Commit 7c9db1a1 authored by Jakub Kicinski's avatar Jakub Kicinski
Browse files

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: default avatarJakub Kicinski <kuba@kernel.org>
parents 5cc61958 74511332

net/ipv4/tcp_input.c

+14 −4
Original line number Diff line number Diff line
@@ -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."

tools/testing/selftests/net/packetdrill/tcp_rcv_zero_wnd_fin.pkt

0 → 100644
+27 −0
Original line number Diff line number Diff line 
// 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