This is a note to let you know that I've just added the patch titled tcp: ensure PMTU updates are processed during fastopen to the 5.15-stable tree which can be found at: http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary The filename of the patch is: tcp-ensure-pmtu-updates-are-processed-during-fastope.patch and it can be found in the queue-5.15 subdirectory. If you, or anyone else, feels it should not be added to the stable tree, please let <stable@xxxxxxxxxxxxxxx> know about it. commit abcf892aa695e18bd8c3c1ddc842023044077bc8 Author: Jakub Kicinski <kuba@xxxxxxxxxx> Date: Mon Mar 21 09:59:57 2022 -0700 tcp: ensure PMTU updates are processed during fastopen [ Upstream commit ed0c99dc0f499ff8b6e75b5ae6092ab42be1ad39 ] tp->rx_opt.mss_clamp is not populated, yet, during TFO send so we rise it to the local MSS. tp->mss_cache is not updated, however: tcp_v6_connect(): tp->rx_opt.mss_clamp = IPV6_MIN_MTU - headers; tcp_connect(): tcp_connect_init(): tp->mss_cache = min(mtu, tp->rx_opt.mss_clamp) tcp_send_syn_data(): tp->rx_opt.mss_clamp = tp->advmss After recent fixes to ICMPv6 PTB handling we started dropping PMTU updates higher than tp->mss_cache. Because of the stale tp->mss_cache value PMTU updates during TFO are always dropped. Thanks to Wei for helping zero in on the problem and the fix! Fixes: c7bb4b89033b ("ipv6: tcp: drop silly ICMPv6 packet too big messages") Reported-by: Andre Nash <alnash@xxxxxx> Reported-by: Neil Spring <ntspring@xxxxxx> Reviewed-by: Wei Wang <weiwan@xxxxxxxxxx> Acked-by: Yuchung Cheng <ycheng@xxxxxxxxxx> Acked-by: Martin KaFai Lau <kafai@xxxxxx> Signed-off-by: Jakub Kicinski <kuba@xxxxxxxxxx> Reviewed-by: Eric Dumazet <edumazet@xxxxxxxxxx> Link: https://lore.kernel.org/r/20220321165957.1769954-1-kuba@xxxxxxxxxx Signed-off-by: Jakub Kicinski <kuba@xxxxxxxxxx> Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx> diff --git a/net/ipv4/tcp_output.c b/net/ipv4/tcp_output.c index 0492f6942778..369752f5f676 100644 --- a/net/ipv4/tcp_output.c +++ b/net/ipv4/tcp_output.c @@ -3734,6 +3734,7 @@ static void tcp_connect_queue_skb(struct sock *sk, struct sk_buff *skb) */ static int tcp_send_syn_data(struct sock *sk, struct sk_buff *syn) { + struct inet_connection_sock *icsk = inet_csk(sk); struct tcp_sock *tp = tcp_sk(sk); struct tcp_fastopen_request *fo = tp->fastopen_req; int space, err = 0; @@ -3748,8 +3749,10 @@ static int tcp_send_syn_data(struct sock *sk, struct sk_buff *syn) * private TCP options. The cost is reduced data space in SYN :( */ tp->rx_opt.mss_clamp = tcp_mss_clamp(tp, tp->rx_opt.mss_clamp); + /* Sync mss_cache after updating the mss_clamp */ + tcp_sync_mss(sk, icsk->icsk_pmtu_cookie); - space = __tcp_mtu_to_mss(sk, inet_csk(sk)->icsk_pmtu_cookie) - + space = __tcp_mtu_to_mss(sk, icsk->icsk_pmtu_cookie) - MAX_TCP_OPTION_SPACE; space = min_t(size_t, space, fo->size);