[TCP]: Make sure write_queue_from does not begin with NULL ptr

NULL ptr can be returned from tcp_write_queue_head to cached_skb
and then assigned to skb if packets_out was zero. Without this,
system is vulnerable to a carefully crafted ACKs which obviously
is remotely triggerable.

Besides, there's very little that needs to be done in sacktag
if there weren't any packets outstanding, just skipping the rest
doesn't hurt.

Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@helsinki.fi>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv4/tcp_input.c

@@ -1269,6 +1269,9 @@ tcp_sacktag_write_queue(struct sock *sk, struct sk_buff *ack_skb, u32 prior_snd_
 	if (before(TCP_SKB_CB(ack_skb)->ack_seq, prior_snd_una - tp->max_window))
 		return 0;
 
+	if (!tp->packets_out)
+		goto out;
+
 	/* SACK fastpath:
 	 * if the only SACK change is the increase of the end_seq of
 	 * the first block then only apply that SACK block
@@ -1515,6 +1518,8 @@ tcp_sacktag_write_queue(struct sock *sk, struct sk_buff *ack_skb, u32 prior_snd_
 	    (!tp->frto_highmark || after(tp->snd_una, tp->frto_highmark)))
 		tcp_update_reordering(sk, tp->fackets_out - reord, 0);
 
+out:
+
 #if FASTRETRANS_DEBUG > 0
 	BUG_TRAP((int)tp->sacked_out >= 0);
 	BUG_TRAP((int)tp->lost_out >= 0);