From: Simon Baatz Commit 2bd99aef1b19 ("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 9ca48d616ed7 ("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: 9ca48d616ed7 ("tcp: do not accept packets beyond window") Signed-off-by: Simon Baatz --- net/ipv4/tcp_input.c | 18 ++++++++++++++---- 1 file changed, 14 insertions(+), 4 deletions(-) diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c index e7b41abb82aad33d8cab4fcfa989cc4771149b41..1c6b8ca67918f0601b9b3e8ef884c5fef0a7ab65 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." -- 2.53.0