--- 1/draft-ietf-nfsv4-acl-mapping-03.txt 2006-05-16 22:12:30.000000000 +0200 +++ 2/draft-ietf-nfsv4-acl-mapping-04.txt 2006-05-16 22:12:31.000000000 +0200 @@ -1,435 +1,563 @@ -Network Working Group Marius Aamodt Eriksen -Internet Draft J. Bruce Fields -Document: draft-ietf-nfsv4-acl-mapping-03.txt February 2005 + +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 Status of this Memo - By submitting this Internet-Draft, I certify that any applicable - patent or other IPR claims of which I am aware have been disclosed, - or will be disclosed, and any of which I become aware will be dis- - closed, in accordance with RFC 3668. - - This document is an Internet-Draft and is in full conformance with - all provisions of Section 10 of RFC2026. + By submitting this Internet-Draft, each author represents that any + applicable patent or other IPR claims of which he or she is aware + have been or will be disclosed, and any of which he or she becomes + aware will be disclosed, in accordance with Section 6 of BCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet- Drafts. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any - time. It is inappropriate to use Internet- Drafts as reference mate- - rial or to cite them other than as "work in progress." + time. It is inappropriate to use Internet-Drafts as reference + material or to cite them other than as "work in progress." The list of current Internet-Drafts can be accessed at - http://www.ietf.org/ietf/1id-abstracts.txt + http://www.ietf.org/ietf/1id-abstracts.txt. The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html. - "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 A number of operating sys- - tems use a different flavor of ACL based on a withdrawn POSIX draft.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 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. + This Internet-Draft will expire on November 16, 2006. -Mapping NFSv4 ACLs February 2005 +Copyright Notice -Table of Contents + Copyright (C) The Internet Society (2006). - 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 . . . . . . . . . 9 - 6. Security Considerations . . . . . . . . . . . . . . . . . 11 - 7. Bibliography . . . . . . . . . . . . . . . . . . . . . . . 12 - 8. Author's Address . . . . . . . . . . . . . . . . . . . . . 13 - 9. Copyright . . . . . . . . . . . . . . . . . . . . . . . . 13 +Abstract -Mapping NFSv4 ACLs February 2005 + 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, 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. We - will describe such a mapping, and discuss its use in NFSv4 clients - and servers. + 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 associ- - ated with the file), and "EVERYONE@". + "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 [rfc3530] for the full list. + 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 - -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. + 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 [rfc3530] for further details. + 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 [posixacl]. We will refer to - such ACLs as "POSIX ACLs". + 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, 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. + 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 + 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 + 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 + 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 match- - ing ACEs is nonempty, and none allow access, then deny access. - Otherwise, if the set of matching ACEs is empty, - -Mapping NFSv4 ACLs February 2005 + 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, + 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 direc- - tory, and one, the "default ACL", used only as the ACL to be inher- - ited by newly created objects in the directory. + 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. Mapping POSIX ACLs to NFSv4 ACLs +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 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. + 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. This is an implementation-dependent process. - It might be done, for example, by 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@", and "EVERYONE@", respectively. + 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. + 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 direc- - tory, set ACE4_DELETE_CHILD as well. + 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. + 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 + 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 + 5. If the ACE is for the special "OWNER@" entity, set ACE4_WRITE_ACL and ACE4_WRITE_ATTRIBUTES. -Mapping NFSv4 ACLs February 2005 - - 6) Clear all other bits in the NFSv4 bitmask. + 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). To - emulate the POSIX ACL permission-checking algorithm, we need to mod- - ify the ACL further, as follows: - - 1) Order the ACL so that the OWNER@ ACE is the first ACE of the ACL, - followed by any user ACEs, followed by the GROUP@ ACE, followed by - any group ACEs, and ending finally with the EVERYONE@ ACE. + entity ACEs). - 2) The POSIX algorithm stops as soon as the 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 for OWNER@, - EVERYONE@, or any named user. The DENY ACE should have the same - entity and flags as the corresponding ALLOW ACE. The bitmask on - the DENY ACE should be the bitwise NOT of the bitmask on the ALLOW - ACE, except that the 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 xdr-encoded ACL that is transmit- - ted, all bits not defined in the protocol should be cleared.) + 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. - 3) Unlike the other ACEs in step 2, all of the ACL_GROUP_OBJ and - ACL_GROUP ACEs are consulted by the 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 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. + Order this NFSv4 ACL in the canonical order: OWNER@, users, GROUP@, + groups, then EVERYONE@. - 4) 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 corre- - sponding ALLOW ACE, but whose bitmask is the inverse of the bit- - mask determined from the mask ACE, with the inverse calculated as - described in step 2. + 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. - 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 required - ACL_USER_OBJ, ACL_GROUP_OBJ, ACL_OTHER, and ACL_MASK ACEs. + Otherwise, we need to insert additional DENY ACE's to emulate the + first-match semantics of the POSIX ACL permission-checking algorithm: - Such an ACL will map to an NFSv4 ACL of the form + 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 + 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.) -Mapping NFSv4 ACLs February 2005 + 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. - 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@ + 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.) - where the ACEs marked with (mask) are those whose bitmask are deter- - mined from the ACL_MASK ACE as described in step 4 above. + 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. - In general, a POSIX ACL with m named users and n named groups will - map to an NFSv4 ACL with (3*(m + n) + 7) ACLs, unless m and n are - both zero, in which case the result will have either 6 or 7 ACLs, - depending on whether the original ACL had an ACL_MASK ACE. + 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. - There is one extremely minor inaccuracy in this mapping: 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 +7. Mapping NFSv4 ACLs to POSIX ACLs - ACL_USER_OBJ: --- - ACL_GROUP_OBJ: --- - g1: r-- - g2: -w- - ACL_MASK: rw- - ACL_OTHER: --- +7.1. Requirements -Mapping NFSv4 ACLs February 2005 + 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). - 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. + 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 NFSv4 ACL permission-checking algorithm has the property that it - permits a group of bits whenever it would permit each bit individu- - ally, so it is impossible to mimic this behaviour with an NFSv4 ACL. + 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." -5. Using the Mapping in NFSv4 Implementations + 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. - Examination of the algorithm described in the previous section shows - that no information is lost; the original POSIX ACL can be recon- - structed from the mapped NFSv4 ACL. Thus we also 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. + We require any server to accept, at least, any NFSv4 ACL that + consists entirely of ALLOW ACLs. - 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. + Clients should also be at least as forgiving, to promote + interoperability when heterogeneous clients share files. - 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 - is not in precisely the format of an ACL produced by this mapping, - then the server may be required to return an error to avoid inaccu- - rately representing the client's intention. The correct error to - return in this case is NFS4ERR_ATTRNOTSUPP. +7.2. Example NFSv4->POSIX Mapping - 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 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." + 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 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 + 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. -Mapping NFSv4 ACLs February 2005 + 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. - that will satisfy a server using the mapping. By making the mapping - more forgiving, the server can simplify that task, improving interop- - erability. + 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: - Servers that implement the full NFSv4 protocol should also handle - carefully ACLs that leave bits neither allowed nor denied. It is - better to 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 because it intends to deny - ACE4_APPEND_DATA. By leaving the bit unspecified, the client leaves - the server the opportunity to provide the reasonable default of set- - ting it to match ACE4_WRITE_DATA. + 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. - Similar issues exist when a client uses NFSv4 ACLs to implement user - interfaces that only deal in POSIX ACLs. When the client translates - ACLs received from the server to POSIX ACLs, some flexibility may - help interopability, but the client must take care not to represent - any ACLs as stricter than they really are. Clients that provide - access to the full set of NFSv4 ACLs may also wish to provide users - with utilities to generate and interpret POSIX-mapped NFSv4 ACLs, to - aid users working with servers using the POSIX mapping. + 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. -Mapping NFSv4 ACLs February 2005 + 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. -6. Security Considerations + 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 + 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. + +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 map- - ping. It is better to return errors than to break any such guaran- - tees. + For this reason, caution is recommended when implementing this + mapping. It is better to return errors than to break any such + guarantees. - Note also that this ACL mapping requires mapping between NFSv4 user- - names and local id's. When the mapping of id's depends on remote + 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. -Mapping NFSv4 ACLs February 2005 - -7. Bibliography +9. References - [rfc3530] - Shepler, S. et. al., "NFS version 4 Protocol", April 2003. + [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. - http://www.ietf.org/rfc/rfc3530.txt + [2] Institute of Electrical and Electronics Engineers, Inc., "IEEE + Draft P1003.1e", October 1997, + . - [posixacl] - IEEE, "IEEE Draft P1003.1e", October 1997 (last draft). +Authors' Addresses - http://wt.xpilot.org/publications/posix.1e/download.html + Marius Aamodt Eriksen + U. of Michigan Center for Information Technology Integration -Mapping NFSv4 ACLs February 2005 + Email: marius@citi.umich.edu -8. Author's Address + J. Bruce Fields + U. of Michigan Center for Information Technology Integration - Address comments related to this memorandum to: + Email: marius@citi.umich.edu - marius@umich.edu bfields@umich.edu +Intellectual Property Statement - Marius Aamodt Eriksen - J. 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