Re: [PATCH net 1/2] tcp: challenge ACK for non-exact RST in SYN-RECEIVED

On Fri, Jul 17, 2026 at 10:15 AM Yuxiang Yang
[off-list ref] wrote:

The SYN-RECEIVED request-socket path in tcp_check_req() accepts an
in-window RST without requiring SEG.SEQ to exactly match RCV.NXT.  A
non-exact RST therefore removes the request instead of eliciting a
challenge ACK.

RFC 9293 section 3.10.7.4 applies the RFC 5961 reset check in
SYN-RECEIVED: an exact RST resets the connection, while a non-exact
in-window RST must trigger a challenge ACK and be dropped.

Apply that check before the ACK-field validation, following the RFC
sequence-number, RST, then ACK processing order.  Factor the per-netns
challenge ACK quota out of tcp_send_challenge_ack() so request sockets
can share it.  Use the request socket's send_ack() callback and its own
out-of-window ACK timestamp to send and rate-limit the response.

Reported-by: Yuxiang Yang <redacted>
Reported-by: Yizhou Zhao <redacted>
Reported-by: Ao Wang <redacted>
Reported-by: Xuewei Feng <redacted>
Reported-by: Qi Li <redacted>
Reported-by: Ke Xu <redacted>
Fixes: 282f23c6ee34 ("tcp: implement RFC 5961 3.2")
Cc: stable@vger.kernel.org
Signed-off-by: Yuxiang Yang <redacted>

Note : Please do not add a Reported-by: Yuxiang Yang
[off-list ref] if you are the patch author.

The amount of noise in this patch footer is quite annoying.

Reviewed-by: Eric Dumazet <edumazet@google.com>