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tcp: send a challenge ACK on SEG.ACK > SND.NXT

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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 354e4aa391 ("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.

Update the existing tcp_ts_recent_invalid_ack.pkt selftest, which
drives this exact path, to consume the new challenge ACK.

Fixes: 354e4aa391 ("tcp: RFC 5961 5.2 Blind Data Injection Attack Mitigation")
Signed-off-by: Jiayuan Chen <jiayuan.chen@linux.dev>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Link: https://patch.msgid.link/20260422123605.320000-2-jiayuan.chen@linux.dev
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
This commit is contained in:
Jiayuan Chen
2026-04-22 20:35:38 +08:00
committed by Jakub Kicinski
parent 4078c5611d
commit 42726ec644
2 changed files with 10 additions and 4 deletions
Download Patch File Download Diff File Expand all files Collapse all files

10
net/ipv4/tcp_input.c
View File

@@ -4286,11 +4286,15 @@ 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;

4
tools/testing/selftests/net/packetdrill/tcp_ts_recent_invalid_ack.pkt
View File

@@ -19,7 +19,9 @@
// bad packet with high tsval (its ACK sequence is above our sndnxt)
+0 < F. 1:1(0) ack 9999 win 20000 <nop,nop,TS val 200000 ecr 100>
// Challenge ACK for SEG.ACK > SND.NXT (RFC 5961 5.2 / RFC 793 3.9).
// ecr=200 (not 200000) proves ts_recent was not updated from the bad packet.
+0 > . 1:1(0) ack 1 <nop,nop,TS val 200 ecr 200>
+0 < . 1:1001(1000) ack 1 win 20000 <nop,nop,TS val 201 ecr 100>
+0 > . 1:1(0) ack 1001 <nop,nop,TS val 200 ecr 201>