tcp: miscounts due to tcp_fragment pcount reset

It seems that trivial reset of pcount to one was not sufficient
in tcp_retransmit_skb. Multiple counters experience a positive
miscount when skb's pcount gets lowered without the necessary
adjustments (depending on skb's sacked bits which exactly), at
worst a packets_out miscount can crash at RTO if the write queue
is empty!

Triggering this requires mss change, so bidir tcp or mtu probe or
like.

Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@helsinki.fi>
Reported-by: Markus Trippelsdorf <markus@trippelsdorf.de>
Tested-by: Uwe Bugla <uwe.bugla@gmx.de>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 6 insertions, 1 deletions

diff --git a/net/ipv4/tcp_output.c b/net/ipv4/tcp_output.c
index f1db89bb3aa7..53300fa2359f 100644
--- a/net/ipv4/tcp_output.c
+++ b/net/ipv4/tcp_output.c
@@ -1893,7 +1893,12 @@ int tcp_retransmit_skb(struct sock *sk, struct sk_buff *skb)
                 if (tcp_fragment(sk, skb, cur_mss, cur_mss))
                         return -ENOMEM; /* We'll try again later. */
         } else {
-                tcp_init_tso_segs(sk, skb, cur_mss);
+                int oldpcount = tcp_skb_pcount(skb);
+
+                if (unlikely(oldpcount > 1)) {
+                        tcp_init_tso_segs(sk, skb, cur_mss);
+                        tcp_adjust_pcount(sk, skb, oldpcount - tcp_skb_pcount(skb));
+                }
         }
 
         tcp_retrans_try_collapse(sk, skb, cur_mss);