACL,
the ACE4_INHERIT_ONLY_ACE, ACE4_DIRECTORY_INHERIT, and
ACE4_FILE_INHERIT flags are set in mod-
ify the NFSv4 ACE. If ACL further, as follows:

1) Order the entity in ACL so that the POSIX OWNER@ ACE refers to a group, the "ACE4_IDENTIFIER_GROUP" flag is
set in the NFSv4 ACE.

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

Completing ACL,
followed by any user ACEs, followed by the mapping reduces to being able to emulate an ACL_MASK GROUP@ ACE, followed by
any group ACEs, and compensate for some differences in the permission-checking algo-
rithms of ending finally with the two ACL implementations. EVERYONE@ ACE.

2) The difference in permission-checking algorithms is handled POSIX algorithm stops as soon as fol-
lows:

Every user ACE in the POSIX ACL maps into 2 NFSv4 ACEs; one requester matches an
ACL_USER_OBJ, ACL_OTHER, or ACL_USER ACE. To emulate this
behaviour, add a single DENY ACE after each ALLOW ACE
which is translated for OWNER@,
EVERYONE@, or any named user. The DENY ACE should have the same
entity and flags as specified by the above scheme, then a comple-
menting corresponding ALLOW ACE. The bitmask on
the DENY ACE which is also translated as specified by should be the above
scheme, with bitwise NOT of the exception that bitmask on the access mask is inverted. Note ALLOW
ACE, except that the ACL_USER_OBJ ACE is placed first in this list.

Every group ACE ACE4_WRITE_OWNER and ACE4_DELETE bits should
be cleared, and the ACE4_DELETE_CHILD bit should be cleared on
non-directories. (Also, in the POSIX xdr-encoded ACL produces a similar pair, but instead
of being that is transmit-
ted, all bits not defined in the protocol should be cleared.)

3) Unlike the other ACEs in sequence, step 2, all of the ALLOW ACL_GROUP_OBJ and
ACL_GROUP ACEs are all in sequence, fol-
lowed consulted by all the DENY ACEs. The ACL_GROUP_OBJ ACE is placed first POSIX algorithm before deter-
mining permissions. However, if the requester matches any one of
them, then it must deny any permissions they do not allow. To
emulate this behaviour, instead of adding a single DENY after each
corresponding GROUP@ or named group ACE, we insert a list of DENY
ACEs at the end of the list of GROUP@ and named group ACEs. Each
DENY ACE is determined from its corresponding ALLOW ACE exactly as
in
both lists.

Lastly, step 2, and should occur in the inserted list in the same posi-
tion as the corresponding ALLOW ACE occurs in the list of ALLOW
ACEs.

4) Finally, we enforce the POSIX ACL_OTHER mask ACE is translated into by prepending each ALLOW
ACE for a pair named user, GROUP@, or named group, with a single DENY
ACE whose entity and flags are the same as those for the corre-
sponding ALLOW ACE, but whose bitmask is the inverse of ACEs the bit-
mask determined from the mask ACE, with the inverse calculated as
described in step 2.

As an example, take a POSIX ACL with two named users (u1 and u2) and
two named groups (g1 and g2), in addition to the user ACE case. required
ACL_USER_OBJ, ACL_GROUP_OBJ, ACL_OTHER, and ACL_MASK ACEs.

Such an ACL will map to an NFSv4 ACL of the form

Mapping NFSv4 ACLs October 2004

With this done, February 2005

ALLOW OWNER@
DENY OWNER@
DENY u1 (mask)
ALLOW u1
DENY u1
DENY u2 (mask)
ALLOW u2
DENY u2
DENY GROUP@ (mask)
ALLOW GROUP@
DENY g1 (mask)
ALLOW g1
DENY g2 (mask)
ALLOW g2
DENY GROUP@
DENY g1
DENY g2
ALLOW EVERYONE@
DENY EVERYONE@

where the NFSv4 permission-checking algorithm applied to ACEs marked with (mask) are those whose bitmask are deter-
mined from the resulting NFSv4 ACL_MASK ACE as described in step 4 above.

In general, a POSIX ACL with m named users and n named groups will produce
map to an NFSv4 ACL with (3*(m + n) + 7) ACLs, unless m and n are
both zero, in which case the same result will have either 6 or 7 ACLs,
depending on whether the original ACL had an ACL_MASK ACE.

On directories with default ACLs, we translate the default ACL as
above, but set the POSIX
permission-checking algorithm did ACE4_INHERIT_ONLY_ACE, ACE4_DIRECTORY_INHERIT_ACE,
and ACE4_FILE_INHERIT_ACE flags on every ACE in the resulting ACL.
On directories with both default and access ACLs, we translate the
two ACLs and then concatenate them. The order of the concatenation
is unimportant.

There is one extremely minor inaccuracy in this mapping: if a
requester that is a member of more than one group listed in the original POSIX ACL.

To handle ACL
requests multiple bits simultaneously, the special POSIX entity ACL_MASK, we slightly modify the
above translation:

With the exception algorithm requires
all of the "OWNER" and "EVERYONE" ACEs, another ACE is
prepended bits to be granted simultaneously by one of the ACE. The prepended ACE is group
ACEs. Thus a DENY ACE with the same
entity POSIX ACL such as the following ALLOW ACE, but with

ACL_USER_OBJ: ---
ACL_GROUP_OBJ: ---
g1: r--
g2: -w-
ACL_MASK: rw-
ACL_OTHER: ---

Mapping NFSv4 ACLs February 2005

will prevent a permission mask set to
the complement user that is a member of groups g1 and g2 from opening
a file for both read and write, even though read and write would be
individually permitted.

The NFSv4 ACL permission-checking algorithm has the POSIX ACL_MASK.

This method allows us to preserve the real permission bits property that it
permits a group of bits whenever it would permit each
ACE should the ACL_MASK be changed. bit individu-
ally, so it is impossible to mimic this behaviour with an NFSv4 ACL.

5. Using the Mapping in NFSv4 Implementations

Note that

Examination of the algorithm described in the previous section not only
provides a way to map any shows
that no information is lost; the original POSIX ACL to can be recon-
structed from the mapped to an NFSv4 ACL with
similar semantics, but ACL. Thus we also provides the reverse mapping have a way to map
NFSv4 ACLs to POSIX ACLs 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
is not 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), mapping,
then the server can map it to a POSIX ACL may be required to store
on the filesystem. But for any other NFSv4 ACL, the server should return an error to avoid any chance of inaccurately inaccu-
rately representing the client's intention. The correct error to
return in this case is NFS4ERR_ATTRNOTSUPP.

In the case where a client sets an ACL that leaves certain bits nei-
ther allowed nor denied, the server may choose to allow or deny those
bits as necessary to make mapping possible. In some situations it
may also be possible for a server to map the ACL if it adds a DENY
ACE or denies a few additional bits. The language of [rfc3530]
allows a server some flexibility in han-
dling handling 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."

Given the choice, as long as the "guiding principle" is not violated,
servers should opt to be forgiving. The complexity of the
POSIX<->NFSv4 mapping makes difficult the task of generating ACLs

Mapping NFSv4 ACLs October 2004

It may therefore be possible for February 2005

that will satisfy a server to accept a wider range of
NFSv4 ACLs, as long as it using the mapping. By making the mapping
more forgiving, the server can ensure simplify that in every case the resulting
POSIX ACL denies at least all access task, improving interop-
erability.

Servers that implement the original full NFSv4 ACL protocol should also handle
carefully ACLs that leave bits neither allowed nor denied. The results of such a mapping may, however, be somewhat
unexpected, and it It is preferable simply
better to refuse all NFSv4 ACLs that
do fall back on some reasonable default rather than to always
allow or always deny. A client that, for example, sets
ACE4_WRITE_DATA but leaves unspecified ACE4_APPEND_DATA probably does
so because its system interfaces are incapable of independently rep-
resenting ACE4_APPEND_DATA, not map accurately, and because it intends to deny
ACE4_APPEND_DATA. By leaving the bit unspecified, the client leaves
the server the opportunity to provide clients with software the reasonable default of set-
ting it to help gen-
erate POSIX-mappable NFSv4 ACLs if necessary.

Similarly, match ACE4_WRITE_DATA.

Similar issues exist when a client that uses NFSv4 ACLS ACLs to implement user
interfaces that only deal in POSIX ACLs. When the client translates
ACLs may handle user requests received from the server to set ACLs
easily enough, POSIX ACLs, some flexibility may
help interopability, but should return errors when the user requests client must take care not to represent
any ACLs
that, on consulting as stricter than they really are. Clients that provide
access to the server, turn out full set of NFSv4 ACLs may also wish to not be mappable provide users
with utilities to generate and interpret POSIX-mapped NFSv4 ACLs, to
aid users working with servers using the POSIX
ACLs. mapping.

Mapping NFSv4 ACLs October 2004 February 2005

6. Security Considerations

Any automatic mapping from one ACL model to another must provide
guarantees that as to how the mapping preserves semantics, affects the meaning of ACLs, or risk
misleading users about the permissions set on filesystem objects.
For this rea-
son, we recommend performing reason, caution is recommended when implementing this map-
ping. It is better to return errors than to break any such guaran-
tees.

Note also that this ACL mapping only when it can be done
accurately, requires mapping between NFSv4 user-
names and returning errors in all other cases. local id's. When the mapping of id's depends on remote
services, the method used for the mapping must be at least as secure
as the method used to set or get ACLs.

Mapping NFSv4 ACLs October 2004 February 2005

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

Mapping NFSv4 ACLs October 2004 February 2005

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
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