tcp: fix indefinite deferral of RTO with SACK reneging

This commit fixes a bug that can cause a TCP data sender to repeatedly
defer RTOs when encountering SACK reneging.

The bug is that when we're in fast recovery in a scenario with SACK
reneging, every time we get an ACK we call tcp_check_sack_reneging()
and it can note the apparent SACK reneging and rearm the RTO timer for
srtt/2 into the future. In some SACK reneging scenarios that can
happen repeatedly until the receive window fills up, at which point
the sender can't send any more, the ACKs stop arriving, and the RTO
fires at srtt/2 after the last ACK. But that can take far too long
(O(10 secs)), since the connection is stuck in fast recovery with a
low cwnd that cannot grow beyond ssthresh, even if more bandwidth is
available.

This fix changes the logic in tcp_check_sack_reneging() to only rearm
the RTO timer if data is cumulatively ACKed, indicating forward
progress. This avoids this kind of nearly infinite loop of RTO timer
re-arming. In addition, this meets the goals of
tcp_check_sack_reneging() in handling Windows TCP behavior that looks
temporarily like SACK reneging but is not really.

Many thanks to Jakub Kicinski and Neil Spring, who reported this issue
and provided critical packet traces that enabled root-causing this
issue. Also, many thanks to Jakub Kicinski for testing this fix.

Fixes: 5ae344c ("tcp: reduce spurious retransmits due to transient SACK reneging")
Reported-by: Jakub Kicinski <[email protected]>
Reported-by: Neil Spring <[email protected]>
Signed-off-by: Neal Cardwell <[email protected]>
Reviewed-by: Eric Dumazet <[email protected]>
Cc: Yuchung Cheng <[email protected]>
Tested-by: Jakub Kicinski <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Jakub Kicinski <[email protected]>

Author: nealcardwell

net/ipv4/tcp_input.c

@@ -2192,7 +2192,8 @@ void tcp_enter_loss(struct sock *sk)
  */
 static bool tcp_check_sack_reneging(struct sock *sk, int flag)
 {
-	if (flag & FLAG_SACK_RENEGING) {
+	if (flag & FLAG_SACK_RENEGING &&
+	    flag & FLAG_SND_UNA_ADVANCED) {
 		struct tcp_sock *tp = tcp_sk(sk);
 		unsigned long delay = max(usecs_to_jiffies(tp->srtt_us >> 4),
 					  msecs_to_jiffies(10));