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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Note the invalid/outdated synack ack number.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 <fw@strlen.de>
Acked-by: Jozsef Kadlecsik <kadlec@netfilter.org>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
This commit is contained in:
Florian Westphal 2022-01-29 17:47:01 +01:00 committed by Pablo Neira Ayuso
parent cc4f9d6203
commit 82b72cb946
1 changed files with 12 additions and 0 deletions
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12
net/netfilter/nf_conntrack_proto_tcp.c
View File

@ -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)) {