tcp: fix infinite cwnd in tcp_complete_cwr()

When the cwnd reduction is done, ssthresh may be infinite
if TCP enters CWR via ECN or F-RTO. If cwnd is not undone, i.e.,
undo_marker is set, tcp_complete_cwr() falsely set cwnd to the
infinite ssthresh value. The correct operation is to keep cwnd
intact because it has been updated in ECN or F-RTO.

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, 6 insertions, 3 deletions

diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
index 2a702e3a4ae6..d99efd7dfb6c 100644
--- a/net/ipv4/tcp_input.c
+++ b/net/ipv4/tcp_input.c
@@ -2868,11 +2868,14 @@ static inline void tcp_complete_cwr(struct sock *sk)
 
         /* Do not moderate cwnd if it's already undone in cwr or recovery. */
         if (tp->undo_marker) {
-                if (inet_csk(sk)->icsk_ca_state == TCP_CA_CWR)
+                if (inet_csk(sk)->icsk_ca_state == TCP_CA_CWR) {
                         tp->snd_cwnd = min(tp->snd_cwnd, tp->snd_ssthresh);
-                else /* PRR */
+                        tp->snd_cwnd_stamp = tcp_time_stamp;
+                } else if (tp->snd_ssthresh < TCP_INFINITE_SSTHRESH) {
+                        /* PRR algorithm. */
                         tp->snd_cwnd = tp->snd_ssthresh;
-                tp->snd_cwnd_stamp = tcp_time_stamp;
+                        tp->snd_cwnd_stamp = tcp_time_stamp;
+                }
         }
         tcp_ca_event(sk, CA_EVENT_COMPLETE_CWR);
 }