netrom: Fix a memory leak in nr_heartbeat_expiry()

syzbot reported a memory leak in nr_create() [0].

Commit 409db27e ("netrom: Fix use-after-free of a listening socket.")
added sock_hold() to the nr_heartbeat_expiry() function, where
a) a socket has a SOCK_DESTROY flag or
b) a listening socket has a SOCK_DEAD flag.

But in the case "a," when the SOCK_DESTROY flag is set, the file descriptor
has already been closed and the nr_release() function has been called.
So it makes no sense to hold the reference count because no one will
call another nr_destroy_socket() and put it as in the case "b."

nr_connect
  nr_establish_data_link
    nr_start_heartbeat

nr_release
  switch (nr->state)
  case NR_STATE_3
    nr->state = NR_STATE_2
    sock_set_flag(sk, SOCK_DESTROY);

                        nr_rx_frame
                          nr_process_rx_frame
                            switch (nr->state)
                            case NR_STATE_2
                              nr_state2_machine()
                                nr_disconnect()
                                  nr_sk(sk)->state = NR_STATE_0
                                  sock_set_flag(sk, SOCK_DEAD)

                        nr_heartbeat_expiry
                          switch (nr->state)
                          case NR_STATE_0
                            if (sock_flag(sk, SOCK_DESTROY) ||
                               (sk->sk_state == TCP_LISTEN
                                 && sock_flag(sk, SOCK_DEAD)))
                               sock_hold()  // ( !!! )
                               nr_destroy_socket()

To fix the memory leak, let's call sock_hold() only for a listening socket.

Found by InfoTeCS on behalf of Linux Verification Center
(linuxtesting.org) with Syzkaller.

[0]: https://syzkaller.appspot.com/bug?extid=d327a1f3b12e1e206c16

Reported-by: syzbot+d327a1f3b12e1e206c16@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=d327a1f3b12e1e206c16
Fixes: 409db27e ("netrom: Fix use-after-free of a listening socket.")
Signed-off-by: Gavrilov Ilia <Ilia.Gavrilov@infotecs.ru>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/netrom/nr_timer.c

@@ -121,7 +121,8 @@ static void nr_heartbeat_expiry(struct timer_list *t)
 		   is accepted() it isn't 'dead' so doesn't get removed. */
 		if (sock_flag(sk, SOCK_DESTROY) ||
 		    (sk->sk_state == TCP_LISTEN && sock_flag(sk, SOCK_DEAD))) {
-			sock_hold(sk);
+			if (sk->sk_state == TCP_LISTEN)
+				sock_hold(sk);
 			bh_unlock_sock(sk);
 			nr_destroy_socket(sk);
 			goto out;