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 <[email protected]>
Reviewed-by: Eric Dumazet <[email protected]>
---
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 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;
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 <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>
--
2.43.0
