4.4 Permission issues

The NFS version 3 protocol, strictly speaking, does not define the permission checking used by servers. However, it is expected that a server will do normal operating system permission checking using AUTH_UNIX style authentication as the basis of its protection mechanism, or another stronger form of authentication such as AUTH_DES or AUTH_KERB. With AUTH_UNIX authentication, the server gets the client's effective uid, effective gid, and groups on each call and uses them to check permission. These are the so-called UNIX credentials. AUTH_DES and AUTH_KERB use a network name, or netname, as the basis for identification (from which a UNIX server derives the necessary standard UNIX credentials). There are problems with this method that have been solved.

Using uid and gid implies that the client and server share the same uid list. Every server and client pair must have the same mapping from user to uid and from group to gid. Since every client can also be a server, this tends to imply that the whole network shares the same uid/gid space. If this is not the case, then it usually falls upon the server to perform some custom mapping of credentials from one authentication domain into another. A discussion of techniques for managing a shared user space or for providing mechanisms for user ID mapping is beyond the scope of this specification.

Another problem arises due to the usually stateful open operation. Most operating systems check permission at open time, and then check that the file is open on each read and write request. With stateless servers, the server cannot detect that the file is open and must do permission checking on each read and write call. UNIX client semantics of access permission checking on open can be provided with the ACCESS procedure call in this revision, which allows a client to explicitly check access permissions without resorting to trying the operation. On a local file system, a user can open a file and then change the permissions so that no one is allowed to touch it, but will still be able to write to the file because it is open. On a remote file system, by contrast, the write would fail. To get around this problem, the server's permission checking algorithm should allow the owner of a file to access it regardless of the permission setting. This is needed in a practical NFS version 3 protocol server implementation, but it does depart from correct local file system semantics. This should not affect the return result of access permissions as returned by the ACCESS procedure, however.

A similar problem has to do with paging in an executable program over the network. The operating system usually checks for execute permission before opening a file for demand paging, and then reads blocks from the open file. In a local UNIX file system, an executable file does not need read permission to execute (pagein). An NFS version 3 protocol server can not tell the difference between a normal file read (where the read permission bit is meaningful) and a demand pagein read (where the server should allow access to the executable file if the execute bit is set for that user or group or public). To make this work, the server allows reading of files if the uid given in the call has either execute or read permission on the file, through ownership, group membership or public access. Again, this departs from correct local file system semantics.

In most operating systems, a particular user (on UNIX, the uid 0) has access to all files, no matter what permission and ownership they have. This superuser permission may not be allowed on the server, since anyone who can become superuser on their client could gain access to all remote files. A UNIX server by default maps uid 0 to a distinguished value (UID_NOBODY), as well as mapping the groups list, before doing its access checking. A server implementation may provide a mechanism to change this mapping. This works except for NFS version 3 protocol root file systems (required for diskless NFS version 3 protocol client support), where superuser access cannot be avoided. Export options are used, on the server, to restrict the set of clients allowed superuser access.