inet: fix race in reqsk_queue_unlink()

reqsk_timer_handler() tests if icsk_accept_queue.listen_opt
is NULL at its beginning.

By the time it calls inet_csk_reqsk_queue_drop() and
reqsk_queue_unlink(), listener might have been closed and
inet_csk_listen_stop() had called reqsk_queue_yank_acceptq()
which sets icsk_accept_queue.listen_opt to NULL

We therefore need to correctly check listen_opt being NULL
after holding syn_wait_lock for proper synchronization.

Fixes: fa76ce73 ("inet: get rid of central tcp/dccp listener timer")
Fixes: b357a364 ("inet: fix possible panic in reqsk_queue_unlink()")
Signed-off-by: NEric Dumazet <edumazet@google.com>
Cc: Yuchung Cheng <ycheng@google.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/ipv4/inet_connection_sock.c

@@ -577,21 +577,22 @@ EXPORT_SYMBOL(inet_rtx_syn_ack);
 static bool reqsk_queue_unlink(struct request_sock_queue *queue,
 			       struct request_sock *req)
 {
-	struct listen_sock *lopt = queue->listen_opt;
 	struct request_sock **prev;
+	struct listen_sock *lopt;
 	bool found = false;
 
 	spin_lock(&queue->syn_wait_lock);
-
-	for (prev = &lopt->syn_table[req->rsk_hash]; *prev != NULL;
-	     prev = &(*prev)->dl_next) {
-		if (*prev == req) {
-			*prev = req->dl_next;
-			found = true;
-			break;
+	lopt = queue->listen_opt;
+	if (lopt) {
+		for (prev = &lopt->syn_table[req->rsk_hash]; *prev != NULL;
+		     prev = &(*prev)->dl_next) {
+			if (*prev == req) {
+				*prev = req->dl_next;
+				found = true;
+				break;
+			}
 		}
 	}
-
 	spin_unlock(&queue->syn_wait_lock);
 	if (timer_pending(&req->rsk_timer) && del_timer_sync(&req->rsk_timer))
 		reqsk_put(req);