tcp: tcp_v4_err() should be more careful

[ 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: NEric Dumazet <edumazet@google.com>
Reported-by: Nsoukjin bae <soukjin.bae@samsung.com>
Acked-by: NNeal Cardwell <ncardwell@google.com>
Acked-by: NSoheil Hassas Yeganeh <soheil@google.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>
Signed-off-by: NSasha Levin <sashal@kernel.org>
Signed-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>

net/ipv4/tcp_ipv4.c

@@ -535,14 +535,15 @@ void tcp_v4_err(struct sk_buff *icmp_skb, u32 info)
 		if (sock_owned_by_user(sk))
 			break;
 
+		skb = tcp_rtx_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_rtx_queue_head(sk);
-		BUG_ON(!skb);
-
 		tcp_mstamp_refresh(tp);
 		delta_us = (u32)(tp->tcp_mstamp - skb->skb_mstamp);
 		remaining = icsk->icsk_rto -