[PATCH net] tcp: fix TFO max_qlen accounting across reuseport migration

[Jiayuan Chen <jiayuan.chen@linux.dev>]

A listener's TCP_FASTOPEN max_qlen stops being accurate and lets through
far more pending Fast Open requests than it was configured for.

This only shows up with SO_REUSEPORT listener migration, where closing a
listener hands its still-pending TFO children over to a surviving one.

fastopenq.qlen is charged in tcp_fastopen_create_child() when the child
is created and uncharged in reqsk_fastopen_remove() when the handshake
completes.  The uncharge follows rsk_listener of the request the child
points at, and inet_reqsk_clone() has repointed the child at a new
request owned by the new listener, so the ++ and the -- land on two
different sockets.  The new listener's qlen drifts negative and its
limit no longer binds.

Charge the new listener during migration, like reqsk_queue_migrated()
already does for queue->young and queue->qlen.

Fixes: 54b92e841937 ("tcp: Migrate TCP_ESTABLISHED/TCP_SYN_RECV sockets in accept queues.")
Signed-off-by: Jiayuan Chen <jiayuan.chen@linux.dev>
---
fastopenq.qlen is read without the lock in tcp_fastopen_queue_check() and
has been since long before this patch, but as this is meant to be
backported I left the READ_ONCE()/WRITE_ONCE() conversion to a separate
patch.
---
 net/ipv4/inet_connection_sock.c | 20 ++++++++++++++++----
 1 file changed, 16 insertions(+), 4 deletions(-)

diff --git a/net/ipv4/inet_connection_sock.c b/net/ipv4/inet_connection_sock.c
index 56902bba5483..6257459bcee2 100644
--- a/net/ipv4/inet_connection_sock.c
+++ b/net/ipv4/inet_connection_sock.c
@@ -943,11 +943,23 @@ static struct request_sock *inet_reqsk_clone(struct request_sock *req,
 
 	nreq->rsk_listener = sk;
 
-	/* We need not acquire fastopenq->lock
-	 * because the child socket is locked in inet_csk_listen_stop().
-	 */
-	if (sk->sk_protocol == IPPROTO_TCP && tcp_rsk(nreq)->tfo_listener)
+	if (sk->sk_protocol == IPPROTO_TCP && tcp_rsk(nreq)->tfo_listener) {
+		struct fastopen_queue *fastopenq;
+
+		/* reqsk_fastopen_remove() will uncharge nreq->rsk_listener,
+		 * that is @sk, so charge it here.  Unlike the listener
+		 * being closed, @sk is live and needs its lock.
+		 */
+		fastopenq = &inet_csk(sk)->icsk_accept_queue.fastopenq;
+		spin_lock_bh(&fastopenq->lock);
+		fastopenq->qlen++;
+		spin_unlock_bh(&fastopenq->lock);
+
+		/* We need not acquire fastopenq->lock
+		 * because the child socket is locked in inet_csk_listen_stop().
+		 */
 		rcu_assign_pointer(tcp_sk(nreq->sk)->fastopen_rsk, nreq);
+	}
 
 	return nreq;
 }
-- 
2.43.0