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: John Johansen <john.johansen@canonical.com>
Acked-by: Kees Cook <kees@ubuntu.com>

1 files changed, 1 insertions, 1 deletions

diff --git a/security/apparmor/include/file.h b/security/apparmor/include/file.h
index ab8c6d87f75..f98fd4701d8 100644
--- a/security/apparmor/include/file.h
+++ b/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;
         }