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 <geneblue.mail@gmail.com>
Signed-off-by: Cong Wang <xiyou.wangcong@gmail.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Manfred Spraul <manfred@colorfullife.com>
Cc: stable@kernel.org
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
static void __do_notify(struct mqueue_inode_info *info)
{
	/* notification
	 * invoked when there is registered process and there isn't process
	 * waiting synchronously for message AND state of queue changed from
	 * empty to not empty. Here we are sure that no one is waiting
	 * synchronously. */
	if (info->notify_owner &&
	    info->attr.mq_curmsgs == 1) {
		struct siginfo sig_i;
		switch (info->notify.sigev_notify) {
		case SIGEV_NONE:
			break;
		case SIGEV_SIGNAL:
			/* sends signal */

			sig_i.si_signo = info->notify.sigev_signo;
			sig_i.si_errno = 0;
			sig_i.si_code = SI_MESGQ;
			sig_i.si_value = info->notify.sigev_value;
			/* map current pid/uid into info->owner's namespaces */
			rcu_read_lock();
			sig_i.si_pid = task_tgid_nr_ns(current,
						ns_of_pid(info->notify_owner));
			sig_i.si_uid = from_kuid_munged(info->notify_user_ns, current_uid());
			rcu_read_unlock();

			kill_pid_info(info->notify.sigev_signo,
				      &sig_i, info->notify_owner);
			break;
		case SIGEV_THREAD:
			set_cookie(info->notify_cookie, NOTIFY_WOKENUP);
			netlink_sendskb(info->notify_sock, info->notify_cookie);
			break;
		}
		/* after notification unregisters process */
		put_pid(info->notify_owner);
		put_user_ns(info->notify_user_ns);
		info->notify_owner = NULL;
		info->notify_user_ns = NULL;
	}
	wake_up(&info->wait_q);
}

static void __do_notify(struct mqueue_inode_info *info)
{
	/* notification
	 * invoked when there is registered process and there isn't process
	 * waiting synchronously for message AND state of queue changed from
	 * empty to not empty. Here we are sure that no one is waiting
	 * synchronously. */
	if (info->notify_owner &&
	    info->attr.mq_curmsgs == 1) {
		struct siginfo sig_i;
		switch (info->notify.sigev_notify) {
		case SIGEV_NONE:
			break;
		case SIGEV_SIGNAL:
			/* sends signal */

			sig_i.si_signo = info->notify.sigev_signo;
			sig_i.si_errno = 0;
			sig_i.si_code = SI_MESGQ;
			sig_i.si_value = info->notify.sigev_value;
			/* map current pid/uid into info->owner's namespaces */
			rcu_read_lock();
			sig_i.si_pid = task_tgid_nr_ns(current,
						ns_of_pid(info->notify_owner));
			sig_i.si_uid = from_kuid_munged(info->notify_user_ns, current_uid());
			rcu_read_unlock();

			kill_pid_info(info->notify.sigev_signo,
				      &sig_i, info->notify_owner);
			break;
		case SIGEV_THREAD:
			set_cookie(info->notify_cookie, NOTIFY_WOKENUP);
			netlink_sendskb(info->notify_sock, info->notify_cookie);
			break;
		}
		/* after notification unregisters process */
		put_pid(info->notify_owner);
		put_user_ns(info->notify_user_ns);
		info->notify_owner = NULL;
		info->notify_user_ns = NULL;
	}
	wake_up(&info->wait_q);
}
