[PATCH 4.4 07/20] tcp: tcp_v4_err() should be more careful

[Greg Kroah-Hartman <gregkh@linuxfoundation.org>]

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

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

From: Eric Dumazet <edumazet@google.com>

[ Upstream commit 2c4cc9712364c051b1de2d175d5fbea6be948ebf ]

ICMP handlers are not very often stressed, we should
make them more resilient to bugs that might surface in
the future.

If there is no packet in retransmit queue, we should
avoid a NULL deref.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Reported-by: soukjin bae <soukjin.bae@samsung.com>
Acked-by: Neal Cardwell <ncardwell@google.com>
Acked-by: Soheil Hassas Yeganeh <soheil@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
 net/ipv4/tcp_ipv4.c |    7 ++++---
 1 file changed, 4 insertions(+), 3 deletions(-)

--- a/net/ipv4/tcp_ipv4.c
+++ b/net/ipv4/tcp_ipv4.c
@@ -466,14 +466,15 @@ void tcp_v4_err(struct sk_buff *icmp_skb
 		if (sock_owned_by_user(sk))
 			break;
 
+		skb = tcp_write_queue_head(sk);
+		if (WARN_ON_ONCE(!skb))
+			break;
+
 		icsk->icsk_backoff--;
 		icsk->icsk_rto = tp->srtt_us ? __tcp_set_rto(tp) :
 					       TCP_TIMEOUT_INIT;
 		icsk->icsk_rto = inet_csk_rto_backoff(icsk, TCP_RTO_MAX);
 
-		skb = tcp_write_queue_head(sk);
-		BUG_ON(!skb);
-
 		remaining = icsk->icsk_rto -
 			    min(icsk->icsk_rto,
 				tcp_time_stamp - tcp_skb_timestamp(skb));