linux-kernel - [PATCH 5.15 568/913] tcp: ensure PMTU updates are processed during fastopen

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Date:   Tue,  5 Apr 2022 09:27:10 +0200
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     linux-kernel@...r.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        stable@...r.kernel.org, Andre Nash <alnash@...com>,
        Neil Spring <ntspring@...com>, Wei Wang <weiwan@...gle.com>,
        Yuchung Cheng <ycheng@...gle.com>,
        Martin KaFai Lau <kafai@...com>,
        Jakub Kicinski <kuba@...nel.org>,
        Eric Dumazet <edumazet@...gle.com>,
        Sasha Levin <sashal@...nel.org>
Subject: [PATCH 5.15 568/913] tcp: ensure PMTU updates are processed during fastopen

From: Jakub Kicinski <kuba@...nel.org>

[ 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@...com>
Reported-by: Neil Spring <ntspring@...com>
Reviewed-by: Wei Wang <weiwan@...gle.com>
Acked-by: Yuchung Cheng <ycheng@...gle.com>
Acked-by: Martin KaFai Lau <kafai@...com>
Signed-off-by: Jakub Kicinski <kuba@...nel.org>
Reviewed-by: Eric Dumazet <edumazet@...gle.com>
Link: https://lore.kernel.org/r/20220321165957.1769954-1-kuba@kernel.org
Signed-off-by: Jakub Kicinski <kuba@...nel.org>
Signed-off-by: Sasha Levin <sashal@...nel.org>
---
 net/ipv4/tcp_output.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

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);
-- 
2.34.1