From 82b72cb94666b3dbd7152bb9f441b068af7a921b Mon Sep 17 00:00:00 2001 From: Florian Westphal Date: Sat, 29 Jan 2022 17:47:01 +0100 Subject: netfilter: conntrack: re-init state for retransmitted syn-ack TCP conntrack assumes that a syn-ack retransmit is identical to the previous syn-ack. This isn't correct and causes stuck 3whs in some more esoteric scenarios. tcpdump to illustrate the problem: client > server: Flags [S] seq 1365731894, win 29200, [mss 1460,sackOK,TS val 2083035583 ecr 0,wscale 7] server > client: Flags [S.] seq 145824453, ack 643160523, win 65535, [mss 8952,wscale 5,TS val 3215367629 ecr 2082921663] Note the invalid/outdated synack ack number. Conntrack marks this syn-ack as out-of-window/invalid, but it did initialize the reply direction parameters based on this packets content. client > server: Flags [S] seq 1365731894, win 29200, [mss 1460,sackOK,TS val 2083036623 ecr 0,wscale 7] ... retransmit... server > client: Flags [S.], seq 145824453, ack 643160523, win 65535, [mss 8952,wscale 5,TS val 3215368644 ecr 2082921663] and another bogus synack. This repeats, then client re-uses for a new attempt: client > server: Flags [S], seq 2375731741, win 29200, [mss 1460,sackOK,TS val 2083100223 ecr 0,wscale 7] server > client: Flags [S.], seq 145824453, ack 643160523, win 65535, [mss 8952,wscale 5,TS val 3215430754 ecr 2082921663] ... but still gets a invalid syn-ack. This repeats until: server > client: Flags [S.], seq 145824453, ack 643160523, win 65535, [mss 8952,wscale 5,TS val 3215437785 ecr 2082921663] server > client: Flags [R.], seq 145824454, ack 643160523, win 65535, [mss 8952,wscale 5,TS val 3215443451 ecr 2082921663] client > server: Flags [S], seq 2375731741, win 29200, [mss 1460,sackOK,TS val 2083115583 ecr 0,wscale 7] server > client: Flags [S.], seq 162602410, ack 2375731742, win 65535, [mss 8952,wscale 5,TS val 3215445754 ecr 2083115583] This syn-ack has the correct ack number, but conntrack flags it as invalid: The internal state was created from the first syn-ack seen so the sequence number of the syn-ack is treated as being outside of the announced window. Don't assume that retransmitted syn-ack is identical to previous one. Treat it like the first syn-ack and reinit state. Signed-off-by: Florian Westphal Acked-by: Jozsef Kadlecsik Signed-off-by: Pablo Neira Ayuso --- net/netfilter/nf_conntrack_proto_tcp.c | 12 ++++++++++++ 1 file changed, 12 insertions(+) (limited to 'net') diff --git a/net/netfilter/nf_conntrack_proto_tcp.c b/net/netfilter/nf_conntrack_proto_tcp.c index 88c89e97d8a2..d1582b888c0d 100644 --- a/net/netfilter/nf_conntrack_proto_tcp.c +++ b/net/netfilter/nf_conntrack_proto_tcp.c @@ -571,6 +571,18 @@ static bool tcp_in_window(struct nf_conn *ct, sender->td_maxwin = (win == 0 ? 1 : win); tcp_options(skb, dataoff, tcph, sender); + } else if (tcph->syn && dir == IP_CT_DIR_REPLY && + state->state == TCP_CONNTRACK_SYN_SENT) { + /* Retransmitted syn-ack, or syn (simultaneous open). + * + * Re-init state for this direction, just like for the first + * syn(-ack) reply, it might differ in seq, ack or tcp options. + */ + tcp_init_sender(sender, receiver, + skb, dataoff, tcph, + end, win); + if (!tcph->ack) + return true; } if (!(tcph->ack)) { -- cgit v1.2.3