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

Commit 2bd99ae ("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
9ca48d6 ("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: 9ca48d6 ("tcp: do not accept packets beyond window")
Signed-off-by: Simon Baatz <gmbnomis@gmail.com>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: Kuniyuki Iwashima <kuniyu@google.com>
Link: https://patch.msgid.link/20260224-fix_zero_wnd_fin-v2-1-a16677ea7cea@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

Author: gmbnomis

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."