AppArmor: Fix underflow in xindex calculation

If the xindex value stored in the accept tables is 0, the extraction of
that value will result in an underflow (0 - 4).

In properly compiled policy this should not happen for file rules but
it may be possible for other rule types in the future.

To exploit this underflow a user would have to be able to load a corrupt
policy, which requires CAP_MAC_ADMIN, overwrite system policy in kernel
memory or know of a compiler error resulting in the flaw being present
for loaded policy (no such flaw is known at this time).
Signed-off-by: NJohn Johansen <john.johansen@canonical.com>
Acked-by: NKees Cook <kees@ubuntu.com>

security/apparmor/include/file.h

@@ -117,7 +117,7 @@ static inline u16 dfa_map_xindex(u16 mask)
 		index |= AA_X_NAME;
 	} else if (old_index == 3) {
 		index |= AA_X_NAME | AA_X_CHILD;
-	} else {
+	} else if (old_index) {
 		index |= AA_X_TABLE;
 		index |= old_index - 4;
 	}