This is a note to let you know that I've just added the patch titled
tcp: fix to allow timestamp undo if no retransmits were sent
to the 6.6-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary
The filename of the patch is:
tcp-fix-to-allow-timestamp-undo-if-no-retransmits-we.patch
and it can be found in the queue-6.6 subdirectory.
If you, or anyone else, feels it should not be added to the stable tree,
please let <stable@xxxxxxxxxxxxxxx> know about it.
commit a549d96e42e3465196812b93037daedfc6baf71f
Author: Neal Cardwell <ncardwell@xxxxxxxxxx>
Date: Tue Oct 1 20:05:15 2024 +0000
tcp: fix to allow timestamp undo if no retransmits were sent
[ Upstream commit e37ab7373696e650d3b6262a5b882aadad69bb9e ]
Fix the TCP loss recovery undo logic in tcp_packet_delayed() so that
it can trigger undo even if TSQ prevents a fast recovery episode from
reaching tcp_retransmit_skb().
Geumhwan Yu <geumhwan.yu@xxxxxxxxxxx> recently reported that after
this commit from 2019:
commit bc9f38c8328e ("tcp: avoid unconditional congestion window undo
on SYN retransmit")
...and before this fix we could have buggy scenarios like the
following:
+ Due to reordering, a TCP connection receives some SACKs and enters a
spurious fast recovery.
+ TSQ prevents all invocations of tcp_retransmit_skb(), because many
skbs are queued in lower layers of the sending machine's network
stack; thus tp->retrans_stamp remains 0.
+ The connection receives a TCP timestamp ECR value echoing a
timestamp before the fast recovery, indicating that the fast
recovery was spurious.
+ The connection fails to undo the spurious fast recovery because
tp->retrans_stamp is 0, and thus tcp_packet_delayed() returns false,
due to the new logic in the 2019 commit: commit bc9f38c8328e ("tcp:
avoid unconditional congestion window undo on SYN retransmit")
This fix tweaks the logic to be more similar to the
tcp_packet_delayed() logic before bc9f38c8328e, except that we take
care not to be fooled by the FLAG_SYN_ACKED code path zeroing out
tp->retrans_stamp (the bug noted and fixed by Yuchung in
bc9f38c8328e).
Note that this returns the high-level behavior of tcp_packet_delayed()
to again match the comment for the function, which says: "Nothing was
retransmitted or returned timestamp is less than timestamp of the
first retransmission." Note that this comment is in the original
2005-04-16 Linux git commit, so this is evidently long-standing
behavior.
Fixes: bc9f38c8328e ("tcp: avoid unconditional congestion window undo on SYN retransmit")
Reported-by: Geumhwan Yu <geumhwan.yu@xxxxxxxxxxx>
Diagnosed-by: Geumhwan Yu <geumhwan.yu@xxxxxxxxxxx>
Signed-off-by: Neal Cardwell <ncardwell@xxxxxxxxxx>
Signed-off-by: Yuchung Cheng <ycheng@xxxxxxxxxx>
Signed-off-by: Eric Dumazet <edumazet@xxxxxxxxxx>
Link: https://patch.msgid.link/20241001200517.2756803-2-ncardwell.sw@xxxxxxxxx
Signed-off-by: Jakub Kicinski <kuba@xxxxxxxxxx>
Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>
diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
index b565c8a8e7bad..31e4f7bce00ce 100644
--- a/net/ipv4/tcp_input.c
+++ b/net/ipv4/tcp_input.c
@@ -2452,8 +2452,22 @@ static bool tcp_skb_spurious_retrans(const struct tcp_sock *tp,
*/
static inline bool tcp_packet_delayed(const struct tcp_sock *tp)
{
- return tp->retrans_stamp &&
- tcp_tsopt_ecr_before(tp, tp->retrans_stamp);
+ const struct sock *sk = (const struct sock *)tp;
+
+ if (tp->retrans_stamp &&
+ tcp_tsopt_ecr_before(tp, tp->retrans_stamp))
+ return true; /* got echoed TS before first retransmission */
+
+ /* Check if nothing was retransmitted (retrans_stamp==0), which may
+ * happen in fast recovery due to TSQ. But we ignore zero retrans_stamp
+ * in TCP_SYN_SENT, since when we set FLAG_SYN_ACKED we also clear
+ * retrans_stamp even if we had retransmitted the SYN.
+ */
+ if (!tp->retrans_stamp && /* no record of a retransmit/SYN? */
+ sk->sk_state != TCP_SYN_SENT) /* not the FLAG_SYN_ACKED case? */
+ return true; /* nothing was retransmitted */
+
+ return false;
}
/* Undo procedures. */
