sctp: do not peel off an assoc from one netns to another one

Now when peeling off an association to the sock in another netns, all
transports in this assoc are not to be rehashed and keep use the old
key in hashtable.

As a transport uses sk->net as the hash key to insert into hashtable,
it would miss removing these transports from hashtable due to the new
netns when closing the sock and all transports are being freeed, then
later an use-after-free issue could be caused when looking up an asoc
and dereferencing those transports.

This is a very old issue since very beginning, ChunYu found it with
syzkaller fuzz testing with this series:

  socket$inet6_sctp()
  bind$inet6()
  sendto$inet6()
  unshare(0x40000000)
  getsockopt$inet_sctp6_SCTP_GET_ASSOC_ID_LIST()
  getsockopt$inet_sctp6_SCTP_SOCKOPT_PEELOFF()

This patch is to block this call when peeling one assoc off from one
netns to another one, so that the netns of all transport would not
go out-sync with the key in hashtable.

Note that this patch didn't fix it by rehashing transports, as it's
difficult to handle the situation when the tuple is already in use
in the new netns. Besides, no one would like to peel off one assoc
to another netns, considering ipaddrs, ifaces, etc. are usually
different.

Reported-by: ChunYu Wang <chunwang@redhat.com>
Signed-off-by: Xin Long <lucien.xin@gmail.com>
Acked-by: Marcelo Ricardo Leitner <marcelo.leitner@gmail.com>
Acked-by: Neil Horman <nhorman@tuxdriver.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
unsigned int sctp_poll(struct file *file, struct socket *sock, poll_table *wait)
{
	struct sock *sk = sock->sk;
	struct sctp_sock *sp = sctp_sk(sk);
	unsigned int mask;

	poll_wait(file, sk_sleep(sk), wait);

	sock_rps_record_flow(sk);

	/* A TCP-style listening socket becomes readable when the accept queue
	 * is not empty.
	 */
	if (sctp_style(sk, TCP) && sctp_sstate(sk, LISTENING))
		return (!list_empty(&sp->ep->asocs)) ?
			(POLLIN | POLLRDNORM) : 0;

	mask = 0;

	/* Is there any exceptional events?  */
	if (sk->sk_err || !skb_queue_empty(&sk->sk_error_queue))
		mask |= POLLERR |
			(sock_flag(sk, SOCK_SELECT_ERR_QUEUE) ? POLLPRI : 0);
	if (sk->sk_shutdown & RCV_SHUTDOWN)
		mask |= POLLRDHUP | POLLIN | POLLRDNORM;
	if (sk->sk_shutdown == SHUTDOWN_MASK)
		mask |= POLLHUP;

	/* Is it readable?  Reconsider this code with TCP-style support.  */
	if (!skb_queue_empty(&sk->sk_receive_queue))
		mask |= POLLIN | POLLRDNORM;

	/* The association is either gone or not ready.  */
	if (!sctp_style(sk, UDP) && sctp_sstate(sk, CLOSED))
		return mask;

	/* Is it writable?  */
	if (sctp_writeable(sk)) {
		mask |= POLLOUT | POLLWRNORM;
	} else {
		sk_set_bit(SOCKWQ_ASYNC_NOSPACE, sk);
		/*
		 * Since the socket is not locked, the buffer
		 * might be made available after the writeable check and
		 * before the bit is set.  This could cause a lost I/O
		 * signal.  tcp_poll() has a race breaker for this race
		 * condition.  Based on their implementation, we put
		 * in the following code to cover it as well.
		 */
		if (sctp_writeable(sk))
			mask |= POLLOUT | POLLWRNORM;
	}
	return mask;
}

unsigned int sctp_poll(struct file *file, struct socket *sock, poll_table *wait)
{
	struct sock *sk = sock->sk;
	struct sctp_sock *sp = sctp_sk(sk);
	unsigned int mask;

	poll_wait(file, sk_sleep(sk), wait);

	sock_rps_record_flow(sk);

	/* A TCP-style listening socket becomes readable when the accept queue
	 * is not empty.
	 */
	if (sctp_style(sk, TCP) && sctp_sstate(sk, LISTENING))
		return (!list_empty(&sp->ep->asocs)) ?
			(POLLIN | POLLRDNORM) : 0;

	mask = 0;

	/* Is there any exceptional events?  */
	if (sk->sk_err || !skb_queue_empty(&sk->sk_error_queue))
		mask |= POLLERR |
			(sock_flag(sk, SOCK_SELECT_ERR_QUEUE) ? POLLPRI : 0);
	if (sk->sk_shutdown & RCV_SHUTDOWN)
		mask |= POLLRDHUP | POLLIN | POLLRDNORM;
	if (sk->sk_shutdown == SHUTDOWN_MASK)
		mask |= POLLHUP;

	/* Is it readable?  Reconsider this code with TCP-style support.  */
	if (!skb_queue_empty(&sk->sk_receive_queue))
		mask |= POLLIN | POLLRDNORM;

	/* The association is either gone or not ready.  */
	if (!sctp_style(sk, UDP) && sctp_sstate(sk, CLOSED))
		return mask;

	/* Is it writable?  */
	if (sctp_writeable(sk)) {
		mask |= POLLOUT | POLLWRNORM;
	} else {
		sk_set_bit(SOCKWQ_ASYNC_NOSPACE, sk);
		/*
		 * Since the socket is not locked, the buffer
		 * might be made available after the writeable check and
		 * before the bit is set.  This could cause a lost I/O
		 * signal.  tcp_poll() has a race breaker for this race
		 * condition.  Based on their implementation, we put
		 * in the following code to cover it as well.
		 */
		if (sctp_writeable(sk))
			mask |= POLLOUT | POLLWRNORM;
	}
	return mask;
}
