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Merge branch 'tcp-exclude-old-acks-from-fast-path'

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Inbal Schussheim says:

====================
tcp: exclude old ACKs from fast path

Exclude ACKs outside [SND.UNA, SND.NXT] from TCP header prediction so
that they fall through to the slow path, where ACK
validation is applied.

Add a packetdrill test for a data segment carrying an
excessively old ACK. The test fails on the unpatched kernel and passes
with the fix.

v2: https://lore.kernel.org/netdev/20260909075644.1408171-1-inbal.lipshtat@mail.huji.ac.il/
v1: https://lore.kernel.org/netdev/20260906123151.1391349-1-inbal.lipshtat@mail.huji.ac.il/T/#u
====================

Link: https://patch.msgid.link/20260914090408.1435080-1-inbal.lipshtat@mail.huji.ac.il
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
This commit is contained in:
Paolo Abeni 2026-09-17 15:18:10 +02:00
commit ad9c65b8f9
2 changed files with 31 additions and 1 deletions
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3
net/ipv4/tcp_input.c
View File

@ -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;

29
tools/testing/selftests/net/packetdrill/tcp_rfc5961_reject-old-ack.pkt Normal file
View File

@ -0,0 +1,29 @@
// SPDX-License-Identifier: GPL-2.0
`./defaults.sh
sysctl -q net.ipv4.tcp_invalid_ratelimit=0
`
// Test rejection of data segments carrying excessively old ACKs
0 socket(..., SOCK_STREAM, IPPROTO_TCP) = 3
+0 setsockopt(3, SOL_SOCKET, SO_REUSEADDR, [1], 4) = 0
+0 bind(3, ..., ...) = 0
+0 listen(3, 1024) = 0
// ---------------- Handshake ------------------- //
+0 < S 0:0(0) win 65535
+0 > S. 0:0(0) ack 1 <...>
+0 < . 1:1(0) ack 1 win 65535
+0 accept(3, ..., ...) = 4
// Populate receive memory so the following segment can use
// header prediction.
+0 < P. 1:501(500) ack 1 win 65535
+0 > . 1:1(0) ack 501
// Send an in-sequence data segment carrying an excessively old ACK.
+0 < P. 501:1501(1000) ack 2794967397 win 65535
// Challenge ACK; RCV.NXT must remain 501.
+0 > . 1:1(0) ack 501