inet: fix races with reqsk timers

reqsk_queue_destroy() and reqsk_queue_unlink() should use
del_timer_sync() instead of del_timer() before calling reqsk_put(),
otherwise we could free a req still used by another cpu.

But before doing so, reqsk_queue_destroy() must release syn_wait_lock
spinlock or risk a dead lock, as reqsk_timer_handler() might
need to take this same spinlock from reqsk_queue_unlink() (called from
inet_csk_reqsk_queue_drop())

Fixes: fa76ce7328b2 ("inet: get rid of central tcp/dccp listener timer")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/core/request_sock.c

@@ -103,10 +103,16 @@ void reqsk_queue_destroy(struct request_sock_queue *queue)
 			spin_lock_bh(&queue->syn_wait_lock);
 			while ((req = lopt->syn_table[i]) != NULL) {
 				lopt->syn_table[i] = req->dl_next;
+				/* Because of following del_timer_sync(),
+				 * we must release the spinlock here
+				 * or risk a dead lock.
+				 */
+				spin_unlock_bh(&queue->syn_wait_lock);
 				atomic_inc(&lopt->qlen_dec);
-				if (del_timer(&req->rsk_timer))
+				if (del_timer_sync(&req->rsk_timer))
 					reqsk_put(req);
 				reqsk_put(req);
+				spin_lock_bh(&queue->syn_wait_lock);
 			}
 			spin_unlock_bh(&queue->syn_wait_lock);
 		}

net/ipv4/inet_connection_sock.c

@@ -593,7 +593,7 @@ static bool reqsk_queue_unlink(struct request_sock_queue *queue,
 	}
 
 	spin_unlock(&queue->syn_wait_lock);
-	if (del_timer(&req->rsk_timer))
+	if (del_timer_sync(&req->rsk_timer))
 		reqsk_put(req);
 	return found;
 }