Network Working Group M. Eriksen
Internet-Draft J. Fields
Expires: November 16, 2006 CITI
May 15, 2006
Mapping Between NFSv4 and Posix Draft ACLs
draft-ietf-nfsv4-acl-mapping-04
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Abstract
A number of filesystems and applications support ACLs based on a
withdrawn POSIX draft [2]. Those ACLs differ significantly from NFS
version 4 (NFSv4) ACLs [1]. We describe how to translate between the
two types of ACLs.
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1. Introduction
Access Control Lists (ACLs) are used to specify fine-grained access
rights to file system objects. An ACL is a list of Access Control
Entries (ACEs), each specifying an entity (such as a user) and some
level of access for that entity.
In the following sections we describe two ACL models: NFSv4 ACLs, and
ACLs based on a withdrawn POSIX draft. We will refer to the latter
as "POSIX ACLs". Since NFSv4 ACLs are 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. However, it is possible to map
any POSIX ACL to a NFSv4 ACL with nearly identical semantics, and it
is possible to map any NFSv4 ACL to a POSIX ACL in a way that
preserves certain guarantees. We will explain how to do this, and
give guidelines for clients and servers performing such translation.
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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 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 type may be ALLOW or DENY. (AUDIT or ALARM are also allowed, but
they are not relevant to our discussion).
The access mask has 14 separate bits, including bits to control read,
write, execute, append, ACL modification, file owner modification,
etc.; consult [1] for the full list.
Of the flags, four are relevant here. The ACE4_IDENTIFIER_GROUP flag
is used to indicate that the entity name is the name of a 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 does the same for new files; and
ACE4_INHERIT_ONLY indicates that the ACE should be ignored when
determining access to the directory itself.
The NFSv4 ACL permission-checking algorithm is straightforward.
Assume a a requester asks for access, as specified by a single bit in
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
succeeds if and only if each bit in the mask is allowed.
We refer the reader to [1] for further details.
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3. POSIX ACLs
A number of operating systems implement ACLs based on the withdrawn
POSIX 1003.1e/1003.2c Draft Standard 17 [2]. 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_OBJ, 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 requester asking for access, permission is
determined as follows:
1. If the requester is the file owner, then allow or deny access
depending on whether the ACL_USER_OBJ ACE allows or denies it.
Otherwise,
2. if the requester's uid matches the uid of one of the ACL_USER
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 ACEs whose gid the requester is
a member of. Add to that set the ACL_GROUP_OBJ ACE, if the
requester is also a member of the file's group. Allow access if
any ACE in the resulting set allows access. If the set of
matching ACEs is nonempty, and none allow access, then deny
access. Otherwise, if the set of matching ACEs is empty,
4. if the requester's access mask is allowed by the ACL_OTHER ACE,
then grant access. Otherwise, deny access.
The above description omits one detail: in steps (2) and (3), the
requested bits must be granted both by the matching ACE and by the
ACL_MASK ACE. The ACL_MASK ACE thus limits the maximum permissions
which may be granted by any ACL_USER or ACL_GROUP ACE, or by the
ACL_GROUP_OBJ ACE.
Each file may have a single POSIX ACL associated with it, used to
determine access to that file. Directories, however, may have two
ACLs: one, the "access ACL", used to determine access to the
directory, and one, the "default ACL", used only as the ACL to be
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inherited by newly created objects in the directory.
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4. Ordering of NFSv4 and POSIX ACLs
POSIX ACLs are unordered--the order in which the POSIX access-
checking algorithm considers the entries is determined entirely by
the type of the entries, so the entries don't need to be kept in any
particular order.
By contrast, the meaning of an NFSv4 ACL can be dramatically changed
by modifying the order that the entries are listed in.
In the following, we will say that an NFSv4 ACL is in the "canonical
order" if its entries are ordered in the order that the POSIX
algorithm would consider them. That is, with all OWNER@ entries
first, followed by user entries, followed by GROUP@ entries, followed
by group entries, with all EVERYONE@ entries at the end.
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5. A Minor Eccentrity of POSIX ACLs
We will see below that it is possible to find an NFSv4 ACL with
precisely the same effect as any given POSIX ACL, with one extremely
minor exception: if a requester that is a member of more than one
group listed in the ACL requests multiple bits simultaneously, the
POSIX algorithm requires all of the bits to be granted simultaneously
by one of the group ACEs. Thus a POSIX ACL such as
ACL_USER_OBJ: ---
ACL_GROUP_OBJ: ---
g1: r--
g2: -w-
ACL_MASK: rw-
ACL_OTHER: ---
will prevent a 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 property that it
permits a group of bits whenever it would permit each bit
individually, so it is impossible to mimic this behaviour with an
NFSv4 ACL.
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6. Mapping POSIX ACLs to NFSv4 ACLs
6.1. Requirements
In the next section we give an example of a mapping of POSIX ACLs
into NFSv4 ACLs. We permit a server or client to use a different
mapping, provided the mapping meets the following requirements:
It must map the POSIX ACL to an NFSv4 ACL with identical access
semantics, ignoring the minor exception described in the previous
section.
It must map the read mode bit to ACE4_READ_DATA, the write bit to
ACE4_WRITE_DATA and ACE4_APPEND_DATA (and ACE4_DELETE_CHILD for
directories), and the EXECUTE bit to ACE4_EXECUTE. It should also
allow ACE4_READ_ACL, ACE4_READ_ATTRIBUTES, and ACE4_SYNCHRONIZE
unconditionally, and allow ACE4_WRITE_ACL and ACE4_WRITE_ATTRIBUTES
to the owner. The handling of other NFSv4 mode bits may depend on
the implementation, but it is preferable to leave them unused.
It should avoid using DENY ACEs. If DENY ACEs are required, it
should attempt to place them at the beginning. (This is not always
possible.)
For simplicity's sake, the translator may choose to handle the mask
by first applying it to the USER, GROUP, and GROUP_OBJ ACEs, and then
mapping the resulting ACL. However, that will result in an ACL from
which it is impossible to determine the original value of the mask or
of the masked USER, GROUP, and GROUP_OBJ bitmasks. If the resulting
ACL is later translated back to a POSIX ACL, the translator will
assume that the value of the mask is the union of the bitmasks
permitted to any USER, GROUP, or GROUP_OBJ. If that would be
incorrect, the original translation should not modify the bitmasks of
the USER, GROUP, and GROUP_OBJ bitmasks, and should instead use
additional DENY ACEs as necessary to give the effect of the mask. It
should also arrange for the first GROUP@ ACE to be a DENY ACE whose
bitmask is determined by the mask, allowing that ACE to be used to
determine the original mask value.
6.2. Example POSIX->NFSv4 Mapping
We now describe an algorithm which maps any POSIX ACL to an NFSv4 ACL
with the same semantics, meeting the above requirements.
First, translate the uid's and gid's on the ACL_USER and ACL_GROUP
ACEs into NFSv4 names, using directory services, etc., as
appropriate, and translate ACL_USER_OBJ, ACL_GROUP_OBJ, and ACL_OTHER
to the special NFSv4 names "OWNER@", "GROUP@", and "EVERYONE@",
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respectively.
Next, map each POSIX ACE (excepting any mask ACE) in the given POSIX
ACL to an NFSv4 ALLOW ACE with an entity determined as above, and
with a bitmask determined from the permission bits on the POSIX ACE
as follows:
1. If the read bit is set in the POSIX ACE, then set ACE4_READ_DATA.
2. If the write bit is set in the POSIX ACE, then set
ACE4_WRITE_DATA and ACE4_APPEND_DATA. If the object carrying the
ACL is a directory, set ACE4_DELETE_CHILD as well.
3. If the execute bit is set in the POSIX ACE, then set
ACE4_EXECUTE.
4. Set ACE4_READ_ACL, ACE4_READ_ATTRIBUTES, and ACE4_SYNCHRONIZE
unconditionally.
5. If the ACE is for the special "OWNER@" entity, set ACE4_WRITE_ACL
and ACE4_WRITE_ATTRIBUTES.
6. Clear all other bits in the NFSv4 bitmask.
In addition, we set the GROUP flag in each ACE which corresponds to a
named group (but not in the GROUP@ ACE, or any of the other special
entity ACEs).
At this point, we've replaced the POSIX ACL by an NFSv4 ACL with the
same number of ACEs (ignoring any mask ACE), all of them ALLOW ACEs.
Order this NFSv4 ACL in the canonical order: OWNER@, users, GROUP@,
groups, then EVERYONE@.
If the bitmasks in the resulting ACEs are non-increasing (so no ACE
allows a bit not allowed by a previous ACE), then we can skip the
next step.
Otherwise, we need to insert additional DENY ACE's to emulate the
first-match semantics of the POSIX ACL permission-checking algorithm:
1. If an ACL_USER_OBJ, ACL_OTHER, or ACL_USER ACE fails to grant
some permissions that are granted later in the ACL, then that ACE
must be prepended by a single DENY ACE. The DENY ACE should have
the same entity and flags as the corresponding ALLOW ACE, but the
bitmask on the DENY ACE should be the bitwise NOT of the bitmask
on the ALLOW ACE, except that the ACE4_WRITE_OWNER, ACE4_DELETE,
ACE4_READ_NAMED_ATTRIBUTES, ACE4_WRITE_NAMED_ATTRIBUTES bits
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should be cleared, and the ACE4_DELETE_CHILD bit should be
cleared on non-directories. (Also, in the xdr-encoded ACL that
is transmitted, all bits not defined in the protocol should be
cleared.)
2. All of the ACL_GROUP_OBJ and ACL_GROUP ACEs are consulted by the
POSIX algorithm before determining permissions. To emulate this
behaviour, instead of adding a single DENY before corresponding
GROUP@ or named group ACEs, we insert a list of DENY ACEs after
the list of GROUP@ and named group ACEs. Each DENY ACE is
determined from its corresponding ALLOW ACE exactly as in the
previous step. As before, these DENY aces should only be added
when they are necessitated by an ACE that is less permissive than
the final EVERYONE@ ace.
Finally, we enforce the POSIX mask ACE by prepending each ALLOW ACE
for a named user, GROUP@, or named group, with a single DENY ACE
whose entity and flags are the same as those for the corresponding
ALLOW ACE, but whose bitmask is the inverse of the bitmask determined
from the mask ACE, with the inverse calculated as described above.
In the case of named users, these DENY aces may be coalesced with any
existing prepended DENY aces. The DENY aces are omitted entirely if
they would have no affect, or if the mask ACE has the same bitmask as
the maximum of the affected ACEs. (With the one exception that if
the POSIX ACL posesses exactly 4 ACEs, then a mask-derived DENY ace
should be inserted before the GROUP@ ace, even if it would not
otherwise be.)
Regardless of what scheme is used to represent the mask, the receiver
will use the first GROUP@ DENY ace to determine the value of the mask
(if it is different from the union of the bitmasks on the affected
ACEs), and use the relevant ALLOWs to determine the pre-mask values
of user and group ACEs.
The implementation may also choose to just mask out the bitmasks on
the relevant ALLOW ACEs. This will produce a simpler ACL (in
particular, an ACL that usually requires no DENY ACE's), at the
expense of losing some ACL information after a chmod.
On directories with default ACLs, we translate the default ACL as
above, but set the 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.
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7. Mapping NFSv4 ACLs to POSIX ACLs
7.1. Requirements
Any mapping of NFSv4 ACLs to POSIX ACLs must map any NFSv4 ACL that
is semantically equivalent to a POSIX ACL (with the exception of the
"minor inaccuracy" mentioned above) to the equivalent POSIX ACL. It
should also extract the mask correctly; as the mask doesn't affect
the semantics of the NFSv4 ACL, and as there is more than one way the
mask might be encoded, we require a convention for this.
Specifically: we require that the mask be computed as the bitmask
used on the first GROUP@ DENY ACE which precedes any GROUP@ allow
ACE, unless no such DENY ACE exists, in which case the mask must be
computed as the union of the bitmasks allowed to all named users,
groups, and GROUP@ (where by the "bitmask allowed to" an entity we
mean the maximum bitmask that the ACL would permit to any user
matching the entity).
Implementations may vary in how they deal with NFSv4 ACLs that are
not precisely semantically equivalent to any POSIX ACL. In
particular they may return errors for such ACLs instead of attempting
to map them. However, when possible without compromising security,
they should attempt to be forgiving.
The language of [1] allows a server some flexibility in 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."
Note that an NFSv4 ACL consisting entirely of ALLOW ACLs can always
be transformed into a POSIX-equivalent ACL by first sorting it into
the canonical order, and then inserting DENY ACEs as necessary to
ensure POSIX first-match semantics. Since inserting DENY ACEs can
only restrict access, it is safe for a server to do this.
We require any server to accept, at least, any NFSv4 ACL that
consists entirely of ALLOW ACLs.
Clients should also be at least as forgiving, to promote
interoperability when heterogeneous clients share files.
7.2. Example NFSv4->POSIX Mapping
We now give an example of an algorithm that meets the above
requirements. We assume it is to be used by a server mapping client-
provided NFSv4 ACLs to POSIX ACLs it can store in its filesystem, so
the translation errs on the side of making the ACL less permissive.
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Given an NFSv4 ACL, first calculate the mask by taking the bitmask
from the first GROUP@ DENY ACE from the original NFSv4 ACL, if it
exists. After doing so, remove that DENY ACE, and clear the bits in
its bitmask from any DENY ACE for a named user, group, or GROUP@
which precedes an ALLOW ACE for the same entity.
In the case where there is no such GROUP@ DENY ACE, continue through
the rest of the algorithm and then calculate the mask as the union of
the calculated permissions of all named users, group, and the
GROUP_OBJ ACE.
Given an NFSv4 ACL, sort it into canonical order (OWNER@ ACEs first,
then user ACEs, then GROUP@ ACEs, then group ACEs, then EVERYONE@
ACEs.) Also, sort the GROUP@ and group ACEs that all ALLOW ACEs
precede all DENY ACEs. To do so, take advantage of the following
observations:
1. If two consecutive ACEs are either both ALLOW ACEs, or both DENY
ACEs, then we can swap their order without changing the effect of
the ACL.
2. If it would be impossible for a single user to match both of the
entities on two consecutive ACEs, then we can swap their order
without changing the effect of the ACL.
3. If an ALLOW ACE is immediately followed by a DENY ACE, then
swapping the order of the two ACEs will not make the ACL any more
permissive.
4. If a DENY ACE is immediately followed by an ALLOW ACE, then
swapping the order of the two ACEs will not make the ACL any more
permissive, *if* we modify the bitmask on the ALLOW ACE by
clearing any bits that are set in the DENY ACE.
The second observation is the trickiest: it may usually be safe to
assume that two distinct user names cannot match the same user. An
implementation with knowledge about group memberships or about the
current value of the file owner might also use that information, but
if it does so it will produce a translation that is no longer
accurate after owners or group memberships change.
Fortunately, observations 1, 3, and 4 are sufficient to sort any ACL
into canonical order, so a paranoid implementation can simply ignore
number 2 completely, while an implementation willing to sacrifice
some accuracy may choose to do something more complex.
Ensure that the resulting ACL posesses at least one each of OWNER@,
GROUP@, and EVERYONE@ ACEs, by inserting an ALLOW ACE with a zero
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bitmask if necessary in the correct position.
Next, for each entity, calculate a bitmask for that entity as
follows: Starting with the first ACE for that entity (ignoring all
previous ACEs), perform the NFSv4 ACL-checking algorithm for a user
that is assumed to match the entity on every DENY ACE that a user
matching the given entity might match, but is assumed to match only
those entities on ALLOW ACEs that *any* user matching the current
entity must match.
Finally, construct the POSIX ACL by translating NFSv4 entity names to
uid's and gid's (and handling special entities in the obvious way),
then assign a POSIX bitmask determined by the NFSv4 bitmask
calculated in the previous step; the bitmask calculation should use
the inverse of the mapping described previously in the POSIX-to-NFSv4
mapping, erring on the side of denying bits if it cannot determine a
sensible mapping. However, if certain bits simply cannot be mapped
in a reasonable way to mode bits, the server may simply ignore them
rather than returning an error. (For example, the server should deny
write if either ACE4_WRITE_DATA or ACE4_APPEND_DATA are denied. But
it may choose to ignore ACE4_READ_ATTRIBUTES entirely.)
The resulting mapping errs on the side of creating a more restrictive
ACE. However it can be modified to produce a mapping that errs on
the side of permissiveness, for the purposes of translating a server-
provided NFSv4 ACL to a POSIX ACL to present to a user or
application, as follows:
1. When sorting ACEs, ALLOW ACEs can always be moved towards the
start of the ACL, but a DENY ACE can be moved towards the start
of the ACL only as long as we clear any of the DENY ACE's bitmask
bits that are set in the intervening ALLOW ACEs.
2. When calculating the NFSv4 bitmask for each entity, err on the
side of assuming that ALLOW ACEs apply and that DENY ACEs don't,
with the one exception that when calculating the GROUP@ and named
group bitmasks, ALLOW ACEs for groups other than the one under
consideration should be ignored.
3. When mapping the NFSv4 bitmask to POSIX mode bits, err on the
side of allowing access.
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8. Security Considerations
Any automatic mapping from one ACL model to another must provide
guarantees as to how the mapping affects the meaning of ACLs, or risk
misleading users about the permissions set on filesystem objects.
For this reason, caution is recommended when implementing this
mapping. It is better to return errors than to break any such
guarantees.
That said, there may be cases where small losses in accuracy can
avoid dramatic interoperability and usability problems; as long as
the losses in accuracy are clearly documented, these tradeoffs may be
found acceptable.
For example, a server unable to support all of the NFSv4 mode bits
does not have a way to communicate its exact limitations to clients,
so clients (and users) may be unable to recover from such errors.
For this reason we recommend ignoring bitmask bits that the server is
completely unable to map to mode bits, and advertising this fact
loudly in the server documentation. If this is considered
insufficient, we should add to the NFSv4 protocol additional
attributes necessary to advertise the server's limitations.
Note also that this ACL mapping requires mapping between NFSv4
usernames and 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.
9. References
[1] Shepler, S., Callaghan, B., Robinson, D., Thurlow, R., Beame,
C., Eisler, M., and D. Noveck, "Network File System (NFS)
version 4 Protocol", RFC 3530, April 2003.
[2] Institute of Electrical and Electronics Engineers, Inc., "IEEE
Draft P1003.1e", October 1997,
<http://wt.xpilot.org/publications/posix.1e/download.html>.
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Authors' Addresses
Marius Aamodt Eriksen
U. of Michigan Center for Information Technology Integration
Email: marius@citi.umich.edu
J. Bruce Fields
U. of Michigan Center for Information Technology Integration
Email: marius@citi.umich.edu
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