tcp: resalt the secret every 10 seconds

In order to limit the ability for an observer to recognize the source
ports sequence used to contact a set of destinations, we should
periodically shuffle the secret. 10 seconds looks effective enough
without causing particular issues.

Cc: Moshe Kol <moshe.kol@mail.huji.ac.il>
Cc: Yossi Gilad <yossi.gilad@mail.huji.ac.il>
Cc: Amit Klein <aksecurity@gmail.com>
Cc: Jason A. Donenfeld <Jason@zx2c4.com>
Tested-by: Willy Tarreau <w@1wt.eu>
Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

Author: edumazet

net/core/secure_seq.c

@@ -22,6 +22,8 @@
 static siphash_aligned_key_t net_secret;
 static siphash_aligned_key_t ts_secret;
 
+#define EPHEMERAL_PORT_SHUFFLE_PERIOD (10 * HZ)
+
 static __always_inline void net_secret_init(void)
 {
 	net_get_random_once(&net_secret, sizeof(net_secret));
@@ -100,11 +102,13 @@ u64 secure_ipv6_port_ephemeral(const __be32 *saddr, const __be32 *daddr,
 	const struct {
 		struct in6_addr saddr;
 		struct in6_addr daddr;
+		unsigned int timeseed;
 		__be16 dport;
 	} __aligned(SIPHASH_ALIGNMENT) combined = {
 		.saddr = *(struct in6_addr *)saddr,
 		.daddr = *(struct in6_addr *)daddr,
-		.dport = dport
+		.timeseed = jiffies / EPHEMERAL_PORT_SHUFFLE_PERIOD,
+		.dport = dport,
 	};
 	net_secret_init();
 	return siphash(&combined, offsetofend(typeof(combined), dport),
@@ -145,8 +149,10 @@ EXPORT_SYMBOL_GPL(secure_tcp_seq);
 u64 secure_ipv4_port_ephemeral(__be32 saddr, __be32 daddr, __be16 dport)
 {
 	net_secret_init();
-	return siphash_3u32((__force u32)saddr, (__force u32)daddr,
-			    (__force u16)dport, &net_secret);
+	return siphash_4u32((__force u32)saddr, (__force u32)daddr,
+			    (__force u16)dport,
+			    jiffies / EPHEMERAL_PORT_SHUFFLE_PERIOD,
+			    &net_secret);
 }
 EXPORT_SYMBOL_GPL(secure_ipv4_port_ephemeral);
 #endif