tcp: syncookies: extend validity range

Now we allow storing more request socks per listener, we might
hit syncookie mode less often and hit following bug in our stack :

When we send a burst of syncookies, then exit this mode,
tcp_synq_no_recent_overflow() can return false if the ACK packets coming
from clients are coming three seconds after the end of syncookie
episode.

This is a way too strong requirement and conflicts with rest of
syncookie code which allows ACK to be aged up to 2 minutes.

Perfectly valid ACK packets are dropped just because clients might be
in a crowded wifi environment or on another planet.

So let's fix this, and also change tcp_synq_overflow() to not
dirty a cache line for every syncookie we send, as we are under attack.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Acked-by: Florian Westphal <fw@strlen.de>
Acked-by: Yuchung Cheng <ycheng@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/net/tcp.h

@@ -326,18 +326,6 @@ static inline bool tcp_too_many_orphans(struct sock *sk, int shift)
 
 bool tcp_check_oom(struct sock *sk, int shift);
 
-/* syncookies: remember time of last synqueue overflow */
-static inline void tcp_synq_overflow(struct sock *sk)
-{
-	tcp_sk(sk)->rx_opt.ts_recent_stamp = jiffies;
-}
-
-/* syncookies: no recent synqueue overflow on this listening socket? */
-static inline bool tcp_synq_no_recent_overflow(const struct sock *sk)
-{
-	unsigned long last_overflow = tcp_sk(sk)->rx_opt.ts_recent_stamp;
-	return time_after(jiffies, last_overflow + TCP_TIMEOUT_FALLBACK);
-}
 
 extern struct proto tcp_prot;
 
@@ -483,13 +471,35 @@ struct sock *cookie_v4_check(struct sock *sk, struct sk_buff *skb);
  * i.e. a sent cookie is valid only at most for 2*60 seconds (or less if
  * the counter advances immediately after a cookie is generated).
  */
-#define MAX_SYNCOOKIE_AGE 2
+#define MAX_SYNCOOKIE_AGE	2
+#define TCP_SYNCOOKIE_PERIOD	(60 * HZ)
+#define TCP_SYNCOOKIE_VALID	(MAX_SYNCOOKIE_AGE * TCP_SYNCOOKIE_PERIOD)
+
+/* syncookies: remember time of last synqueue overflow
+ * But do not dirty this field too often (once per second is enough)
+ */
+static inline void tcp_synq_overflow(struct sock *sk)
+{
+	unsigned long last_overflow = tcp_sk(sk)->rx_opt.ts_recent_stamp;
+	unsigned long now = jiffies;
+
+	if (time_after(now, last_overflow + HZ))
+		tcp_sk(sk)->rx_opt.ts_recent_stamp = now;
+}
+
+/* syncookies: no recent synqueue overflow on this listening socket? */
+static inline bool tcp_synq_no_recent_overflow(const struct sock *sk)
+{
+	unsigned long last_overflow = tcp_sk(sk)->rx_opt.ts_recent_stamp;
+
+	return time_after(jiffies, last_overflow + TCP_SYNCOOKIE_VALID);
+}
 
 static inline u32 tcp_cookie_time(void)
 {
 	u64 val = get_jiffies_64();
 
-	do_div(val, 60 * HZ);
+	do_div(val, TCP_SYNCOOKIE_PERIOD);
 	return val;
 }