net/x25: Fix null-ptr-deref caused by x25_disconnect

When the link layer is terminating, x25->neighbour will be set to NULL
in x25_disconnect(). As a result, it could cause null-ptr-deref bugs in
x25_sendmsg(),x25_recvmsg() and x25_connect(). One of the bugs is
shown below.

    (Thread 1)                 |  (Thread 2)
x25_link_terminated()          | x25_recvmsg()
 x25_kill_by_neigh()           |  ...
  x25_disconnect()             |  lock_sock(sk)
   ...                         |  ...
   x25->neighbour = NULL //(1) |
   ...                         |  x25->neighbour->extended //(2)

The code sets NULL to x25->neighbour in position (1) and dereferences
x25->neighbour in position (2), which could cause null-ptr-deref bug.

This patch adds lock_sock() in x25_kill_by_neigh() in order to synchronize
with x25_sendmsg(), x25_recvmsg() and x25_connect(). What`s more, the
sock held by lock_sock() is not NULL, because it is extracted from x25_list
and uses x25_list_lock to synchronize.

Fixes: 4becb7ee ("net/x25: Fix x25_neigh refcnt leak when x25 disconnect")
Signed-off-by: Duoming Zhou <duoming@zju.edu.cn>
Reviewed-by: Lin Ma <linma@zju.edu.cn>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/x25/af_x25.c

@@ -1765,10 +1765,15 @@ void x25_kill_by_neigh(struct x25_neigh *nb)
 
 	write_lock_bh(&x25_list_lock);
 
-	sk_for_each(s, &x25_list)
-		if (x25_sk(s)->neighbour == nb)
+	sk_for_each(s, &x25_list) {
+		if (x25_sk(s)->neighbour == nb) {
+			write_unlock_bh(&x25_list_lock);
+			lock_sock(s);
 			x25_disconnect(s, ENETUNREACH, 0, 0);
-
+			release_sock(s);
+			write_lock_bh(&x25_list_lock);
+		}
+	}
 	write_unlock_bh(&x25_list_lock);
 
 	/* Remove any related forwards */