apparmor: mitigate parser generating large xtables

Some versions of the parser are generating an xtable transition per
state in the state machine, even when the state machine isn't using
the transition table.

The parser bug is triggered by
commit 2e12c5f ("apparmor: add additional flags to extended permission.")

In addition to fixing this in userspace, mitigate this in the kernel
as part of the policy verification checks by detecting this situation
and adjusting to what is actually used, or if not used at all freeing
it, so we are not wasting unneeded memory on policy.

Fixes: 2e12c5f ("apparmor: add additional flags to extended permission.")
Signed-off-by: John Johansen <john.johansen@canonical.com>

Author: jrjohansen

security/apparmor/include/lib.h

@@ -125,6 +125,7 @@ struct aa_str_table {
 };
 
 void aa_free_str_table(struct aa_str_table *table);
+bool aa_resize_str_table(struct aa_str_table *t, int newsize, gfp_t gfp);
 
 struct counted_str {
 	struct kref count;

security/apparmor/lib.c

@@ -116,6 +116,29 @@ int aa_print_debug_params(char *buffer)
 			       aa_g_debug);
 }
 
+bool aa_resize_str_table(struct aa_str_table *t, int newsize, gfp_t gfp)
+{
+	char **n;
+	int i;
+
+	if (t->size == newsize)
+		return true;
+	n = kcalloc(newsize, sizeof(*n), gfp);
+	if (!n)
+		return false;
+	for (i = 0; i < min(t->size, newsize); i++)
+		n[i] = t->table[i];
+	for (; i < t->size; i++)
+		kfree_sensitive(t->table[i]);
+	if (newsize > t->size)
+		memset(&n[t->size], 0, (newsize-t->size)*sizeof(*n));
+	kfree_sensitive(t->table);
+	t->table = n;
+	t->size = newsize;
+
+	return true;
+}
+
 /**
  * aa_free_str_table - free entries str table
  * @t: the string table to free  (MAYBE NULL)

security/apparmor/policy_unpack.c

@@ -802,8 +802,12 @@ static int unpack_pdb(struct aa_ext *e, struct aa_policydb **policy,
 	if (!pdb->dfa && pdb->trans.table)
 		aa_free_str_table(&pdb->trans);
 
-	/* TODO: move compat mapping here, requires dfa merging first */
-	/* TODO: move verify here, it has to be done after compat mappings */
+	/* TODO:
+	 * - move compat mapping here, requires dfa merging first
+	 * - move verify here, it has to be done after compat mappings
+	 * - move free of unneeded trans table here, has to be done
+	 *   after perm mapping.
+	 */
 out:
 	*policy = pdb;
 	return 0;
@@ -1242,21 +1246,32 @@ static bool verify_perm(struct aa_perms *perm)
 static bool verify_perms(struct aa_policydb *pdb)
 {
 	int i;
+	int xidx, xmax = -1;
 
 	for (i = 0; i < pdb->size; i++) {
 		if (!verify_perm(&pdb->perms[i]))
 			return false;
 		/* verify indexes into str table */
-		if ((pdb->perms[i].xindex & AA_X_TYPE_MASK) == AA_X_TABLE &&
-		    (pdb->perms[i].xindex & AA_X_INDEX_MASK) >= pdb->trans.size)
-			return false;
+		if ((pdb->perms[i].xindex & AA_X_TYPE_MASK) == AA_X_TABLE) {
+			xidx = pdb->perms[i].xindex & AA_X_INDEX_MASK;
+			if (xidx >= pdb->trans.size)
+				return false;
+			if (xmax < xidx)
+				xmax = xidx;
+		}
 		if (pdb->perms[i].tag && pdb->perms[i].tag >= pdb->trans.size)
 			return false;
 		if (pdb->perms[i].label &&
 		    pdb->perms[i].label >= pdb->trans.size)
 			return false;
 	}
-
+	/* deal with incorrectly constructed string tables */
+	if (xmax == -1) {
+		aa_free_str_table(&pdb->trans);
+	} else if (pdb->trans.size > xmax + 1) {
+		if (!aa_resize_str_table(&pdb->trans, xmax + 1, GFP_KERNEL))
+			return false;
+	}
 	return true;
 }