tcp: fix to allow timestamp undo if no retransmits were sent

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@samsung.com> recently reported that after
this commit from 2019:

commit bc9f38c8 ("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 bc9f38c8 ("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 bc9f38c8, 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
bc9f38c8).

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: bc9f38c8 ("tcp: avoid unconditional congestion window undo on SYN retransmit")
Reported-by: Geumhwan Yu <geumhwan.yu@samsung.com>
Diagnosed-by: Geumhwan Yu <geumhwan.yu@samsung.com>
Signed-off-by: Neal Cardwell <ncardwell@google.com>
Signed-off-by: Yuchung Cheng <ycheng@google.com>
Signed-off-by: Eric Dumazet <edumazet@google.com>
Link: https://patch.msgid.link/20241001200517.2756803-2-ncardwell.sw@gmail.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>