audit: Fix use after free in audit_remove_watch_rule()

audit_remove_watch_rule() drops watch's reference to parent but then
continues to work with it. That is not safe as parent can get freed once
we drop our reference. The following is a trivial reproducer:

mount -o loop image /mnt
touch /mnt/file
auditctl -w /mnt/file -p wax
umount /mnt
auditctl -D
<crash in fsnotify_destroy_mark()>

Grab our own reference in audit_remove_watch_rule() earlier to make sure
mark does not get freed under us.

CC: stable@vger.kernel.org
Reported-by: Tony Jones <tonyj@suse.de>
Signed-off-by: Jan Kara <jack@suse.cz>
Tested-by: Tony Jones <tonyj@suse.de>
Signed-off-by: Paul Moore <paul@paul-moore.com>

kernel/audit_watch.c

@@ -457,14 +457,16 @@ void audit_remove_watch_rule(struct audit_krule *krule)
 	list_del(&krule->rlist);
 
 	if (list_empty(&watch->rules)) {
-		audit_remove_watch(watch);
-
-		if (list_empty(&parent->watches)) {
+		/*
+		 * audit_remove_watch() drops our reference to 'parent' which
+		 * can get freed. Grab our own reference to be safe.
+		 */
 		audit_get_parent(parent);
+		audit_remove_watch(watch);
+		if (list_empty(&parent->watches))
 			fsnotify_destroy_mark(&parent->mark, audit_watch_group);
 		audit_put_parent(parent);
 	}
-	}
 }
 
 /* Update watch data in audit rules based on fsnotify events. */