tcp: undo spurious timeout after SACK reneging

On SACK reneging the sender immediately retransmits and forces a
timeout but disables Eifel (undo). If the (buggy) receiver does not
drop any packet this can trigger a false slow-start retransmit storm
driven by the ACKs of the original packets. This can be detected with
undo and TCP timestamps.

Signed-off-by: Yuchung Cheng <ycheng@google.com>
Acked-by: Neal Cardwell <ncardwell@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 2 insertions, 5 deletions

diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
index 0d9bdacce99f..3bd55bad230a 100644
--- a/net/ipv4/tcp_input.c
+++ b/net/ipv4/tcp_input.c
@@ -2059,11 +2059,8 @@ void tcp_enter_loss(struct sock *sk, int how)
         if (tcp_is_reno(tp))
                 tcp_reset_reno_sack(tp);
 
-        if (!how) {
-                /* Push undo marker, if it was plain RTO and nothing
-                 * was retransmitted. */
-                tp->undo_marker = tp->snd_una;
-        } else {
+        tp->undo_marker = tp->snd_una;
+        if (how) {
                 tp->sacked_out = 0;
                 tp->fackets_out = 0;
         }