[PATCH linux-stable-4.14] tcp: clear tp->packets_out when purging write queue

[Soheil Hassas Yeganeh <soheil.kdev@gmail.com>]

From: Soheil Hassas Yeganeh <soheil@google.com>

Clear tp->packets_out when purging the write queue, otherwise
tcp_rearm_rto() mistakenly assumes TCP write queue is not empty.
This results in NULL pointer dereference.

Also, remove the redundant `tp->packets_out = 0` from
tcp_disconnect(), since tcp_disconnect() calls
tcp_write_queue_purge().

Fixes: a27fd7a8ed38 (tcp: purge write queue upon RST)
Reported-by: Subash Abhinov Kasiviswanathan <subashab@codeaurora.org>
Reported-by: Sami Farin <hvtaifwkbgefbaei@gmail.com>
Tested-by: Sami Farin <hvtaifwkbgefbaei@gmail.com>
Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: Soheil Hassas Yeganeh <soheil@google.com>
Acked-by: Yuchung Cheng <ycheng@google.com>
Acked-by: Neal Cardwell <ncardwell@google.com>
---
 include/net/tcp.h | 1 +
 net/ipv4/tcp.c    | 1 -
 2 files changed, 1 insertion(+), 1 deletion(-)

diff --git a/include/net/tcp.h b/include/net/tcp.h
index d323d4fa742ca..fb653736f3353 100644
--- a/include/net/tcp.h
+++ b/include/net/tcp.h
@@ -1616,6 +1616,7 @@ static inline void tcp_write_queue_purge(struct sock *sk)
 	sk_mem_reclaim(sk);
 	tcp_clear_all_retrans_hints(tcp_sk(sk));
 	tcp_init_send_head(sk);
+	tcp_sk(sk)->packets_out = 0;
 }
 
 static inline struct sk_buff *tcp_write_queue_head(const struct sock *sk)
diff --git a/net/ipv4/tcp.c b/net/ipv4/tcp.c
index 38b9a6276a9de..4dda8d301802e 100644
--- a/net/ipv4/tcp.c
+++ b/net/ipv4/tcp.c
@@ -2354,7 +2354,6 @@ int tcp_disconnect(struct sock *sk, int flags)
 	icsk->icsk_backoff = 0;
 	tp->snd_cwnd = 2;
 	icsk->icsk_probes_out = 0;
-	tp->packets_out = 0;
 	tp->snd_ssthresh = TCP_INFINITE_SSTHRESH;
 	tp->snd_cwnd_cnt = 0;
 	tp->window_clamp = 0;
-- 
2.17.0.484.g0c8726318c-goog