netfilter: conntrack: always store window size un-scaled

[ Upstream commit 959b69ef57db00cb33e9c4777400ae7183ebddd3 ]

Jakub Jankowski reported following oddity:

After 3 way handshake completes, timeout of new connection is set to
max_retrans (300s) instead of established (5 days).

shortened excerpt from pcap provided:
25.070622 IP (flags [DF], proto TCP (6), length 52)
10.8.5.4.1025 > 10.8.1.2.80: Flags [S], seq 11, win 64240, [wscale 8]
26.070462 IP (flags [DF], proto TCP (6), length 48)
10.8.1.2.80 > 10.8.5.4.1025: Flags [S.], seq 82, ack 12, win 65535, [wscale 3]
27.070449 IP (flags [DF], proto TCP (6), length 40)
10.8.5.4.1025 > 10.8.1.2.80: Flags [.], ack 83, win 512, length 0

Turns out the last_win is of u16 type, but we store the scaled value:
512 << 8 (== 0x20000) becomes 0 window.

The Fixes tag is not correct, as the bug has existed forever, but
without that change all that this causes might cause is to mistake a
window update (to-nonzero-from-zero) for a retransmit.

Fixes: fbcd253d2448b8 ("netfilter: conntrack: lower timeout to RETRANS seconds if window is 0")
Reported-by: Jakub Jankowski <shasta@toxcorp.com>
Tested-by: Jakub Jankowski <shasta@toxcorp.com>
Signed-off-by: Florian Westphal <fw@strlen.de>
Acked-by: Jozsef Kadlecsik <kadlec@blackhole.kfki.hu>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>

net/netfilter/nf_conntrack_proto_tcp.c

@@ -480,6 +480,7 @@ static bool tcp_in_window(const struct nf_conn *ct,
 	struct ip_ct_tcp_state *receiver = &state->seen[!dir];
 	const struct nf_conntrack_tuple *tuple = &ct->tuplehash[dir].tuple;
 	__u32 seq, ack, sack, end, win, swin;
+	u16 win_raw;
 	s32 receiver_offset;
 	bool res, in_recv_win;
 
@@ -488,7 +489,8 @@ static bool tcp_in_window(const struct nf_conn *ct,
 	 */
 	seq = ntohl(tcph->seq);
 	ack = sack = ntohl(tcph->ack_seq);
-	win = ntohs(tcph->window);
+	win_raw = ntohs(tcph->window);
+	win = win_raw;
 	end = segment_seq_plus_len(seq, skb->len, dataoff, tcph);
 
 	if (receiver->flags & IP_CT_TCP_FLAG_SACK_PERM)
@@ -663,14 +665,14 @@ static bool tcp_in_window(const struct nf_conn *ct,
 			    && state->last_seq == seq
 			    && state->last_ack == ack
 			    && state->last_end == end
-			    && state->last_win == win)
+			    && state->last_win == win_raw)
 				state->retrans++;
 			else {
 				state->last_dir = dir;
 				state->last_seq = seq;
 				state->last_ack = ack;
 				state->last_end = end;
-				state->last_win = win;
+				state->last_win = win_raw;
 				state->retrans = 0;
 			}
 		}