diff options
| author | Jakub Kicinski <kuba@kernel.org> | 2026-02-26 06:07:07 +0300 |
|---|---|---|
| committer | Jakub Kicinski <kuba@kernel.org> | 2026-02-26 06:07:07 +0300 |
| commit | 7c9db1a1cd3be140f7807aa9f405cc17be59dc2a (patch) | |
| tree | cd8a998015198a0a4172a6ba8a7b1f7ca3062ff3 | |
| parent | 5cc619583c7e735c4fb801bede671fb6f9c79425 (diff) | |
| parent | 74511332309844c3de64970841b3c250d85f34ce (diff) | |
| download | linux-7c9db1a1cd3be140f7807aa9f405cc17be59dc2a.tar.xz | |
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>
| -rw-r--r-- | net/ipv4/tcp_input.c | 18 | ||||
| -rw-r--r-- | tools/testing/selftests/net/packetdrill/tcp_rcv_zero_wnd_fin.pkt | 27 |
2 files changed, 41 insertions, 4 deletions
diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c index e7b41abb82aa..1c6b8ca67918 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." 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 000000000000..e245359a1a91 --- /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 |