[PATCH net] tcp: fix NULL ref in tail loss probe

[Yuchung Cheng]

TCP loss probe timer may fire when the retranmission queue is empty but
has a non-zero tp->packets_out counter. tcp_send_loss_probe will call
tcp_rearm_rto which triggers NULL pointer reference by fetching the
retranmission queue head in its sub-routines.

Add a more detailed warning to help catch the root cause of the inflight
accounting inconsistency.

Reported-by: Rafael Tinoco <rafael.tin...@linaro.org>
Signed-off-by: Yuchung Cheng <ych...@google.com>
Signed-off-by: Eric Dumazet <eduma...@google.com>
Signed-off-by: Neal Cardwell <ncardw...@google.com>
---
 net/ipv4/tcp_output.c | 11 +++++++----
 1 file changed, 7 insertions(+), 4 deletions(-)

diff --git a/net/ipv4/tcp_output.c b/net/ipv4/tcp_output.c
index 68b5326f7321..9a1101095298 100644
--- a/net/ipv4/tcp_output.c
+++ b/net/ipv4/tcp_output.c
@@ -2494,15 +2494,18 @@ void tcp_send_loss_probe(struct sock *sk)
                goto rearm_timer;
        }
        skb = skb_rb_last(&sk->tcp_rtx_queue);
+       if (unlikely(!skb)) {
+               WARN_ONCE(tp->packets_out,
+                         "invalid inflight: %u state %u cwnd %u mss %d\n",
+                         tp->packets_out, sk->sk_state, tp->snd_cwnd, mss);
+               inet_csk(sk)->icsk_pending = 0;
+               return;
+       }
 
        /* At most one outstanding TLP retransmission. */
        if (tp->tlp_high_seq)
                goto rearm_timer;
 
-       /* Retransmit last segment. */
-       if (WARN_ON(!skb))
-               goto rearm_timer;
-
        if (skb_still_in_host_queue(sk, skb))
                goto rearm_timer;
 
-- 
2.20.0.rc1.387.gf8505762e3-goog