Commit 762674f8 authored by Trond Myklebust's avatar Trond Myklebust

NFSv4: Don't perform cached access checks before we've OPENed the file

Donald Buczek reports that a nfs4 client incorrectly denies
execute access based on outdated file mode (missing 'x' bit).
After the mode on the server is 'fixed' (chmod +x) further execution
attempts continue to fail, because the nfs ACCESS call updates
the access parameter but not the mode parameter or the mode in
the inode.

The root cause is ultimately that the VFS is calling may_open()
before the NFS client has a chance to OPEN the file and hence revalidate
the access and attribute caches.

Al Viro suggests:
>>> Make nfs_permission() relax the checks when it sees MAY_OPEN, if you know
>>> that things will be caught by server anyway?
>>
>> That can work as long as we're guaranteed that everything that calls
>> inode_permission() with MAY_OPEN on a regular file will also follow up
>> with a vfs_open() or dentry_open() on success. Is this always the
>> case?
>
> 1) in do_tmpfile(), followed by do_dentry_open() (not reachable by NFS since
> it doesn't have ->tmpfile() instance anyway)
>
> 2) in atomic_open(), after the call of ->atomic_open() has succeeded.
>
> 3) in do_last(), followed on success by vfs_open()
>
> That's all.  All calls of inode_permission() that get MAY_OPEN come from
> may_open(), and there's no other callers of that puppy.
Reported-by: default avatarDonald Buczek <buczek@molgen.mpg.de>
Link: https://bugzilla.kernel.org/show_bug.cgi?id=109771
Link: http://lkml.kernel.org/r/1451046656-26319-1-git-send-email-buczek@molgen.mpg.de
Cc: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: default avatarTrond Myklebust <trond.myklebust@primarydata.com>
parent 95864c91
...@@ -2449,6 +2449,9 @@ int nfs_permission(struct inode *inode, int mask) ...@@ -2449,6 +2449,9 @@ int nfs_permission(struct inode *inode, int mask)
case S_IFLNK: case S_IFLNK:
goto out; goto out;
case S_IFREG: case S_IFREG:
if ((mask & MAY_OPEN) &&
nfs_server_capable(inode, NFS_CAP_ATOMIC_OPEN))
return 0;
break; break;
case S_IFDIR: case S_IFDIR:
/* /*
......
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