From f81e6c3fb06327bc49cdd6e559845293ba06a704 Mon Sep 17 00:00:00 2001 From: Inbal Schussheim Date: Mon, 14 Sep 2026 12:04:07 +0300 Subject: [PATCH] tcp: exclude old ACKs from tcp fast path Exclude old ACKs before SND.UNA from the tcp fast path as well as ACKs after SND.NXT. Such ACKs will fall through to the slow path, where tcp_ack() performs the appropriate validation and challenge ACK handling according to RFC5961 and Commit 3d501dd326fb1c7 ("tcp: do not accept ACK of bytes we never sent"). This prevents old ACKs from being accepted or modifying connection state as part of the fast path before appropriate ACK validation is applied. In particular, this prevents payload carried by a segment with an excessively old ACK from advancing RCV.NXT before the ACK is rejected. Fixes: 31770e34e43d ("tcp: Revert "tcp: remove header prediction"") Reported-by: Amit Klein Reported-by: Tamir Shahar Reported-by: Inbal Schussheim Suggested-by: Eric Dumazet Cc: stable@vger.kernel.org Signed-off-by: Inbal Schussheim Reviewed-by: Eric Dumazet Link: https://patch.msgid.link/20260914090408.1435080-2-inbal.lipshtat@mail.huji.ac.il Signed-off-by: Paolo Abeni --- net/ipv4/tcp_input.c | 3 ++- 1 file changed, 2 insertions(+), 1 deletion(-) diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c index 0f60a1dbf927..92bc60716f33 100644 --- a/net/ipv4/tcp_input.c +++ b/net/ipv4/tcp_input.c @@ -6490,6 +6490,7 @@ static bool tcp_validate_incoming(struct sock *sk, struct sk_buff *skb, * or pure receivers (this means either the sequence number or the ack * value must stay constant) * - Unexpected TCP option. + * - ACK sequence number is outside [SND.UNA, SND.NXT]. * * When these conditions are not satisfied it drops into a standard * receive procedure patterned after RFC793 to handle all cases. @@ -6539,7 +6540,7 @@ void tcp_rcv_established(struct sock *sk, struct sk_buff *skb) if ((tcp_flag_word(th) & TCP_HP_BITS) == tp->pred_flags && TCP_SKB_CB(skb)->seq == tp->rcv_nxt && - !after(TCP_SKB_CB(skb)->ack_seq, tp->snd_nxt)) { + between(TCP_SKB_CB(skb)->ack_seq, tp->snd_una, tp->snd_nxt)) { int tcp_header_len = tp->tcp_header_len; s32 delta = 0; int flag = 0;