[PATCH] tipc: fix shutdown() of connectionless socket

syzbot is reporting hung task at nbd_ioctl() [1], for there are two
problems regarding TIPC's connectionless socket's shutdown() operation.
I found C reproducer for this problem (shown below) from "no output from
test machine (2)" report.

----------

int main(int argc, char *argv[])
{
        const int fd = open("/dev/nbd0", 3);
        ioctl(fd, NBD_SET_SOCK, socket(PF_TIPC, SOCK_DGRAM, 0));
        ioctl(fd, NBD_DO_IT, 0);
        return 0;
}
----------

One problem is that wait_for_completion() from flush_workqueue() from
nbd_start_device_ioctl() from nbd_ioctl() cannot be completed when
nbd_start_device_ioctl() received a signal at wait_event_interruptible(),
for tipc_shutdown() from kernel_sock_shutdown(SHUT_RDWR) from
nbd_mark_nsock_dead() from sock_shutdown() from nbd_start_device_ioctl()
is failing to wake up a WQ thread sleeping at wait_woken() from
tipc_wait_for_rcvmsg() from sock_recvmsg() from sock_xmit() from
nbd_read_stat() from recv_work() scheduled by nbd_start_device() from
nbd_start_device_ioctl(). Fix this problem by always invoking
sk->sk_state_change() (like inet_shutdown() does) when tipc_shutdown() is
called.

The other problem is that tipc_wait_for_rcvmsg() cannot return when
tipc_shutdown() is called, for tipc_shutdown() sets sk->sk_shutdown to
SEND_SHUTDOWN (despite "how" is SHUT_RDWR) while tipc_wait_for_rcvmsg()
needs sk->sk_shutdown set to RCV_SHUTDOWN or SHUTDOWN_MASK. Fix this
problem by setting sk->sk_shutdown to SHUTDOWN_MASK (like inet_shutdown()
does) when the socket is connectionless.

[1] https://syzkaller.appspot.com/bug?id=3fe51d307c1f0a845485cf1798aa059d12bf18b2

Reported-by: syzbot <redacted>
Signed-off-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
---
 net/tipc/socket.c | 9 ++++++---
 1 file changed, 6 insertions(+), 3 deletions(-)

diff --git a/net/tipc/socket.c b/net/tipc/socket.c
index 2679e97e0389..ebd280e767bd 100644
--- a/net/tipc/socket.c
+++ b/net/tipc/socket.c

@@ -2771,18 +2771,21 @@ static int tipc_shutdown(struct socket *sock, int how)
 
 	trace_tipc_sk_shutdown(sk, NULL, TIPC_DUMP_ALL, " ");
 	__tipc_shutdown(sock, TIPC_CONN_SHUTDOWN);
-	sk->sk_shutdown = SEND_SHUTDOWN;
+	if (tipc_sk_type_connectionless(sk))
+		sk->sk_shutdown = SHUTDOWN_MASK;
+	else
+		sk->sk_shutdown = SEND_SHUTDOWN;
 
 	if (sk->sk_state == TIPC_DISCONNECTING) {
 		/* Discard any unreceived messages */
 		__skb_queue_purge(&sk->sk_receive_queue);
 
-		/* Wake up anyone sleeping in poll */
-		sk->sk_state_change(sk);
 		res = 0;
 	} else {
 		res = -ENOTCONN;
 	}
+	/* Wake up anyone sleeping in poll. */
+	sk->sk_state_change(sk);
 
 	release_sock(sk);
 	return res;

-- 
2.18.4