From c31b70c9968fe9c4194d1b5d06d07596a3b680de Mon Sep 17 00:00:00 2001 From: Alexander Duyck Date: Sat, 12 Dec 2020 12:31:24 -0800 Subject: [PATCH] tcp: Add logic to check for SYN w/ data in tcp_simple_retransmit There are cases where a fastopen SYN may trigger either a ICMP_TOOBIG message in the case of IPv6 or a fragmentation request in the case of IPv4. This results in the socket stalling for a second or more as it does not respond to the message by retransmitting the SYN frame. Normally a SYN frame should not be able to trigger a ICMP_TOOBIG or ICMP_FRAG_NEEDED however in the case of fastopen we can have a frame that makes use of the entire MSS. In the case of fastopen it does, and an additional complication is that the retransmit queue doesn't contain the original frames. As a result when tcp_simple_retransmit is called and walks the list of frames in the queue it may not mark the frames as lost because both the SYN and the data packet each individually are smaller than the MSS size after the adjustment. This results in the socket being stalled until the retransmit timer kicks in and forces the SYN frame out again without the data attached. In order to resolve this we can reduce the MSS the packets are compared to in tcp_simple_retransmit to -1 for cases where we are still in the TCP_SYN_SENT state for a fastopen socket. Doing this we will mark all of the packets related to the fastopen SYN as lost. Signed-off-by: Alexander Duyck Signed-off-by: Eric Dumazet Signed-off-by: Yuchung Cheng Link: https://lore.kernel.org/r/160780498125.3272.15437756269539236825.stgit@localhost.localdomain Signed-off-by: Jakub Kicinski --- net/ipv4/tcp_input.c | 17 ++++++++++++++++- 1 file changed, 16 insertions(+), 1 deletion(-) diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c index 48ee476aa031..c7e16b0ed791 100644 --- a/net/ipv4/tcp_input.c +++ b/net/ipv4/tcp_input.c @@ -2689,7 +2689,22 @@ void tcp_simple_retransmit(struct sock *sk) const struct inet_connection_sock *icsk = inet_csk(sk); struct tcp_sock *tp = tcp_sk(sk); struct sk_buff *skb; - unsigned int mss = tcp_current_mss(sk); + int mss; + + /* A fastopen SYN request is stored as two separate packets within + * the retransmit queue, this is done by tcp_send_syn_data(). + * As a result simply checking the MSS of the frames in the queue + * will not work for the SYN packet. + * + * Us being here is an indication of a path MTU issue so we can + * assume that the fastopen SYN was lost and just mark all the + * frames in the retransmit queue as lost. We will use an MSS of + * -1 to mark all frames as lost, otherwise compute the current MSS. + */ + if (tp->syn_data && sk->sk_state == TCP_SYN_SENT) + mss = -1; + else + mss = tcp_current_mss(sk); skb_rbtree_walk(skb, &sk->tcp_rtx_queue) { if (tcp_skb_seglen(skb) > mss)