[PATCH net 1/2] tcp: clear icsk_backoff in tcp_write_queue_purge()

[Eric Dumazet]

soukjin bae reported a crash in tcp_v4_err() handling
ICMP_DEST_UNREACH after tcp_write_queue_head(sk)
returned a NULL pointer.

Current logic should have prevented this :

  if (seq != tp->snd_una  || !icsk->icsk_retransmits ||
      !icsk->icsk_backoff || fastopen)
      break;

Problem is the write queue might have been purged
and icsk_backoff has not been cleared.

Signed-off-by: Eric Dumazet <eduma...@google.com>
Reported-by: soukjin bae <soukjin....@samsung.com>
---
 net/ipv4/tcp.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/net/ipv4/tcp.c b/net/ipv4/tcp.c
index 
2079145a3b7c5f498af429c9a8289342e4421fca..cf3c5095c10e8e7e56621beae2f93c93de184489
 100644
--- a/net/ipv4/tcp.c
+++ b/net/ipv4/tcp.c
@@ -2528,6 +2528,7 @@ void tcp_write_queue_purge(struct sock *sk)
        sk_mem_reclaim(sk);
        tcp_clear_all_retrans_hints(tcp_sk(sk));
        tcp_sk(sk)->packets_out = 0;
+       inet_csk(sk)->icsk_backoff = 0;
 }
 
 int tcp_disconnect(struct sock *sk, int flags)
@@ -2576,7 +2577,6 @@ int tcp_disconnect(struct sock *sk, int flags)
        tp->write_seq += tp->max_window + 2;
        if (tp->write_seq == 0)
                tp->write_seq = 1;
-       icsk->icsk_backoff = 0;
        tp->snd_cwnd = 2;
        icsk->icsk_probes_out = 0;
        tp->snd_ssthresh = TCP_INFINITE_SSTHRESH;
-- 
2.21.0.rc0.258.g878e2cd30e-goog