Network Working Group Marius Aamodt Eriksen Document:
draft-ietf-nfsv4-acl-mapping-00.txtdraft-ietf-nfsv4-acl-mapping-01.txt Mapping Between NFSv4 and Posix Draft ACLs Status of this MemoSSttaattuuss ooff tthhiiss MMeemmoo 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 materialmate- 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 (2003).(2002-2004). All Rights Reserved." AbstractAAbbssttrraacctt The NFS (Network File System) version 4[rfc3530]4[rfc3010bis] (NFSv4) specifies a flavor of Access Control Lists (ACLs) that applyresembles that of Win- dows NT's. ACLs are used to specify fine grained control of access to file system objects. ACLsAn ACL consists of a number of Access Con- trol Entries (ACEs), each specify an accesssome level of access for an entity; an entity can be a number of entities.a user, group or a special entity. The NFSv4 ACLs model resembles that of Windows NT. A POSIX draft[posixacl] proposes another, more restrictive ACL model. Many systems implement this proposed standard. Differing in syntax, semantics and extensiveness, itaccess level is described using an access mask, which is only feasible to createa correct representation for POSIX ACLs with NFSv4 ACLs. This does not holdbitmask where each bit describes a level of access, for an attempt to represent arbitrary NFSv4 ACLs with POSIX ACLs.example read, write and execute permissions on the file system object. Mapping NFSv4 ACLs August 2003 Table of Contents 1. Introduction . . . . . . . . . . . . . .February 2004 TTaabbllee ooff CCoonntteennttss 11.. IInnttrroodduuccttiioonn . . . . . . . . . . 3 2. NFSv4 ACLs . . . . . . . . . . .22.. NNFFSSvv44 AACCLLss . . . . . . . . . . . . . . 3 3. POSIX ACLs . . . . . . . . . . .33.. PPOOSSIIXX AACCLLss . . . . . . . . . . . . . . 4 4. Mapping Posix ACLs to NFSv4 ACLs . . . .44.. MMaappppiinngg PPoossiixx AACCLLss ttoo NNFFSSvv44 AACCLLss . . . . . . . . . . 5 5. Security Considerations .55.. SSeeccuurriittyy CCoonnssiiddeerraattiioonnss . . . . . . . 7 66.. BBiibblliiooggrraapphhyy . . . . . . . . . . 8 6. Bibliography . . . . . . . . . . . . . . . . . . .77.. AAcckknnoowwlleeddggmmeennttss . . . . . 9 7. Author's Address . . . . . . . . . . . . . . . . .88.. AAuutthhoorr''ss AAddddrreessss . . . . 10 8. Copyright . . . . . . . . . . .99.. CCooppyyrriigghhtt . . . . . . . . . . . . . 10 Mapping NFSv4 ACLs August 2003 1. IntroductionFebruary 2004 11.. IInnttrroodduuccttiioonn The NFS (Network File System) version 4 [rfc3530][rfc3010bis] (NFSv4) specifiesspeci- fies a flavor of Access Control Lists (ACLs) that resembles that of Windows NT's. ACLs are used to specify fine grained control of access to file system objects. An ACL consists of a number of Access Control Entries (ACEs), each specifyspecifying some level of access for an entity; an entity can be a a user, group or a special entity. The access level is described using an access mask, which is a bitmask where each bit describes a level of access, for example read, write and execute permissions on the file system object. The POSIX Draft Standard 17[posixacl] proposes a simpler, more limited ACL model. Due to the difference in syntax, semantics and extensiveness, it is only feasible to correctly represent POSIX ACLs using NFSv4 ACLs, and not the other way around. Thus, we provide such a mapping for use in systems that implement NFSv4, already have POSIX Draft Standard ACL support and wish to continue to use this interface with NFSv4 and interoperate with other such systems. A client may also use the mapping for storing, retrieving and interpreting ACLs on an NFSv4 server that supports the storage, retrieval and interpretation of arbitrary NFSv4 ACLs. 2. NFSv4 ACLs22.. NNFFSSvv44 AACCLLss NFSv4 ACLs are rich in nature and expandsexpand upon the traditional idea of ACLs. An NFSv4 ACE can be of type ALLOW, DENY, LOG or ALARM; each specifies a different action to take should the ACE match a current request. NFSv4 ACLs also have a rich set of access types that complementcom- plements the traditional types. These include appending data to the file systemobject, deleting children of the file system object andobject, deleting the file systemobject, etc [rfc3530].[rfc3010bis]. NFSv4 ACLs are interpreted in a straightforward manner. 1) Walk through the list of ACEs from the ACL in order 2) If the "who" (entity) field in the ACE does not match that of the requester, the particular ACE is not processed. 3) Process all ACEs until all the bits in the requested access mask have been ALLOWed; that is, the bits have entries in matching ALLOW ACEs that are not flagged ACE4_INHERIT_ONLY_ACE. Onceonce a particular bit has been ALLOWed by an ACE, it is no longer considered in further processing. 4) If a particular access is DENYed (while that bit is still under Mapping NFSv4 ACLs August 2003consideration), the request is denied. 5) If all bits have been ALLOWed, the access is granted, otherwise the accessor else behavior is deniedundefined. NFSv4 ACLs also specify a number of special entities such as OWNER, GROUPGROUP, and EVERYONE. These refer to the traditional UNIX permissions.mode bits. Others include DIALUP, BATCHBATCH, and AUTHENTICATED, which have specializedspecial- ized uses. Mapping NFSv4 ACLs February 2004 Additionally the NFSv4 ACLs specify a number of flags that can be applied to individual ACEs.an ACL. These include a specification ofon how an ACEACL on a directory may be propagated to newly created files or directories inside of said directory. TheIt is clear that the granularity of access control provided bythat NFSv4 ACLs specify is well beyond that provided bythe standard UNIX capability of expressing file system object permissions. 3.33.. PPOOSSIIXX AACCLLss POSIX ACLs "POSIX ACLs"refer to POSIX 1003.1e/1003.2c Draft Standard 17 [posixacl]. It[posix- acl], which was meant to specify a POSIX standard for ACLs, but unfortunately never materialized. However, many systems still use it, both in formsthe form of itsit's latest anddraft as well as earlier drafts. POSIX ACLs are simpler than itstheir NFSv4 equivalent.equivalents. Each ACE an has an entity and the traditional UNIX mode bits that are assigned to the particular entity. The entity may be an arbitrary UID or GID or one of a few special entities, the most notable of which is the ACL_MASK entity. POSIX ACLs are also interepreted differently than their NFSv4 equivalents. POSIX ACLs are interpreted as follows: 1) Process the ACL_USER_OBJ (equivalent to UNIX file owner) ACE first; if the UID of the requester does not match that of the ACL_USER_OBJ, then the ACE is ignored. Otherwise, the request is granted if and onlyif the requestrequester's access mask is allowed by the access mask of the ACE.ACE, the request is granted, else the request is denied. 2) Process all of the ACL_USER ACEs; the entity of these ACEs specifiesspecify a user on the system. If the UID of the requester does not match that of the ACE, then the ACE is ignored. Otherwise, the request is granted if and onlyif the requestrequester's access mask is allowed by the access mask of the ACE. Mapping NFSv4 ACLs August 2003ACE, the request is granted, else the request is denied. 3) Process the ACL_GROUP_OBJ ACE and all of the ACL_GROUP ACEs; the entity of each ACEthese ACEs specify a group on the system. If none of the GIDs of the requester match the entity of thecurrent ACE, the particular ACE is ignored. For any matching ACE, if the the requester's access mask is allowed by the ACEs access mask, then access is permitted. If there are matching ACEs, but none allow access, then access is denied. Mapping NFSv4 ACLs February 2004 4) If the requester's access mask is allowed by the ACL_OTHER ACE, then grant access. 5) Deny access. Steps (2) and (3) have an additional restriction;criteria; in addition to checkingcheck- 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. AThis default ACL governs the ACL a file system object will be assigned initially when it is created as a child of the particular directory. 4. Mapping Posix ACLs to NFSv4 ACLs44.. MMaappppiinngg PPoossiixx AACCLLss ttoo NNFFSSvv44 AACCLLss Given the difference in both extensiveness and interpretation of POSIX and NFSv4 ACLs, any conversion of arbitrary NFSv4 ACLs to POSIX ACLsbetween the two is infeasible.difficult. However, POSIX ACLs are a subset of NFSv4 ACLs. Any POSIX ACL can be emulated with an NFSv4 ACL. The difference inACL using the format of POSIX ACEs and NFSv4 ACEs can be compensated for by a directfollowing mapping. The ACE entity isentities are translated as follows. The non-special entityentities in form of UIDs and GIDs is translated to equivalent strings (a systemsys- tem dependent process, typically done by lookups to /etc/passwd in UNIX). The POSIX ACL_USER_OBJ entity is translated to the "OWNER" NFSv4 entity. Similary, the POSIX ACL_GROUP_OBJ is translated to the "GROUP" NFSv4 entity. The ACL_OTHER entity is translated to the "EVERYONE" NFSv4 entity. The ACE access mask is translated as follows. The read bit of the POSIX Mapping NFSv4 ACLs August 2003access mask is translated to the "ACE4_GENERIC_READ"logical OR of the ACE4_READ_DATA and ACE4_READ_NAMED_ATTRS NFSv4 access mask field.fields. The write bit of the POSIX access mask is translated to "ACE4_GENERIC_WRITE"the logical OR of the ACE4_WRITE_DATA, ACE4_WRITE_NAMED_ATTRIBUTES and ACE4_APPEND_DATA NFSv4 access mask field. thefields. The execute bit of the POSIX access mask is translated into the "ACE4_GENERIC_EXECUTE"ACE4_EXECUTE and ACE4_READ_DATA NFSv4 accessacess mask field. Defined in [rfc3530], "ACE4_GENERIC_READ" is a logical OR of "ACE4_SYNCHRONIZE," "ACE4_READ_ACL," "ACE4_READ_ATTRIBUTES," "ACE4_READ_DATA" and "ACE4_LIST_DIRECTORY." "ACE4_GENERIC_WRITE" is a logical OR of "ACE4_SYNCHRONIZE," "ACE4_READ_ACL," "ACE4_WRITE_ACL," "ACE4_WRITE_ATTRIBUTES," "ACE4_WRITE_DATA," "ACE4_ADD_FILE," "ACE4_APPEND_DATA" and "ACE4_ADD_SUBDIRECTORY." Finally, "ACE4_GENERIC_EXECUTE" is a logical OR of "ACE4_SYNCHRONIZE," "ACE4_READ_ACL," "ACE4_READ_ATTRIBUTES"fields. Note that NFSv4 defines ACE4_READ_DATA, ACE4_WRITE_DATA, and "ACE4_EXECUTE." These were chosenACE4_APPEND_DATA to be equal to representACE4_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 true meaningtranslation of the UNIX mode which are used byPOSIX ACLs.write bit. Mapping NFSv4 ACLs February 2004 In 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_ACL and ACE4_READ_ATTRIBUTES. 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 is set in the NFSv4 ACE. If the entity in the POSIX ACE refers to a group, the "ACE4_IDENTIFIER_GROUP""ACE4_IDENTI- FIER_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 difference in interpretation between totwo ACL implementations. The difference in interpretation of the two ACL types call for a translation scheme. The scheme follows: 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 complementingcomple- menting DENY ACE which is also translated as specified by the above scheme, with the exception that the access mask is inverted. TheNote that the ACL_USER_OBJ ACE is placed first in thethis list. Every group ACE in the POSIX ACL produces a similar pair, but instead of being in sequence, all of the ALLOW ACEs are placed first, followedall in sequence, fol- lowed by all the DENY ACEs. The ACL_GROUP_OBJ ACE is placed first in the list.both lists. Lastly, the POSIX ACL_OTHER ACE translates directly into one NFSv4 ACE at the end of the group ACEs. This is an allow ACE whichis translated into a pair of ACEs as specified byin the above scheme.user ACE case. This translation strategy allows us to emulate POSIX ACL interpretationinterpreta- tion in an NFSv4 ACL. To handle the special POSIX entity ACL_MASK, we slightly modify the Mapping NFSv4 ACLs August 2003above translation: With the exception of the "OWNER," "GROUP,""OWNER" and "EVERYONE" ACEs, another ACE is prepended to everythe 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. The fact that POSIX ACLs use separate ACLs for determining access to the file system object and determining inheritance of the ACL needs compensation in the translation scheme. Whenever the server receives a request for an ACL, if the file system object in question is a directory, the server appends the default ACL to the access ACL. It is then up to the client to separate the two ACLs and translate them individually. Similarly, when the client wishes to set an ACL, it either sends the access and default ACLs individually in separate requests, or concatenates them. Again the server should separate default and access ACLs, translating and setting them individually. The reverse mapping follows from the forward mapping described here. The forward mapping obeys a very strict template, and the implementer must ensure that when performing the reverse mapping, the ACL strictly adheres to this template.Mapping NFSv4 ACLs August 2003 5. Security ConsiderationsFebruary 2004 55.. SSeeccuurriittyy CCoonnssiiddeerraattiioonnss Since this draft deals with the mapping of Access Control Lists, it is deeply involved with security. The body of this document needs to address the issue of mapping ACLs in a way as to not disobey the intent of or mislead the user. It is therefore important that ACLs that do not match the above scheme are explicitly rejected. Also, neither optimistic nor pessimistic translation between POSIX and NFSv4 ACLs should be carried out. This can potentially lead to unintended granting or revoking of priveliges.Mapping NFSv4 ACLs August 2003 6. Bibliography [rfc3530]February 2004 66.. BBiibblliiooggrraapphhyy [rfc3010bis] Shepler, S. et. al., "NFS version 4 Protocol", http://www.ietf.org/rfc/rfc3530.txt,draft-ietf- nfsv4-rfc3010bis-05.txt, April 2003. http://www.ietf.org/internet-drafts/draft-ietf- nfsv4-rfc3010bis-05.txt [posixacl] IEEE, "IEEE Draft P1003.1e", October 1997 (last draft). http://wt.xpilot.org/publications/posix.1e/download.html Mapping NFSv4 ACLs August 2003 7. Author's AddressFebruary 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 February 2004 88.. AAuutthhoorr''ss AAddddrreessss Address comments related to this memorandum to: email@example.com Marius Aamodt Eriksen University of Michigan / CITI 535 West William Ann Arbor, Michigan E-mail: firstname.lastname@example.org 8. Copyright99.. CCooppyyrriigghhtt Copyright (C) The Internet Society (2002, 2003).(2002-2004). All Rights Reserved. This document and translations of it may be copied and furnished to others, and derivative works that comment on or otherwise explain it or assist in its implmentation may be prepared, copied, published and distributed, in whole or in part, without restriction of any kind, provided that the above copyright notice and this paragraph are included on all such copies and derivative works. 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