Merge branch 'tcp-symmetric-challenge-ack-for-seg-ack-snd-nxt'

Jiayuan Chen says:

====================
tcp: symmetric challenge ACK for SEG.ACK > SND.NXT

Commit 354e4aa ("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).
====================

Link: https://patch.msgid.link/20260422123605.320000-1-jiayuan.chen@linux.dev
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

Author: kuba-moo

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;

tools/testing/selftests/net/packetdrill/tcp_rfc5961_ack-out-of-window.pkt

@@ -0,0 +1,48 @@
+// SPDX-License-Identifier: GPL-2.0
+//
+// RFC 5961 Section 5.2 / RFC 793 Section 3.9: an incoming segment's
+// ACK value must lie in [SND.UNA - MAX.SND.WND, SND.NXT]; otherwise
+// the receiver MUST discard the segment and send a challenge ACK
+// back.  Exercise both edges of that window in a single connection.
+
+`./defaults.sh
+sysctl -q net.ipv4.tcp_invalid_ratelimit=0
+`
+
+   0 socket(..., SOCK_STREAM, IPPROTO_TCP) = 3
+  +0 setsockopt(3, SOL_SOCKET, SO_REUSEADDR, [1], 4) = 0
+  +0 bind(3, ..., ...) = 0
+  +0 listen(3, 1) = 0
+
+// Three-way handshake.  Peer advertises rwnd = 1000 (no wscale), so
+// MAX.SND.WND is tracked as 1000.
+  +0 < S 0:0(0) win 1000 <mss 1000,sackOK,nop,nop,nop,wscale 0>
+  +0 > S. 0:0(0) ack 1 <...>
++.1 < . 1:1(0) ack 1 win 1000
+  +0 accept(3, ..., ...) = 4
+
+// ---- Upper edge: SEG.ACK > SND.NXT --------------------------------
+// Server has sent nothing yet, so SND.UNA = SND.NXT = 1.
+// Peer sends a pure ACK with SEG.ACK = 2, beyond SND.NXT.
+  +0 < . 1:1(0) ack 2 win 1000
+// Expect a challenge ACK: <SEQ = SND.NXT = 1, ACK = RCV.NXT = 1>.
+  +0 > . 1:1(0) ack 1
+
+// Advance SND.UNA past MAX.SND.WND so that the lower edge becomes
+// reachable.  Issue two 1-MSS writes so each skb is exactly one MSS
+// and PSH is set by tcp_push() at the end of each sendmsg, keeping
+// the setup independent of the TSO / tcp_fragment split path.
+  +0 write(4, ..., 1000) = 1000
+  +0 > P. 1:1001(1000) ack 1
++.01 < . 1:1(0) ack 1001 win 1000
+  +0 write(4, ..., 1000) = 1000
+  +0 > P. 1001:2001(1000) ack 1
++.01 < . 1:1(0) ack 2001 win 1000
+// Now SND.UNA = SND.NXT = 2001, MAX.SND.WND = 1000, bytes_acked = 2000.
+
+// ---- Lower edge: SEG.ACK < SND.UNA - MAX.SND.WND ------------------
+// SND.UNA - MAX.SND.WND = 2001 - 1000 = 1001, so SEG.ACK = 1000 falls
+// below the acceptable range.
+  +0 < . 1:1(0) ack 1000 win 1000
+// Expect a challenge ACK: <SEQ = SND.NXT = 2001, ACK = RCV.NXT = 1>.
+  +0 > . 2001:2001(0) ack 1

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 <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>