Subject: [PATCH net 0/2] tcp: symmetric challenge ACK for SEG.ACK > SND.NXT

From: Jiayuan Chen <[email-masked]>
Date: Mon, 20 Apr 2026 10:54:07 +0800

Commit 354e4aa391ed ("tcp: RFC 5961 5.2 Blind Data Injection Attack
Mitigation") quotes RFC 5961 Section 5.2 in full, which requires
that any incoming segment whose ACK value falls outside
[SND.UNA - MAX.SND.WND, SND.NXT] MUST be discarded and an ACK sent
back.  Linux currently sends that challenge ACK only on the lower
edge (SEG.ACK < SND.UNA - MAX.SND.WND); on the symmetric upper edge
(SEG.ACK > SND.NXT) the segment is silently dropped with
SKB_DROP_REASON_TCP_ACK_UNSENT_DATA.

Patch 1 completes the mitigation by emitting a rate-limited challenge
ACK on that branch, reusing tcp_send_challenge_ack() and honouring
FLAG_NO_CHALLENGE_ACK for consistency with the lower-edge case.

Patch 2 adds a packetdrill selftest under
tools/testing/selftests/net/packetdrill/ that verifies the new
behaviour.

Jiayuan Chen (2):
  tcp: send a challenge ACK on SEG.ACK > SND.NXT
  selftests/net: packetdrill: cover challenge ACK on SEG.ACK > SND.NXT

 net/ipv4/tcp_input.c                          | 10 ++++--
 .../tcp_rfc5961_ack-beyond-snd-nxt.pkt        | 31 +++++++++++++++++++
 2 files changed, 38 insertions(+), 3 deletions(-)
 create mode 100644 tools/testing/selftests/net/packetdrill/tcp_rfc5961_ack-beyond-snd-nxt.pkt

-- 
2.43.0