Linux kernel mirror (for testing)
git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git
kernel
os
linux
Merge branch 'tcp-re-enable-acceptance-of-fin-packets-when-rwin-is-0'
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>
+41
-4
+14
-4
net/ipv4/tcp_input.c
+14
-4
net/ipv4/tcp_input.c
···
4858
4858
*/
4859
4859
4860
4860
static enum skb_drop_reason tcp_sequence(const struct sock *sk,
4861
-
u32 seq, u32 end_seq)
4861
+
u32 seq, u32 end_seq,
4862
+
const struct tcphdr *th)
4862
4863
{
4863
4864
const struct tcp_sock *tp = tcp_sk(sk);
4865
+
u32 seq_limit;
4864
4866
4865
4867
if (before(end_seq, tp->rcv_wup))
4866
4868
return SKB_DROP_REASON_TCP_OLD_SEQUENCE;
4867
4869
4868
-
if (after(end_seq, tp->rcv_nxt + tcp_receive_window(tp))) {
4869
-
if (after(seq, tp->rcv_nxt + tcp_receive_window(tp)))
4870
+
seq_limit = tp->rcv_nxt + tcp_receive_window(tp);
4871
+
if (unlikely(after(end_seq, seq_limit))) {
4872
+
/* Some stacks are known to handle FIN incorrectly; allow the
4873
+
* FIN to extend beyond the window and check it in detail later.
4874
+
*/
4875
+
if (!after(end_seq - th->fin, seq_limit))
4876
+
return SKB_NOT_DROPPED_YET;
4877
+
4878
+
if (after(seq, seq_limit))
4870
4879
return SKB_DROP_REASON_TCP_INVALID_SEQUENCE;
4871
4880
4872
4881
/* Only accept this packet if receive queue is empty. */
···
6388
6379
6389
6380
step1:
6390
6381
/* Step 1: check sequence number */
6391
-
reason = tcp_sequence(sk, TCP_SKB_CB(skb)->seq, TCP_SKB_CB(skb)->end_seq);
6382
+
reason = tcp_sequence(sk, TCP_SKB_CB(skb)->seq,
6383
+
TCP_SKB_CB(skb)->end_seq, th);
6392
6384
if (reason) {
6393
6385
/* RFC793, page 37: "In all states except SYN-SENT, all reset
6394
6386
* (RST) segments are validated by checking their SEQ-fields."
+27
tools/testing/selftests/net/packetdrill/tcp_rcv_zero_wnd_fin.pkt
+27
tools/testing/selftests/net/packetdrill/tcp_rcv_zero_wnd_fin.pkt
···
1
+
// SPDX-License-Identifier: GPL-2.0
2
+
3
+
// Some TCP stacks send FINs even though the window is closed. We break
4
+
// a possible FIN/ACK loop by accepting the FIN.
5
+
6
+
--mss=1000
7
+
8
+
`./defaults.sh`
9
+
10
+
// Establish a connection.
11
+
+0 socket(..., SOCK_STREAM, IPPROTO_TCP) = 3
12
+
+0 setsockopt(3, SOL_SOCKET, SO_REUSEADDR, [1], 4) = 0
13
+
+0 setsockopt(3, SOL_SOCKET, SO_RCVBUF, [20000], 4) = 0
14
+
+0 bind(3, ..., ...) = 0
15
+
+0 listen(3, 1) = 0
16
+
17
+
+0 < S 0:0(0) win 32792 <mss 1000,nop,wscale 7>
18
+
+0 > S. 0:0(0) ack 1 <mss 1460,nop,wscale 0>
19
+
+0 < . 1:1(0) ack 1 win 257
20
+
21
+
+0 accept(3, ..., ...) = 4
22
+
23
+
+0 < P. 1:60001(60000) ack 1 win 257
24
+
* > . 1:1(0) ack 60001 win 0
25
+
26
+
+0 < F. 60001:60001(0) ack 1 win 257
27
+
+0 > . 1:1(0) ack 60002 win 0