4.4-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Yuchung Cheng <ych...@google.com>


[ Upstream commit ed254971edea92c3ac5c67c6a05247a92aa6075e ]

If the sender switches the congestion control during ECN-triggered
cwnd-reduction state (CA_CWR), upon exiting recovery cwnd is set to
the ssthresh value calculated by the previous congestion control. If
the previous congestion control is BBR that always keep ssthresh
to TCP_INIFINITE_SSTHRESH, cwnd ends up being infinite. The safe
step is to avoid assigning invalid ssthresh value when recovery ends.

Signed-off-by: Yuchung Cheng <ych...@google.com>
Signed-off-by: Neal Cardwell <ncardw...@google.com>
Acked-by: Eric Dumazet <eduma...@google.com>
Signed-off-by: David S. Miller <da...@davemloft.net>
Signed-off-by: Greg Kroah-Hartman <gre...@linuxfoundation.org>
---
 net/ipv4/tcp_input.c |    4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

--- a/net/ipv4/tcp_input.c
+++ b/net/ipv4/tcp_input.c
@@ -2503,8 +2503,8 @@ static inline void tcp_end_cwnd_reductio
        struct tcp_sock *tp = tcp_sk(sk);
 
        /* Reset cwnd to ssthresh in CWR or Recovery (unless it's undone) */
-       if (inet_csk(sk)->icsk_ca_state == TCP_CA_CWR ||
-           (tp->undo_marker && tp->snd_ssthresh < TCP_INFINITE_SSTHRESH)) {
+       if (tp->snd_ssthresh < TCP_INFINITE_SSTHRESH &&
+           (inet_csk(sk)->icsk_ca_state == TCP_CA_CWR || tp->undo_marker)) {
                tp->snd_cwnd = tp->snd_ssthresh;
                tp->snd_cwnd_stamp = tcp_time_stamp;
        }


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