apparmor: validate DFA start states are in bounds in unpack_pdb

commit 9063d7e upstream.

Start states are read from untrusted data and used as indexes into the
DFA state tables. The aa_dfa_next() function call in unpack_pdb() will
access dfa->tables[YYTD_ID_BASE][start], and if the start state exceeds
the number of states in the DFA, this results in an out-of-bound read.

==================================================================
 BUG: KASAN: slab-out-of-bounds in aa_dfa_next+0x2a1/0x360
 Read of size 4 at addr ffff88811956fb90 by task su/1097
 ...

Reject policies with out-of-bounds start states during unpacking
to prevent the issue.

Fixes: ad5ff3d ("AppArmor: Add ability to load extended policy")
Reported-by: Qualys Security Advisory <qsa@qualys.com>
Tested-by: Salvatore Bonaccorso <carnil@debian.org>
Reviewed-by: Georgia Garcia <georgia.garcia@canonical.com>
Reviewed-by: Cengiz Can <cengiz.can@canonical.com>
Signed-off-by: Massimiliano Pellizzer <massimiliano.pellizzer@canonical.com>
Signed-off-by: John Johansen <john.johansen@canonical.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Author: m-pellizzer

security/apparmor/policy_unpack.c

@@ -762,7 +762,17 @@ static int unpack_pdb(struct aa_ext *e, struct aa_policydb **policy,
 		if (!aa_unpack_u32(e, &pdb->start[AA_CLASS_FILE], "dfa_start")) {
 			/* default start state for xmatch and file dfa */
 			pdb->start[AA_CLASS_FILE] = DFA_START;
-		}	/* setup class index */
+		}
+
+		size_t state_count = pdb->dfa->tables[YYTD_ID_BASE]->td_lolen;
+
+		if (pdb->start[0] >= state_count ||
+		    pdb->start[AA_CLASS_FILE] >= state_count) {
+			*info = "invalid dfa start state";
+			goto fail;
+		}
+
+		/* setup class index */
 		for (i = AA_CLASS_FILE + 1; i <= AA_CLASS_LAST; i++) {
 			pdb->start[i] = aa_dfa_next(pdb->dfa, pdb->start[0],
 						    i);