Commit 1e3bb184 authored by Simon Baatz's avatar Simon Baatz Committed by Jakub Kicinski
Browse files

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



Commit 2bd99aef ("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
9ca48d61 ("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: 9ca48d61 ("tcp: do not accept packets beyond window")
Signed-off-by: default avatarSimon Baatz <gmbnomis@gmail.com>
Reviewed-by: default avatarEric Dumazet <edumazet@google.com>
Reviewed-by: default avatarKuniyuki Iwashima <kuniyu@google.com>
Link: https://patch.msgid.link/20260224-fix_zero_wnd_fin-v2-1-a16677ea7cea@gmail.com


Signed-off-by: default avatarJakub Kicinski <kuba@kernel.org>
parent 5cc61958

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."