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>

Author: kuba-moo

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."

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