Subject: [PATCH net 1/2] tcp: send a challenge ACK on SEG.ACK > SND.NXT

From: Jiayuan Chen <[email-masked]>
Date: Mon, 20 Apr 2026 10:54:08 +0800

RFC 5961 Section 5.2 validates an incoming segment's ACK value
against the range [SND.UNA - MAX.SND.WND, SND.NXT] and states:

  "All incoming segments whose ACK value doesn't satisfy the above
   condition MUST be discarded and an ACK sent back."

Commit 354e4aa391ed ("tcp: RFC 5961 5.2 Blind Data Injection Attack
Mitigation") opted Linux into this mitigation and implements the
challenge ACK on the lower side (SEG.ACK < SND.UNA - MAX.SND.WND),
but the symmetric upper side (SEG.ACK > SND.NXT) still takes the
pre-RFC-5961 path and silently returns
SKB_DROP_REASON_TCP_ACK_UNSENT_DATA, even though RFC 793 Section 3.9
(now RFC 9293 Section 3.10.7.4) has always required:

  "If the ACK acknowledges something not yet sent (SEG.ACK > SND.NXT)
   then send an ACK, drop the segment, and return."

Complete the mitigation by sending a challenge ACK on that branch,
reusing the existing tcp_send_challenge_ack() path which already
enforces the per-socket RFC 5961 Section 7 rate limit via
__tcp_oow_rate_limited().  FLAG_NO_CHALLENGE_ACK is honoured for
symmetry with the lower-edge case.

Fixes: 354e4aa391ed ("tcp: RFC 5961 5.2 Blind Data Injection Attack Mitigation")
Signed-off-by: Jiayuan Chen <[email-masked]>

---
I'm not sure if 'blamed commit' is appropriate, because I think
it's due to missing parts of the implementation, or it might be
directly targeted to net-next.
---
 net/ipv4/tcp_input.c | 10 +++++++---
 1 file changed, 7 insertions(+), 3 deletions(-)

diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
index 021f745747c5..c2b6f05acdfa 100644
--- a/net/ipv4/tcp_input.c
+++ b/net/ipv4/tcp_input.c
@@ -4284,11 +4284,15 @@ static int tcp_ack(struct sock *sk, const struct sk_buff *skb, int flag)
 		goto old_ack;
 	}
 
-	/* If the ack includes data we haven't sent yet, discard
-	 * this segment (RFC793 Section 3.9).
+	/* If the ack includes data we haven't sent yet, drop the
+	 * segment.  RFC 793 Section 3.9 and RFC 5961 Section 5.2
+	 * require us to send an ACK back in that case.
 	 */
-	if (after(ack, tp->snd_nxt))
+	if (after(ack, tp->snd_nxt)) {
+		if (!(flag & FLAG_NO_CHALLENGE_ACK))
+			tcp_send_challenge_ack(sk, false);
 		return -SKB_DROP_REASON_TCP_ACK_UNSENT_DATA;
+	}
 
 	if (after(ack, prior_snd_una)) {
 		flag |= FLAG_SND_UNA_ADVANCED;
-- 
2.43.0