net/x25: Fix x25_neigh refcnt leak when x25 disconnect

BugLink: https://bugs.launchpad.net/bugs/1892822

commit 4becb7ee upstream.

x25_connect() invokes x25_get_neigh(), which returns a reference of the
specified x25_neigh object to "x25->neighbour" with increased refcnt.

When x25 connect success and returns, the reference still be hold by
"x25->neighbour", so the refcount should be decreased in
x25_disconnect() to keep refcount balanced.

The reference counting issue happens in x25_disconnect(), which forgets
to decrease the refcnt increased by x25_get_neigh() in x25_connect(),
causing a refcnt leak.

Fix this issue by calling x25_neigh_put() before x25_disconnect()
returns.
Signed-off-by: Xiyu Yang <xiyuyang19@fudan.edu.cn>
Signed-off-by: Xin Tan <tanxin.ctf@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Khalid Elmously <khalid.elmously@canonical.com>
Signed-off-by: Ian May <ian.may@canonical.com>
Signed-off-by: Kelsey Skunberg <kelsey.skunberg@canonical.com>

net/x25/x25_subr.c

@@ -368,6 +368,10 @@ void x25_disconnect(struct sock *sk, int reason, unsigned char cause,
 		sk->sk_state_change(sk);
 		sock_set_flag(sk, SOCK_DEAD);
 	}
+	read_lock_bh(&x25_list_lock);
+	x25_neigh_put(x25->neighbour);
+	x25->neighbour = NULL;
+	read_unlock_bh(&x25_list_lock);
 }
 
 /*