Network Working Group Marius Aamodt Eriksen Internet Draft J. Bruce Fields Document:
draft-ietf-nfsv4-acl-mapping-01.txtdraft-ietf-nfsv4-acl-mapping-02.txt October 2004 Mapping Between NFSv4 and Posix Draft ACLs SSttaattuuss ooff tthhiiss MMeemmooStatus 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. 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." The list of current Internet-Drafts can be accessed at 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." AAbbssttrraacctt TheAbstract NFS (Network File System)version 4[rfc3010bis]4 [rfc3530] (NFSv4) specifies a flavor of Access Control Lists (ACLs) that resembles that of Win- dows NT's. ACLs are used to specify fine grained control of access to file system objects. An ACL consists of aresembling Windows NT ACLs. A number of Access Con- trol Entries (ACEs), each specify some level of access for an entity; an entity can be aoperating sys- tems use a user, group ordifferent flavor of ACL based on a special entity. The access level is described using an access mask, which iswithdrawn POSIX draft. NFSv4 clients and servers on such operating systems may wish to map Mapping NFSv4 ACLs October 2004 between NFSv4 ACLs and their native ACLs. To this end, we describe a bitmask where each bit describesmapping from POSIX draft ACLs to a levelsubset of access, for example read, write and execute permissions on the file system object.NFSv4 ACLs. Mapping NFSv4 ACLs FebruaryOctober 2004 TTaabbllee ooff CCoonntteennttss 11.. IInnttrroodduuccttiioonnTable of Contents 1. Introduction . . . . . . . . . . . . . 3 22.. NNFFSSvv44 AACCLLss. . . . . . . . . . . 4 2. NFSv4 ACLs . . . . . . . . . . . 3 33.. PPOOSSIIXX AACCLLss. . . . . . . . . . . . . . 4 44.. MMaappppiinngg PPoossiixx AACCLLss ttoo NNFFSSvv44 AACCLLss3. POSIX ACLs . . . . . . . . . . . . . . . . . . . . . . . . . 5 55.. SSeeccuurriittyy CCoonnssiiddeerraattiioonnss4. Mapping Posix ACLs to NFSv4 ACLs . . . . . . . . . . . . . 7 66.. BBiibblliiooggrraapphhyy. 6 5. Using the Mapping in NFSv4 Implementations . . . . . . . . . 8 77.. AAcckknnoowwlleeddggmmeennttss6. Security Considerations . . . . . . . . . . . . . 9 88.. AAuutthhoorr''ss AAddddrreessss. . . . 10 99.. CCooppyyrriigghhtt7. Bibliography . . . . . . . . . . . . . 10. . . . . . . . . . 11 8. Author's Address . . . . . . . . . . . . . . . . . . . . . 12 9. Copyright . . . . . . . . . . . . . . . . . . . . . . . . 12 Mapping NFSv4 ACLs FebruaryOctober 2004 11.. IInnttrroodduuccttiioonn The NFS (Network File System) version 4 [rfc3010bis] (NFSv4) speci- fies a flavor of1. Introduction Access Control Lists (ACLs) that resembles that of Windows NT's. ACLsare used to specify fine grained control offine-grained access rights to file system objects. An ACL consists of a number of Access Control Entries (ACEs), each specifying some level of access for an entity; anentity. The entity canmay be a auser, groupa group, or a special entity.entity (such as "everyone"). The accesslevel of access is described using an access mask, which is a bitmask wherewith each bit describescorresponding to a leveltype of access, for example read, writeaccess (such as "read" or "append"). In the following sections we describe two ACL models: NFSv4 ACLs, and execute permissionsACLs based on the file system object. 22.. NNFFSSvv44 AACCLLssa withdrawn POSIX draft, which we will refer to as "POSIX ACLs". Since NFSv4 ACLs are richmuch finer-grained than POSIX ACLs, it is not possible in nature and expand upongeneral to map an arbitrary NFSv4 ACL to a POSIX ACL with the traditional idea of ACLs. Ansame semantics. It does, however, turn out to be possible to map any POSIX ACL to a NFSv4 ACE canACL that has nearly iden- tical semantics. We will describe such a mapping, and discuss how it might be used in NFSv4 client and server implementations. 2. NFSv4 ACLs An NFSv4 ACL is an ordered sequence of type ALLOW, DENY, LOG or ALARM;ACEs, each specifieshaving an entity, a different action to take shouldtype, and an access mask. The entity may be the ACE matchname of a current request. NFSv4 ACLsuser or group, or may also havebe one of a richsmall set of special entities. Among the special entities are "OWNER" (the current owner of the file), "GROUP" (the group associated with the file), and "EVERYONE". The access mask includes bits for access types that com- plementsare more fine- grained than the traditional types. These include appending data"read", "write", and "execute" permis- sions used in UNIX mode bits. The type may be ALLOW or DENY. (AUDIT or ALARM are also allowed, but they are not relevant to the file object, deleting children of the file object, deleting the file object, etc [rfc3010bis].our discussion). The NFSv4 ACLs are interpreted inACL permission-checking algorithm is straightforward. Given an ACL and a requestor asking for a straightforward manner.set of permissions speci- fied by an access mask: 1) Walk through the list of ACEs from the ACL in orderorder. 2) If the "who" (entity) field in the ACE does not match that of the requester, the particularIgnore any ACE isfor with an entity not processed.matching requestor. Mapping NFSv4 ACLs October 2004 3) Process all ACEs until all the bits in the requested access mask have been ALLOWed; onceALLOWed by an ALLOW ace with that bit set. Once a particularpar- ticular bit has been ALLOWed by an ACE, it is no longer considered in further processing. 4) If a particularbit in the requested access mask is DENYed (while that bit is still under consideration), the request is denied. 5) If all bits have been ALLOWed, the access is granted, or elsegranted. Otherwise behavior is undefined. NFSv4 ACLsThere are also specify a number of special entities such as OWNER, GROUP, and EVERYONE. These refer to the traditional UNIX mode bits. Others include DIALUP, BATCH, and AUTHENTICATED, which have special- ized uses. Mapping NFSv4 ACLs February 2004 Additionally the NFSv4 ACLs specifya number of flags that can be applied to an ACL. These include a specification on how an ACL onNFSv4 ACE. Three flags that we will need to use in the following discussion apply to ACEs in a directory mayACL. They are: ACE4_DIREC- TORY_INHERIT_ACE, which indicates that the ACE should be propagatedadded to newly created files or directories insidenew subdirectories of said directory. It is clearthe directory; ACE4_FILE_INHERIT_ACE, which does the same for new files; and ACE4_INHERIT_ONLY, which indicates that the granularity ofACE should be ignored when determining access control that NFSv4 ACLs specify is well beyondto the standard UNIX capability of expressing file system object permissions. 33.. PPOOSSIIXX AACCLLss POSIX ACLsdirectory itself. We refer the reader to [rfc3530] for further details. 3. POSIX ACLs A number of operating systems, including Linux and FreeBSD, implement ACLs based on the withdrawn POSIX 1003.1e/1003.2c Draft Standard 17 [posix- acl], which was meant[posixacl]. We will refer to specify asuch ACLs as "POSIX ACLs". POSIX standard for ACLs, but unfortunately never materialized. However, many systems stillACLs use it, both inaccess masks with only the form of it's latest draft as well as earlier drafts.traditional "read", "write", and "execute" bits. Each ACE in a POSIX ACLs are simpler than their NFSv4 equivalents.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 anhas an entitya uid associated with it, and the traditional UNIX mode bits that are assigned to the particular entity.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 entityACL_MASK ace is required if the ACL has any ACL_USER or ACL_GROUP aces. There may not be an arbitrary UID or GID or one of a few special entities,two ACL_USER aces with the most notable of which issame uid, and there may not be two ACL_GROUP aces with the ACL_MASK entity. POSIX ACLs are also interepreted differently than their NFSv4 equivalents.same gid. Given a POSIX ACLs are interpretedACL and a requestor asking for access, permission is determined as follows: 1) ProcessIf the ACL_USER_OBJ (equivalent to UNIX file owner) ACE first; ifrequestor is the UID of the requester does not match that of the ACL_USER_OBJ,file owner, then allow or deny access depending on whether the ACL_USER_OBJ ACE is ignored.allows or denies it. Mapping NFSv4 ACLs October 2004 Otherwise, 2) if the requester's access mask is allowed byrequestor's uid matches the access maskuid of the ACE, the request is granted, else the request is denied. 2) Process allone of the ACL_USER ACEs; the entity of these ACEs specify a userACE's, then allow or deny access depending on whether the system. IfACL_USER_OBJ ACE allows or denies it. Otherwise, 3) Consider the UIDset of all ACL_GROUP ACE's whose gid the requester does not matchrequestor is a member of. Add to that ofset the ACL_GROUP_OBJ ACE, then the ACE is ignored. Otherwise,if the requester's access maskrequestor is allowed by the access maskalso a member of the ACE, the request is granted, else the request is denied. 3) Process the ACL_GROUP_OBJ ACE and allthat group. Allow access if one of the ACL_GROUP ACEs; the entity of these ACEs specify a group onACE's in the system.resulting set allows access. If none ofthe GIDsset of the requester match the current ACE, the particular ACE is ignored. For anymatching ACE, if the the requester's access mask is allowed by theACEs access mask, then accessis permitted. If there are matching ACEs, butnonempty, and none allow access, then access is denied. Mapping NFSv4 ACLs February 2004deny access. Otherwise, if none of these ACEs match, 4) Ifif the requester's access mask is allowed by the ACL_OTHER ACE, then grant access. 5) DenyOtherwise, deny access. Steps (2) and (3) have an additional criteria; in addition to check- ing whether the requested access mask is allowed by the access mask in the ACE, the requested bits also have to be in the access mask of the special ACE with the ACL_MASK entity. This allows file owners to specify a maximum level of access allowed by any other user or group that has any access to the file system object. In addition to a regular POSIX ACL, a directory in the file system may also have associated with it a default ACL. This default ACL does not affect permissions to the directory itself. Instead, it governs the ACL a file system object will be assigned initially when it is created as a child of the particular directory. 44.. MMaappppiinngg PPoossiixx AACCLLss ttoo NNFFSSvv44 AACCLLss4. Mapping Posix ACLs to NFSv4 ACLs Given the difference in both extensiveness and interpretation ofdifferences between POSIX and NFSv4 ACLs, any conversion between the two is difficult. However, POSIX ACLs are a subset of NFSv4 ACLs. AnyACLs, and any POSIX ACL can be emulated with an NFSv4 ACL using the following mapping. The ACE entities are translated as follows. The non-special entities in form of UIDsFirst, the uid's and GIDs isgid's on the ACL_USER and ACL_GROUP ACEs must be translated to equivalent strings (a sys- tem dependentinto NFSv4 names--a system-dependent process, typicallywhich, on UNIX for example, may be done by lookups to /etc/passwd in UNIX). The POSIX ACL_USER_OBJ entity is/etc/passwd. Also, the special ACL_USER_OBJ, ACL_GROUP_OBJ, and ACL_OTHER ACEs must be translated to the "OWNER"NFSv4 entity. Similary,ACEs with the POSIX ACL_GROUP_OBJ is translated to the "GROUP" NFSv4 entity. The ACL_OTHER entity is translated to the "EVERYONE" NFSv4 entity.special entities "OWNER", "GROUP", and "EVERYONE", respectively. The ACE access mask is translated as follows. The read bit of the POSIX access mask is translated to the logical OR of the Mapping NFSv4 ACLs October 2004 ACE4_READ_DATA and ACE4_READ_NAMED_ATTRS NFSv4 access mask fields. The write bit of the POSIX access mask is translated to the logical OR of the ACE4_WRITE_DATA, ACE4_WRITE_NAMED_ATTRIBUTESACE4_WRITE_NAMED_ATTRS and ACE4_APPEND_DATA NFSv4 access mask fields. The execute bit of the POSIX access mask is translated into the ACE4_EXECUTE and ACE4_READ_DATA NFSv4 acess mask fields. Note that NFSv4 defines ACE4_READ_DATA, ACE4_WRITE_DATA, and ACE4_APPEND_DATA to be equal to ACE4_LIST_DIRECTORY, ACE4_ADD_FILE, and ACE4_ADD_SUBDIRECTORY, respectively, so this translation makes sense for directories as well. However, on directories the ACE4_DELETE_CHILD field must be included in the translation of the POSIX write bit. Mapping NFSv4 ACLs February 2004In addition to the above, the OWNER entity must always be given ACE4_WRITE_ACL and ACE4_WRITE_ATTRIBUTES, and all entities must be given ACE4_READ_ACLACE4_READ_ACL, ACE4_READ_ATTRIBUTES, and ACE4_READ_ATTRIBUTES.ACE4_SYNCHRONIZE. The ACE4_DELETE bit should be neither allowed nor denied by any ACE. The ACE flag field also has a simple translation. If the file system object is a directory, and the POSIX ACE belongs to a default ACL, the "ACE4_INHERIT_ONLY_ACE" flag isACE4_INHERIT_ONLY_ACE, ACE4_DIRECTORY_INHERIT, and ACE4_FILE_INHERIT flags are set in the NFSv4 ACE. If the entity in the POSIX ACE refers to a group, the "ACE4_IDENTI- FIER_GROUP""ACE4_IDENTIFIER_GROUP" flag is set in the NFSv4 ACE. The POSIX ACL_USER_OBJ ACE is also always given the permission bits "ACE4_READ_ACL" and "ACE4_WRITE_ACL." Completing the mapping reduces to being able to emulate an ACL_MASK and compensate for the differencesome differences in interpretation betweenthe permission-checking algo- rithms of the two ACL implementations. The difference in interpretation of the two ACL types call for a translation scheme. The scheme follows:permission-checking algorithms is handled as fol- lows: Every user ACE in the POSIX ACL maps into 2 NFSv4 ACEs; one ALLOW ACE which is translated as specified by the above scheme, then a comple- menting DENY ACE which is also translated as specified by the above scheme, with the exception that the access mask is inverted. Note that the ACL_USER_OBJ ACE is placed first in this list. Every group ACE in the POSIX ACL produces a similar pair, but instead of being in sequence, all of the ALLOW ACEs are all in sequence, fol- lowed by all the DENY ACEs. The ACL_GROUP_OBJ ACE is placed first in both lists. Lastly, the POSIX ACL_OTHER ACE is translated into a pair of ACEs as in the user ACE case. This translation strategy allows usMapping NFSv4 ACLs October 2004 With this done, the NFSv4 permission-checking algorithm applied to emulate POSIX ACL interpreta- tion in anthe resulting NFSv4 ACL will produce the same result as the POSIX permission-checking algorithm did on the original POSIX ACL. To handle the special POSIX entity ACL_MASK, we slightly modify the above translation: With the exception of the "OWNER" and "EVERYONE" ACEs, another ACE is prepended to the ACE.ACE. The prepended ACE is a DENY ACE with the same entity as the following ALLOW ACE, but with a permission mask set to the complement of the POSIX ACL_MASK. This method allows us to preserve the real permission bits of each ACE should the ACL_MASK be changed. 5. Using the Mapping in NFSv4 Implementations Note that the algorithm described in the previous section not only provides a way to map any POSIX ACL to be mapped to an NFSv4 ACL with similar semantics, but also provides the reverse mapping in the case where the NFSv4 ACL is precisely in the format of an ACL produced by the algorithm above. The algorithm can therefore be used to implement a subset of the NFSv4 ACL model. This may be useful to NFSv4 clients and servers with preexisting system interfaces that support POSIX ACLs and that cannot be modified to support NFSv4 ACLs. A server, for example, that wishes to export via NFSv4 a filesystem that supports only POSIX ACLs, may use this mapping to answer client requests for existing ACLs by translating POSIX ACLs on its filesys- tem to NFSv4 ACLs to send to the client. However, when a client attempts to set an ACL, the server faces a problem. If the given ACL happens to be in precisely the format of an ACL produced by this map- ping (as would happen if, for example, the client was performing the same translation), then the server can map it to a POSIX ACL to store on the filesystem. But for any other NFSv4 ACL, the server should return an error to avoid any chance of inaccurately representing the client's intention. The language of [rfc3530] allows a server some flexibility in han- dling ACLs that it cannot enforce completely accurately, as long as it adheres to "the guiding principle... that the server must not accept ACLs that appear to make [a file] more secure than it really is." Mapping NFSv4 ACLs October 2004 It may therefore be possible for a server to accept a wider range of NFSv4 ACLs, as long as it can ensure that in every case the resulting POSIX ACL denies at least all access that the original NFSv4 ACL denied. The prepended ACE isresults of such a DENY ACE with the same entity as the following ALLOW ACE, butmapping may, however, be somewhat unexpected, and it is preferable simply to refuse all NFSv4 ACLs that do not map accurately, and provide clients with software to help gen- erate POSIX-mappable NFSv4 ACLs if necessary. Similarly, a permission mask setclient that uses NFSv4 ACLS to the complement of theimplement user interfaces that only deal in POSIX ACL_MASK. This method allows usACLs may handle user requests to preserve the real permission bits of each ACEset ACLs easily enough, but should return errors when the ACL_MASKuser requests ACLs that, on consulting the server, turn out to not be changed.mappable to POSIX ACLs. Mapping NFSv4 ACLs FebruaryOctober 2004 55.. SSeeccuurriittyy CCoonnssiiddeerraattiioonnss Since this draft deals with the6. Security Considerations Any automatic mapping of Access Control Lists, it is deeply involved with security. The body of this document needsfrom one ACL model to addressanother must provide guarantees that the issue ofmapping ACLs in a way as to not disobey the intent ofpreserves semantics, or misleadrisk misleading users about the user.permissions set on filesystem objects. For this rea- son, we recommend performing such mapping only when it can be done accurately, and returning errors in all other cases. Mapping NFSv4 ACLs FebruaryOctober 2004 66.. BBiibblliiooggrraapphhyy [rfc3010bis]7. Bibliography [rfc3530] Shepler, S. et. al., "NFS version 4 Protocol", draft-ietf- nfsv4-rfc3010bis-05.txt,April 2003. http://www.ietf.org/internet-drafts/draft-ietf- nfsv4-rfc3010bis-05.txthttp://www.ietf.org/rfc/rfc3530.txt [posixacl] IEEE, "IEEE Draft P1003.1e", October 1997 (last draft). http://wt.xpilot.org/publications/posix.1e/download.html Mapping NFSv4 ACLs February 2004 77.. AAcckknnoowwlleeddggmmeennttss The author would like to thank and acknowledge Bruce Fields for his careful scrutiny and excellent comments and suggestions. Mapping NFSv4 ACLs FebruaryOctober 2004 88.. AAuutthhoorr''ss AAddddrreessss8. Author's Address Address comments related to this memorandum to: email@example.com firstname.lastname@example.org Marius Aamodt Eriksen J. Bruce Fields University of Michigan / CITI 535 West William Ann Arbor, Michigan E-mail: email@example.com 99.. CCooppyyrriigghhttE-mail: firstname.lastname@example.org 9. Copyright Copyright (C) The Internet Society (2002-2004). All Rights Reserved.(2004). 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