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Merge branch 'tcp-re-enable-acceptance-of-fin-packets-when-rwin-is-0'

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Simon Baatz says:

====================
tcp: re-enable acceptance of FIN packets when RWIN is 0

this series restores the ability to accept in‑sequence FIN packets
even when the advertised receive window is zero, and adds a
packetdrill test to guard the behavior.
====================

Link: https://patch.msgid.link/20260224-fix_zero_wnd_fin-v2-0-a16677ea7cea@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
This commit is contained in:
Jakub Kicinski 2026-02-25 19:07:07 -08:00
commit 7c9db1a1cd
2 changed files with 41 additions and 4 deletions
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18
net/ipv4/tcp_input.c
View file

@ -4858,15 +4858,24 @@ static enum skb_drop_reason tcp_disordered_ack_check(const struct sock *sk,
*/
static enum skb_drop_reason tcp_sequence(const struct sock *sk,
u32 seq, u32 end_seq)
u32 seq, u32 end_seq,
const struct tcphdr *th)
{
const struct tcp_sock *tp = tcp_sk(sk);
u32 seq_limit;
if (before(end_seq, tp->rcv_wup))
return SKB_DROP_REASON_TCP_OLD_SEQUENCE;
if (after(end_seq, tp->rcv_nxt + tcp_receive_window(tp))) {
if (after(seq, tp->rcv_nxt + tcp_receive_window(tp)))
seq_limit = tp->rcv_nxt + tcp_receive_window(tp);
if (unlikely(after(end_seq, seq_limit))) {
/* Some stacks are known to handle FIN incorrectly; allow the
* FIN to extend beyond the window and check it in detail later.
*/
if (!after(end_seq - th->fin, seq_limit))
return SKB_NOT_DROPPED_YET;
if (after(seq, seq_limit))
return SKB_DROP_REASON_TCP_INVALID_SEQUENCE;
/* Only accept this packet if receive queue is empty. */
@ -6379,7 +6388,8 @@ static bool tcp_validate_incoming(struct sock *sk, struct sk_buff *skb,
step1:
/* Step 1: check sequence number */
reason = tcp_sequence(sk, TCP_SKB_CB(skb)->seq, TCP_SKB_CB(skb)->end_seq);
reason = tcp_sequence(sk, TCP_SKB_CB(skb)->seq,
TCP_SKB_CB(skb)->end_seq, th);
if (reason) {
/* RFC793, page 37: "In all states except SYN-SENT, all reset
* (RST) segments are validated by checking their SEQ-fields."

27
tools/testing/selftests/net/packetdrill/tcp_rcv_zero_wnd_fin.pkt Normal file
View file

@ -0,0 +1,27 @@
// SPDX-License-Identifier: GPL-2.0
// Some TCP stacks send FINs even though the window is closed. We break
// a possible FIN/ACK loop by accepting the FIN.
--mss=1000
`./defaults.sh`
// Establish a connection.
+0 socket(..., SOCK_STREAM, IPPROTO_TCP) = 3
+0 setsockopt(3, SOL_SOCKET, SO_REUSEADDR, [1], 4) = 0
+0 setsockopt(3, SOL_SOCKET, SO_RCVBUF, [20000], 4) = 0
+0 bind(3, ..., ...) = 0
+0 listen(3, 1) = 0
+0 < S 0:0(0) win 32792 <mss 1000,nop,wscale 7>
+0 > S. 0:0(0) ack 1 <mss 1460,nop,wscale 0>
+0 < . 1:1(0) ack 1 win 257
+0 accept(3, ..., ...) = 4
+0 < P. 1:60001(60000) ack 1 win 257
* > . 1:1(0) ack 60001 win 0
+0 < F. 60001:60001(0) ack 1 win 257
+0 > . 1:1(0) ack 60002 win 0