mqueue: fix a use-after-free in sys_mq_notify()

The retry logic for netlink_attachskb() inside sys_mq_notify()
is nasty and vulnerable:

1) The sock refcnt is already released when retry is needed
2) The fd is controllable by user-space because we already
   release the file refcnt

so we when retry but the fd has been just closed by user-space
during this small window, we end up calling netlink_detachskb()
on the error path which releases the sock again, later when
the user-space closes this socket a use-after-free could be
triggered.

Setting 'sock' to NULL here should be sufficient to fix it.

Reported-by: GeneBlue <[email protected]>
Signed-off-by: Cong Wang <[email protected]>
Cc: Andrew Morton <[email protected]>
Cc: Manfred Spraul <[email protected]>
Cc: [email protected]
Signed-off-by: Linus Torvalds <[email protected]>

ipc/mqueue.c

@@ -1270,8 +1270,10 @@ static int do_mq_notify(mqd_t mqdes, const struct sigevent *notification)
 
 			timeo = MAX_SCHEDULE_TIMEOUT;
 			ret = netlink_attachskb(sock, nc, &timeo, NULL);
-			if (ret == 1)
+			if (ret == 1) {
+				sock = NULL;
 				goto retry;
+			}
 			if (ret) {
 				sock = NULL;
 				nc = NULL;