tcp: fix NULL ref in tail loss probe

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.tinoco@linaro.org>
Signed-off-by: Yuchung Cheng <ycheng@google.com>
Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: Neal Cardwell <ncardwell@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 7 insertions, 4 deletions

diff --git a/net/ipv4/tcp_output.c b/net/ipv4/tcp_output.c
index 318690234758..5aa600900695 100644
--- a/net/ipv4/tcp_output.c
+++ b/net/ipv4/tcp_output.c
@@ -2497,15 +2497,18 @@ 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;