apparmor: ensure that dfa state tables have entries

Currently it is possible to specify a state machine table with 0 length,
this is not valid as optional tables are specified by not defining
the table as present. Further this allows by-passing the base tables
range check against the next/check tables.

Fixes: d901d6a2 ("apparmor: dfa split verification of table headers")
Reported-by: Mike Salvatore <mike.salvatore@canonical.com>
Signed-off-by: John Johansen <john.johansen@canonical.com>

security/apparmor/match.c

@@ -97,6 +97,9 @@ static struct table_header *unpack_table(char *blob, size_t bsize)
 	      th.td_flags == YYTD_DATA8))
 		goto out;
 
+	/* if we have a table it must have some entries */
+	if (th.td_lolen == 0)
+		goto out;
 	tsize = table_size(th.td_lolen, th.td_flags);
 	if (bsize < tsize)
 		goto out;
@@ -198,6 +201,8 @@ static int verify_dfa(struct aa_dfa *dfa)
 
 	state_count = dfa->tables[YYTD_ID_BASE]->td_lolen;
 	trans_count = dfa->tables[YYTD_ID_NXT]->td_lolen;
+	if (state_count == 0)
+		goto out;
 	for (i = 0; i < state_count; i++) {
 		if (!(BASE_TABLE(dfa)[i] & MATCH_FLAG_DIFF_ENCODE) &&
 		    (DEFAULT_TABLE(dfa)[i] >= state_count))