posix_acl: handle NULL ACL in posix_acl_equiv_mode

Various filesystems don't bother checking for a NULL ACL in
posix_acl_equiv_mode, and thus can dereference a NULL pointer when it
gets passed one. This usually happens from the NFS server, as the ACL tools
never pass a NULL ACL, but instead of one representing the mode bits.

Instead of adding boilerplat to all filesystems put this check into one place,
which will allow us to remove the check from other filesystems as well later
on.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reported-by: Ben Greear <greearb@candelatech.com>
Reported-by: Marco Munderloh <munderl@tnt.uni-hannover.de>,
Cc: Chuck Lever <chuck.lever@oracle.com>
Cc: stable@vger.kernel.org
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

diff --git a/fs/posix_acl.c b/fs/posix_acl.c
index 9e363e41dacc8c2ebc007661a598c01e84da45c9..0855f772cd41599d6c1d1091e7da616d32cccf53 100644 (file)
--- a/fs/posix_acl.c
+++ b/fs/posix_acl.c
@@ -246,6 +246,12 @@ posix_acl_equiv_mode(const struct posix_acl *acl, umode_t *mode_p)
        umode_t mode = 0;
        int not_equiv = 0;
 
+       /*
+        * A null ACL can always be presented as mode bits.
+        */
+       if (!acl)
+               return 0;
+
        FOREACH_ACL_ENTRY(pa, acl, pe) {
                switch (pa->e_tag) {
                        case ACL_USER_OBJ: