[PATCH net v2 0/2] tcp: re-enable acceptance of FIN packets when RWIN is 0

[PATCH net v2 0/2] tcp: re-enable acceptance of FIN packets when RWIN is 0

[Simon Baatz via B4 Relay]

Hi,

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.

Signed-off-by: Simon Baatz <gmbnomis@gmail.com>
---
Changes in v2:
- Move the FIN handling out of the default execution path as suggested by Eric Dumazet
- Link to v1: https://lore.kernel.org/r/20260222-fix_zero_wnd_fin-v1-0-5f4034952f3c@gmail.com

---
Simon Baatz (2):
      tcp: re-enable acceptance of FIN packets when RWIN is 0
      selftests/net: packetdrill: Verify acceptance of FIN packets when RWIN is 0

 net/ipv4/tcp_input.c                               | 18 +++++++++++----
 .../net/packetdrill/tcp_rcv_zero_wnd_fin.pkt       | 27 ++++++++++++++++++++++
 2 files changed, 41 insertions(+), 4 deletions(-)
---
base-commit: d4f687fbbce45b5e88438e89b5e26c0c15847992
change-id: 20260221-fix_zero_wnd_fin-d1ba11cd3b07

Best regards,
-- 
Simon Baatz <gmbnomis@gmail.com>

[PATCH net v2 1/2] tcp: re-enable acceptance of FIN packets when RWIN is 0

[Simon Baatz via B4 Relay]

From: Simon Baatz <gmbnomis@gmail.com>

Commit 2bd99aef1b19 ("tcp: accept bare FIN packets under memory
pressure") allowed accepting FIN packets in tcp_data_queue() even when
the receive window was closed, to prevent ACK/FIN loops with broken
clients.

Such a FIN packet is in sequence, but because the FIN consumes a
sequence number, it extends beyond the window. Before commit
9ca48d616ed7 ("tcp: do not accept packets beyond window"),
tcp_sequence() only required the seq to be within the window. After
that change, the entire packet (including the FIN) must fit within the
window. As a result, such FIN packets are now dropped and the handling
path is no longer reached.

Be more lenient by not counting the sequence number consumed by the
FIN when calling tcp_sequence(), restoring the previous behavior for
cases where only the FIN extends beyond the window.

Fixes: 9ca48d616ed7 ("tcp: do not accept packets beyond window")
Signed-off-by: Simon Baatz <gmbnomis@gmail.com>
---
 net/ipv4/tcp_input.c | 18 ++++++++++++++----
 1 file changed, 14 insertions(+), 4 deletions(-)

diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
index e7b41abb82aad33d8cab4fcfa989cc4771149b41..1c6b8ca67918f0601b9b3e8ef884c5fef0a7ab65 100644
--- a/net/ipv4/tcp_input.c
+++ b/net/ipv4/tcp_input.c
@@ -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."

-- 
2.53.0

Re: [PATCH net v2 1/2] tcp: re-enable acceptance of FIN packets when RWIN is 0

[Eric Dumazet]

On Tue, Feb 24, 2026 at 9:20 AM Simon Baatz via B4 Relay
<devnull+gmbnomis.gmail.com@kernel.org> wrote:
>
> From: Simon Baatz <gmbnomis@gmail.com>
>
> Commit 2bd99aef1b19 ("tcp: accept bare FIN packets under memory
> pressure") allowed accepting FIN packets in tcp_data_queue() even when
> the receive window was closed, to prevent ACK/FIN loops with broken
> clients.
>
> Such a FIN packet is in sequence, but because the FIN consumes a
> sequence number, it extends beyond the window. Before commit
> 9ca48d616ed7 ("tcp: do not accept packets beyond window"),
> tcp_sequence() only required the seq to be within the window. After
> that change, the entire packet (including the FIN) must fit within the
> window. As a result, such FIN packets are now dropped and the handling
> path is no longer reached.
>
> Be more lenient by not counting the sequence number consumed by the
> FIN when calling tcp_sequence(), restoring the previous behavior for
> cases where only the FIN extends beyond the window.
>
> Fixes: 9ca48d616ed7 ("tcp: do not accept packets beyond window")
> Signed-off-by: Simon Baatz <gmbnomis@gmail.com>
> ---

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

Re: [PATCH net v2 1/2] tcp: re-enable acceptance of FIN packets when RWIN is 0

[Kuniyuki Iwashima]

On Tue, Feb 24, 2026 at 12:20 AM Simon Baatz via B4 Relay
<devnull+gmbnomis.gmail.com@kernel.org> wrote:
>
> From: Simon Baatz <gmbnomis@gmail.com>
>
> Commit 2bd99aef1b19 ("tcp: accept bare FIN packets under memory
> pressure") allowed accepting FIN packets in tcp_data_queue() even when
> the receive window was closed, to prevent ACK/FIN loops with broken
> clients.
>
> Such a FIN packet is in sequence, but because the FIN consumes a
> sequence number, it extends beyond the window. Before commit
> 9ca48d616ed7 ("tcp: do not accept packets beyond window"),
> tcp_sequence() only required the seq to be within the window. After
> that change, the entire packet (including the FIN) must fit within the
> window. As a result, such FIN packets are now dropped and the handling
> path is no longer reached.
>
> Be more lenient by not counting the sequence number consumed by the
> FIN when calling tcp_sequence(), restoring the previous behavior for
> cases where only the FIN extends beyond the window.
>
> Fixes: 9ca48d616ed7 ("tcp: do not accept packets beyond window")
> Signed-off-by: Simon Baatz <gmbnomis@gmail.com>

Reviewed-by: Kuniyuki Iwashima <kuniyu@google.com>

[PATCH net v2 2/2] selftests/net: packetdrill: Verify acceptance of FIN packets when RWIN is 0

[Simon Baatz via B4 Relay]

From: Simon Baatz <gmbnomis@gmail.com>

Add a packetdrill test that verifies we accept bare FIN packets when
the advertised receive window is zero.

Signed-off-by: Simon Baatz <gmbnomis@gmail.com>
---
 .../net/packetdrill/tcp_rcv_zero_wnd_fin.pkt       | 27 ++++++++++++++++++++++
 1 file changed, 27 insertions(+)

diff --git a/tools/testing/selftests/net/packetdrill/tcp_rcv_zero_wnd_fin.pkt b/tools/testing/selftests/net/packetdrill/tcp_rcv_zero_wnd_fin.pkt
new file mode 100644
index 0000000000000000000000000000000000000000..e245359a1a91d4b6d7ef64496681cea57792cb71
--- /dev/null
+++ b/tools/testing/selftests/net/packetdrill/tcp_rcv_zero_wnd_fin.pkt
@@ -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

-- 
2.53.0

Re: [PATCH net v2 2/2] selftests/net: packetdrill: Verify acceptance of FIN packets when RWIN is 0

[Eric Dumazet]

On Tue, Feb 24, 2026 at 9:20 AM Simon Baatz via B4 Relay
<devnull+gmbnomis.gmail.com@kernel.org> wrote:
>
> From: Simon Baatz <gmbnomis@gmail.com>
>
> Add a packetdrill test that verifies we accept bare FIN packets when
> the advertised receive window is zero.
>
> Signed-off-by: Simon Baatz <gmbnomis@gmail.com>

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

Thanks !

Re: [PATCH net v2 2/2] selftests/net: packetdrill: Verify acceptance of FIN packets when RWIN is 0

[Kuniyuki Iwashima]

On Tue, Feb 24, 2026 at 12:20 AM Simon Baatz via B4 Relay
<devnull+gmbnomis.gmail.com@kernel.org> wrote:
>
> From: Simon Baatz <gmbnomis@gmail.com>
>
> Add a packetdrill test that verifies we accept bare FIN packets when
> the advertised receive window is zero.
>
> Signed-off-by: Simon Baatz <gmbnomis@gmail.com>

Reviewed-by: Kuniyuki Iwashima <kuniyu@google.com>

Re: [PATCH net v2 0/2] tcp: re-enable acceptance of FIN packets when RWIN is 0

[patchwork-bot+netdevbpf]

Hello:

This series was applied to netdev/net.git (main)
by Jakub Kicinski <kuba@kernel.org>:

On Tue, 24 Feb 2026 09:20:11 +0100 you wrote:
> Hi,
> 
> 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.
> 
> Signed-off-by: Simon Baatz <gmbnomis@gmail.com>
> 
> [...]

Here is the summary with links:
  - [net,v2,1/2] tcp: re-enable acceptance of FIN packets when RWIN is 0
    https://git.kernel.org/netdev/net/c/1e3bb184e941
  - [net,v2,2/2] selftests/net: packetdrill: Verify acceptance of FIN packets when RWIN is 0
    https://git.kernel.org/netdev/net/c/745113323098

You are awesome, thank you!
-- 
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