tcp: relax tcp_paws_check()

Some windows versions have wrong RFC1323 implementations, with SYN and
SYNACKS messages containing zero tcp timestamps.

We relaxed in commit fc1ad92 the passive connection case
(Windows connects to a linux machine), but the reverse case (linux
connects to a Windows machine) has an analogue problem when tsvals from
windows machine are 'negative' (high order bit set) : PAWS triggers and
we drops incoming messages.

Fix this by making zero ts_recent value special, allowing frame to be
processed.

Based on a report and initial patch from Dmitiy Balakin

Bugzilla reference : https://bugzilla.kernel.org/show_bug.cgi?id=24842

Reported-by: dmitriy.balakin@nicneiron.ru
Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/net/tcp.h

@@ -1038,7 +1038,13 @@ static inline int tcp_paws_check(const struct tcp_options_received *rx_opt,
 		return 1;
 	if (unlikely(get_seconds() >= rx_opt->ts_recent_stamp + TCP_PAWS_24DAYS))
 		return 1;
-
+	/*
+	 * Some OSes send SYN and SYNACK messages with tsval=0 tsecr=0,
+	 * then following tcp messages have valid values. Ignore 0 value,
+	 * or else 'negative' tsval might forbid us to accept their packets.
+	 */
+	if (!rx_opt->ts_recent)
+		return 1;
 	return 0;
 }