From: Jiayuan Chen Date: Wed, 22 Apr 2026 12:35:38 +0000 (+0800) Subject: tcp: send a challenge ACK on SEG.ACK > SND.NXT X-Git-Url: http://git.ipfire.org/gitweb.cgi?a=commitdiff_plain;h=42726ec644cbdde0035c3e0417fee8ed9547e120;p=thirdparty%2Fkernel%2Fstable.git tcp: send a challenge ACK on SEG.ACK > SND.NXT 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. Update the existing tcp_ts_recent_invalid_ack.pkt selftest, which drives this exact path, to consume the new challenge ACK. Fixes: 354e4aa391ed ("tcp: RFC 5961 5.2 Blind Data Injection Attack Mitigation") Signed-off-by: Jiayuan Chen Reviewed-by: Eric Dumazet Link: https://patch.msgid.link/20260422123605.320000-2-jiayuan.chen@linux.dev Signed-off-by: Jakub Kicinski --- diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c index e04ae105893c..d5c9e65d9760 100644 --- a/net/ipv4/tcp_input.c +++ b/net/ipv4/tcp_input.c @@ -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; diff --git a/tools/testing/selftests/net/packetdrill/tcp_ts_recent_invalid_ack.pkt b/tools/testing/selftests/net/packetdrill/tcp_ts_recent_invalid_ack.pkt index 174ce9a1bfc0..ee6baf7c36cf 100644 --- a/tools/testing/selftests/net/packetdrill/tcp_ts_recent_invalid_ack.pkt +++ b/tools/testing/selftests/net/packetdrill/tcp_ts_recent_invalid_ack.pkt @@ -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 - +// 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 +0 < . 1:1001(1000) ack 1 win 20000 +0 > . 1:1(0) ack 1001