Subject: [PATCH net v2 0/2] tcp: symmetric challenge ACK for SEG.ACK > SND.NXT

From: Jiayuan Chen <[email-masked]>
Date: Tue, 21 Apr 2026 09:40:59 +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.  It
also updates the existing tcp_ts_recent_invalid_ack.pkt selftest,
which drives this exact path, to consume the new challenge ACK so
bisect stays clean.

Patch 2 adds a new packetdrill selftest that exercises RFC 5961
Section 5.2 on both edges of the acceptable window, filling a gap in
the selftests tree (neither edge had dedicated coverage before).

---

Changelog
=========

v1 -> v2:
  - Add Reviewed-by tag.
  - Fold the tcp_ts_recent_invalid_ack.pkt update into patch 1 so
    that bisect stays clean and the fix is self-contained for
    backport.
  - Extend the new selftest to cover both edges of RFC 5961
    Section 5.2 (SEG.ACK > SND.NXT and SEG.ACK < SND.UNA -
    MAX.SND.WND) in a single connection, and rename it to
    tcp_rfc5961_ack-out-of-window.pkt.  Neither edge had explicit
    packetdrill coverage before.

v1: https://lore.kernel.org/netdev/[email-masked]/

Jiayuan Chen (2):
  tcp: send a challenge ACK on SEG.ACK > SND.NXT
  selftests/net: packetdrill: cover RFC 5961 5.2 challenge ACK on both
    edges

 net/ipv4/tcp_input.c                          | 10 ++--
 .../tcp_rfc5961_ack-out-of-window.pkt         | 46 +++++++++++++++++++
 .../packetdrill/tcp_ts_recent_invalid_ack.pkt |  4 +-
 3 files changed, 56 insertions(+), 4 deletions(-)
 create mode 100644 tools/testing/selftests/net/packetdrill/tcp_rfc5961_ack-out-of-window.pkt

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2.43.0