From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-alma10-1.taild15c8.ts.net [100.103.45.18]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 43B4C3F4114; Wed, 20 May 2026 17:03:16 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=100.103.45.18 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1779296597; cv=none; b=Xc17bEKlS1+62HEi7gMG7imftZICPCC9vJd0VQ56Bgbc53jGs/WOO1EecupQDqCRBjvQOU4eMuht8C3PXAcBsSUDY8rlZUs3zpTXPSpnZQpA0NS+2Fsbd7kJAVoF8SCtPxBm+xME9YMkrVKCxlEniuK4IcVUJOOuSg/HdEX4kgM= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1779296597; c=relaxed/simple; bh=VI//Vnt5Yfd5xlk0gCn66FUnZeYbzWNQqyAW8O5jsoI=; h=From:To:Cc:Subject:Date:Message-ID:In-Reply-To:References: MIME-Version; b=eWlJEPkfrD8pzjml59PdSRZL9nBKc1YuazWNBSXIpkC46Ja1T6pQHo5tAGthB/Vu73CaN9rTJtr+SDmHwsr6AItr4oxLlBYVmLUSzQPj/ASk03danvTQGE1mTPpkzbmg5dSYwBQCxeWnSocBNZViDVwn6yLDej+P+nxls3AMC9A= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linuxfoundation.org header.i=@linuxfoundation.org header.b=UiLUzYYz; arc=none smtp.client-ip=100.103.45.18 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linuxfoundation.org header.i=@linuxfoundation.org header.b="UiLUzYYz" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 6F7D91F000E9; Wed, 20 May 2026 17:03:15 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linuxfoundation.org; s=korg; t=1779296595; bh=/dcnPKswT5K/Ccxu/sXoRvqdjIViaFfQmwamXWBFXcM=; h=From:To:Cc:Subject:Date:In-Reply-To:References; b=UiLUzYYz3K99nY5l2vR78bIfADYVpx7oaT3zdF3K5wDcJ9a6AdkzRv5CfBk0pWLbU vDecw0vRlV0SdMtiNw9l5h0/D9Nw5D0wLzzDIOaz0FC/odoYzdav7e9IsfXR7DGzeD guA7ydunZkvQATDQKY5vdryZFBfK5kSVBlZ5cm4w= From: Greg Kroah-Hartman To: stable@vger.kernel.org Cc: Greg Kroah-Hartman , patches@lists.linux.dev, Jiayuan Chen , Eric Dumazet , Jakub Kicinski , Sasha Levin Subject: [PATCH 7.0 0858/1146] tcp: send a challenge ACK on SEG.ACK > SND.NXT Date: Wed, 20 May 2026 18:18:28 +0200 Message-ID: <20260520162207.657460616@linuxfoundation.org> X-Mailer: git-send-email 2.54.0 In-Reply-To: <20260520162148.390695140@linuxfoundation.org> References: <20260520162148.390695140@linuxfoundation.org> User-Agent: quilt/0.69 X-stable: review X-Patchwork-Hint: ignore Precedence: bulk X-Mailing-List: patches@lists.linux.dev List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit 7.0-stable review patch. If anyone has any objections, please let me know. ------------------ From: Jiayuan Chen [ Upstream commit 42726ec644cbdde0035c3e0417fee8ed9547e120 ] 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 Signed-off-by: Sasha Levin --- net/ipv4/tcp_input.c | 10 +++++++--- .../net/packetdrill/tcp_ts_recent_invalid_ack.pkt | 4 +++- 2 files changed, 10 insertions(+), 4 deletions(-) diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c index c6c55c51a6409..cb4bcc5a85787 100644 --- a/net/ipv4/tcp_input.c +++ b/net/ipv4/tcp_input.c @@ -4288,11 +4288,15 @@ 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 174ce9a1bfc07..ee6baf7c36cfa 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 -- 2.53.0