audit: fix use-after-free in audit_add_watch

audit_add_watch stores locally krule->watch without taking a reference
on watch. Then, it calls audit_add_to_parent, and uses the watch stored
locally.

Unfortunately, it is possible that audit_add_to_parent updates
krule->watch.
When it happens, it also drops a reference of watch which
could free the watch.

How to reproduce (with KASAN enabled):

    auditctl -w /etc/passwd -F success=0 -k test_passwd
    auditctl -w /etc/passwd -F success=1 -k test_passwd2

The second call to auditctl triggers the use-after-free, because
audit_to_parent updates krule->watch to use a previous existing watch
and drops the reference to the newly created watch.

To fix the issue, we grab a reference of watch and we release it at the
end of the function.

Signed-off-by: Ronny Chevalier <ronny.chevalier@hp.com>
Reviewed-by: Richard Guy Briggs <rgb@redhat.com>
Signed-off-by: Paul Moore <paul@paul-moore.com>

kernel/audit_watch.c

@@ -420,6 +420,13 @@ int audit_add_watch(struct audit_krule *krule, struct list_head **list)
 	struct path parent_path;
 	int h, ret = 0;
 
+	/*
+	 * When we will be calling audit_add_to_parent, krule->watch might have
+	 * been updated and watch might have been freed.
+	 * So we need to keep a reference of watch.
+	 */
+	audit_get_watch(watch);
+
 	mutex_unlock(&audit_filter_mutex);
 
 	/* Avoid calling path_lookup under audit_filter_mutex. */
@@ -428,8 +435,10 @@ int audit_add_watch(struct audit_krule *krule, struct list_head **list)
 	/* caller expects mutex locked */
 	mutex_lock(&audit_filter_mutex);
 
-	if (ret)
+	if (ret) {
+		audit_put_watch(watch);
 		return ret;
+	}
 
 	/* either find an old parent or attach a new one */
 	parent = audit_find_parent(d_backing_inode(parent_path.dentry));
@@ -447,6 +456,7 @@ int audit_add_watch(struct audit_krule *krule, struct list_head **list)
 	*list = &audit_inode_hash[h];
 error:
 	path_put(&parent_path);
+	audit_put_watch(watch);
 	return ret;
 }