wdiff draft-ietf-nfsv4-acl-mapping-04.txt draft-ietf-nfsv4-acl-mapping-05.txt

Network Working Group M. Eriksen
Internet-Draft J. Fields
Expires: November 16, 2006 February 23, 2007 CITI
May 15,
August 22, 2006

Mapping Between NFSv4 and Posix Draft ACLs
draft-ietf-nfsv4-acl-mapping-04
draft-ietf-nfsv4-acl-mapping-05

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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withdrawn POSIX draft [2].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.

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, 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withdrawn POSIX draft.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.

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

An ACL may be have a "type" of 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
First, assume a requester asks for access, as access 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 access is undefined. normally
denied.

If
an a requester asks for access requiring multiple bits from the
access bitmask simutaneously, then we allow the access mask consisting of more than one bit is requested, it
succeeds if and only if
each bit in the mask is allowed. requested bitmask would be allowed individually.

We refer the reader to [1] for further details.

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". ACLs", though they are not part of any published POSIX
standard.

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: by consulting the ACEs in the order ACL_USER_OBJ,
ACL_USER, ACL_GROUP_OBJ, ACL_GROUP, ACL_OTHER, and allowing or
denying access based on the first ACE encountered that the requester
matches, except that we never allow the ACL_USER, ACL_OWNER_OBJ, or
ACL_GROUP objects to grant more than the ACL_MASK object does, and in
the case of ACL_GROUP_OBJ and ACL_GROUP ACEs, we allow access if any
one of those ACEs allows access.

In more detail:

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 matches the file's group, and the ACL mask ACE
would deny the requested access, then skip to step 5. Otherwise,

3. 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,

4. 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.
5. 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
inherited by newly created objects in the directory.

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.

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.

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 A server or client to may use a different mapping, provided but
the mapping meets should meet 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 resulting NFSv4 ACL must take into account the mask ACE, by first applying
ensuring that it to the USER, GROUP, and GROUP_OBJ ACEs, and then
mapping does not give the resulting ACL. However, that will result group file owner or any users or
groups named in an the 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 more permissions than permitted by the resulting
ACL is later translated back mask.
It would also be possible to specify a POSIX ACL, mapping that encoded the translator will
assume mask
in such a way that the original value of the mask is the union of the bitmasks
permitted to any USER, GROUP, or GROUP_OBJ. If that would could be
incorrect, the original translation should not modify recovered
by someone that knew the bitmasks of ACL was produced by our algorithm. However,
the USER, GROUP, and GROUP_OBJ bitmasks, added complexity and should instead use
additional DENY ACEs as necessary to give the effect fragility of the mask. It
should also arrange for the first GROUP@ ACE to be such a DENY ACE whose
bitmask mapping is determined by not worth the mask, allowing that ACE to be used to
determine
small benefit of preserving the original mask value. information, so we do not
attempt that here.

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 modify all ACL_USER, ACL_GROUP, and ACL_GROUP_OBJ ACEs by
removing any permissions not granted by the mask ACE. The mask ACE
may then be ignored for the rest of this process.

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@", 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 preceded 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, and ACE4_WRITE_NAMED_ATTRIBUTES bits
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 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, ACE.

On directories with a single DENY ACE
whose entity and flags are default ACLs, we translate the same default ACL as those for the corresponding
ALLOW ACE,
above, but whose bitmask is the inverse of set the bitmask determined
from ACE4_INHERIT_ONLY_ACE, ACE4_DIRECTORY_INHERIT_ACE,
and ACE4_FILE_INHERIT_ACE flags on every ACE in the mask ACE, resulting ACL.
On directories 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 both default and access ACLs, we translate the
two ACLs and then concatenate them. The order of the concatenation
is unimportant.

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 an equivalent POSIX ACL. It
should also extract the mask correctly; as the mask doesn't affect

However, a more difficult problem is presented by NFSv4 ACLs that are
not precisely equivalant to any POSIX ACL.

The only way that the semantics NFSv4 protocol gives servers to indicate that
they support only a subset of the NFSv4 ACL, and as there ACL model is more than one way the
mask might be encoded, we require "aclsupport"
attribute, which allows a convention for this.
Specifically: we require server to advertise that the mask be computed as the bitmask
used on the first GROUP@ DENY it only supports
certain ACE which precedes any GROUP@ allow
ACE, unless no such DENY ACE exists, in which case the mask must types. This allows a server to report that it only
supports ALLOW ACEs, or that it does not support AUDIT or ALARM ACEs
(which will be
computed as the union of the bitmasks allowed case for most servers with only POSIX ACLs). But
it does not give a way to all named users,
groups, and GROUP@ (where by the "bitmask allowed to" an entity we
mean the maximum bitmask that claim support for more complex subsets of
the ACL would permit model.

While it is possible for a server to reject any user
matching the entity).

Implementations may vary in how they deal with NFSv4 ACLs that are do not precisely semantically equivalent fit
its ACL model, this places a large burden on clients and users, since
the server has no way to any POSIX ACL. In explain why it rejected a particular they may return errors for such ACLs instead of attempting ACL.
Therefore, it is preferable to map them. However, when be more forgiving, whenever that is
possible without compromising security,
they should attempt and to be forgiving. limit any restrictions
to those that are easily documented and verified by users.

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

ACLs with arbitrary sequences of ALLOWs and DENYs may be particularly
troublesome; but 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

Therefore servers should 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.

Given an NFSv4 ACL, first calculate more restrictive.

In fact, if we ignore some loss of information in the mask by taking the bitmask
from ACE, this
mapping takes an NFSv4 ACL to the first GROUP@ DENY ACE from unique most permissive POSIX ACL
that is no more permissive than the original given NFSv4 ACL, ACL.

Before starting, if it
exists. After doing so, remove that DENY ACE, and clear the bits ACL in
its bitmask from any DENY ACE question is 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 directory, we split
it into canonical order (OWNER@ ACEs first,
then user ACEs, then GROUP@ ACEs, then group ACEs, then EVERYONE@
ACEs.) Also, sort the GROUP@ two ACLs, one purely effective and group ACEs that all ALLOW ACEs
precede all DENY ACEs. To do so, take advantage of the following
observations: one purely inherited, as
follows:

1. If two consecutive ACEs with no inheritance flags are either put in the purely effective
ACL.

2. Aces with FILE_INHERIT and DIRECTORY_INHERIT both ALLOW ACEs, or set are put in
both DENY
ACEs, then we can swap their order without changing the effect of effective and the inherited ACL

3. Aces with FILE_INHERIT, DIRECTORY_INHERIT, and INHERIT_ONLY all
set are put only in the inherited ACL.

2. If it would

Other combinations of ineritance flags may be impossible for a single user rejected or silently
modified to match both one of the
entities on two consecutive ACEs, above.

The main algorithm that follows is then we can swap their order
without changing performed on each ACL, with
one used to set the effect of effictive ACL, and one the default ACL.

3. If an ALLOW

First, we calculate the OTHER mode as follows:

1. Initialize the bitmasks other_allow and other_deny both to zero.

2. For each ACE is immediately followed by a DENY ACE, then
swapping in the order of ACL, starting from the two ACEs will top:

1. If the ACE is not make an EVERYONE@ ACE, ignore it and move to the ACL any more
permissive.

4.
next ACE.

2. If a DENY the ACE is immediately followed by an EVERYONE@ 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 add to other_allow
any bits that are set in this ACE but not set in other_deny.

3. If the DENY ACE.

The second observation ACE 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
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 ACE, then add to
uid's and gid's (and handling special entities other_deny
any bits set in the obvious way),
then assign a POSIX bitmask determined by the NFSv4 bitmask
calculated this ACE but not set in other_allow.

3. Discard other_deny. Set the previous step; the bitmask calculation should use USER_OBJ mask from other_allow using
the inverse of the mapping described previously in the POSIX-to-NFSv4 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 entirely; though in that case it can be modified may at
least want to produce a mapping that errs on treat specially the side of permissiveness, for the purposes of translating case where such bits are
explicitly denied by some DENY ACE.)

Note that the bits determined above are exactly the maximum bits that
will always be permitted to a user that doesn't match the file owner
or group, or any of the named owners or groups. Thus this choice of
the OTHER mode is exactly the maximum choice we can safely make.

Next we calculate the GROUP_OBJ and GROUP masks.

1. Initialize to zero an allow and deny bitmask for each GROUP_OBJ
and for each GROUP mask.

2. For each ACE in the ACL, starting from the top:

1. If the ACE is an OWNER@ or named user ACE, ignore it and move
to the next ACE.

2. If the ACE is an EVERYONE@ ALLOW ACE, then, for each GROUP or
GROUP_OBJ allow mask, set the bits allowed in the EVERYONE
ACE but not already in this GROUP or GROUP_OBJ's deny mask.

3. If the ACE is an EVERYONE@ DENY ACE, then, for each GROUP or
GROUP_OBJ deny mask, set the bits denied in the EVERYONE ACE
but not already allowed in this GROUP or GROUP_OBJ's deny
mask.

4. If the ACE is a GROUP or GROUP@ ALLOW ACE, then set the allow
bits in the corresponding GROUP or GROUP_OBJ allow mask that
are allowed by this ACE but not already denied by the
corresponding GROUP or GROUP_OBJ deny mask.

5. If the ACE is a GROUP or GROUP@ DENY ACE, then set the deny
bits in the corresponding GROUP or GROUP_OBJ deny mask that
are denied by this ACE but not already allowed by the
corresponding GROUP or GROUP_OBJ allow mask. Call the
resulting deny mask "m". In each GROUP or GROUP_OBJ deny
mask, set every bit that is in m and not already in that
GROUP or GROUP_OBJ allow mask.

3. Having calculated allow and deny masks for GROUP_OBJ and each
GROUP, we now set the corresponding modes from the allow masks as
we did in the last step of the USER_OBJ mask calculation above.

Note that the bits thus determined for a group are exactly the
maximum bits that will always be permitted to a user that matches the
group in question, and that is denied any bits that could be denied
by matching other groups, without out being allowed bits by matching
any such groups. This is the most permissive mode we can choose that
will never permit more permissions than the original NFSv4 ACL, for
any possible choice of group memberships.

An implementation with special knowledge about the current gowning
group or about group memberships may choose to use that knowledge to
calculate a more permissive mode. However, doing so may render
resulting POSIX ACL inaccurate after the owning group changes, or
after any group memberships change.

Next, we calculate USER modes by first calculating allow and deny
masks for each USER as above, this time assuming we are a user that
does not match the file owner, that matches no user except for the
one user under consideration, and that matches groups only when they
would deny some permissions that they have not allowed yet. (To
ensure this last step it will also be necessary to maintain group
allow and deny ACEs, as we did in the previous calculation.) We omit
the detailed steps, which are similar. Again, the implementation may
choose to use special knowledge about group memberships at the risk
of increased complexity and of loss of some accuracy.

Next, we calculate the USER_OBJ mode by calculating allow and deny
masks for a user that matches the file owner and any user or group
that denies bits that it does not first allow.

Finally, if the resulting ACL has any named user or group ACEs, add a
mask ACE with bitmask equal to the union of the calculated
permissions of all named users, group, and the GROUP_OBJ ACE.

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 performing the final mapping from the allow bitmask to a server-
provided
mode, we instead using a mapping that errs on the side of
permissiveness; for example, we allow write permissions even if
only one of WRITE_DATA, APPEND_DATA, or (in the case of
directories) DELETE_CHILD is allowed.

2. Wherever in the above we pessimistically assume that a user will
match any entity that has permissions denied to it before they
are first allowed, we instead assume that the user will match any
entity that has permissions allowed to it before they are first
denied.

Once again, the resulting mapping may be seen to produce the unique
(up to choice of mask) POSIX ACL which is the most restrictive among
all POSIX ACLs no more restrictive than the given NFSv4 ACL.

Note that the above algorithms may be optimized in a number of ways:
for example, although they are described in terms of multiple passes,
it will be simpler and more efficient to calculate the entire POSIX
ACL in a single pass.

8. Backwards Compatibility

Previous versions of this document recommended a different
POSIX->NFSv4 mapping, which enforces POSIX semantics by inserting
DENYs into the ACL even when those DENY's would have no effect, and
which represents the POSIX mask ACE using additional DENYs. The
resulting ACLs are overly complex and create problems for Windows
clients, because the default Windows ACL editor prefers to order
DENYs before ALLOWs.

The NFSv4 to POSIX mapping we describe in this document can accept
the NFSv4 ACLs produced by the old mapping.

However, previous versions of this document also recommended
accepting only NFSv4 ACLs that were precisely those produced by the
old POSIX->NFSv4 mapping; therefore, existing implementations of that
recommendation will reject the NFSv4 ACLs produced by the newer
mapping.

We strongly recommend fixing implementations to accept a wider range
of NFSv4 ACLs. However, we briefly document the old mapping here in
case that is impossible:

Names, bitmasks, and flags are determined as in the the current
mapping.

Whenever the following instructions requiring taking "the complement"
of an NFSv4 ACL to a POSIX ACL to present to a user or
application, bitmask, do so as follows:

1. When sorting ACEs, ALLOW ACEs can always be moved towards first, take the
start bitwise NOT of
the ACL, but a DENY ACE can be moved towards bitmask. Then clear the start
of ACE4_WRITE_OWNER, ACE4_DELETE,
ACE4_READ_NAMED_ATTRIBUTES, and ACE4_WRITE_NAMED_ATTRIBUTES bits.
Also, clear the ACL only as long as we ACE4_DELETE_CHILD bit on non-directories, and clear
any of bits not defined in the protocol.

Create one ALLOW ACE for each entity (OWNER@, GROUP@, and EVERYONE@,
and each user and group named in the given POSIX ACL). After each
OWNER@, EVERYONE@, and named user ACE, append a DENY ACE's ACE with the
same entity and flags as the corresponding ALLOW ACE, but with
bitmask
bits that are set in to the complement (as defined above) of the intervening ALLOW ACEs.

2. When calculating ACE.

Do the NFSv4 bitmask same for each entity, err on the
side GROUP@ and named group ACE, but instead of assuming that
inserting each new DENY ACE after the corresponding ALLOW ACEs apply and that ACE, insert
all of the DENY ACEs don't,
with at the one exception end of the list of GROUP@ and named group
ACEs, in the same order that when calculating the GROUP@ and named group bitmasks, ALLOW ACEs for groups other than
occur in.

Finally, prepend each GROUP@, named user, and named group ACE by a
single DENY whose entity and flags are the one under
consideration should be ignored.

3. When mapping same as the NFSv4 corresponding
ALLOW, but whose bitmask to POSIX mode bits, err on is the
side complement (as defined above) of allowing access.

8. the
bitmask determined from the mask ACE in the given POSIX ACL. Skip
this step if the given POSIX ACL has no mask ACE.

9. 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 at least when no ACE
explicitly contradicts the server documentation. server's default behavior. 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 any ACL mapping also 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.

10. 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>.

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 bfields@citi.umich.edu

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