[PATCH 4.20 17/32] tcp: clear icsk_backoff in tcp_write_queue_purge()

[Greg Kroah-Hartman]

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

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

[ Upstream commit 04c03114be82194d4a4858d41dba8e286ad1787c ]

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 <edumazet@xxxxxxxxxx>
Reported-by: soukjin bae <soukjin.bae@xxxxxxxxxxx>
Acked-by: Neal Cardwell <ncardwell@xxxxxxxxxx>
Signed-off-by: David S. Miller <davem@xxxxxxxxxxxxx>
Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>
---
 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 40cbe5609663f..b102973102b9e 100644
--- a/net/ipv4/tcp.c
+++ b/net/ipv4/tcp.c
@@ -2532,6 +2532,7 @@ 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)
@@ -2580,7 +2581,6 @@ 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;
-- 
2.19.1