Merge branch 'tcp-rfc-6056'

Eric Dumazet says:

====================
tcp: RFC 6056 induced changes

This is based on a report from David Dworken.

First patch implements RFC 6056 3.3.4 proposal.

Second patch is adding a little bit of noise to make
attacker life a bit harder.
===================
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv4/inet_hashtables.c

@@ -709,6 +709,17 @@ void inet_unhash(struct sock *sk)
 }
 EXPORT_SYMBOL_GPL(inet_unhash);
 
+/* RFC 6056 3.3.4.  Algorithm 4: Double-Hash Port Selection Algorithm
+ * Note that we use 32bit integers (vs RFC 'short integers')
+ * because 2^16 is not a multiple of num_ephemeral and this
+ * property might be used by clever attacker.
+ * RFC claims using TABLE_LENGTH=10 buckets gives an improvement,
+ * we use 256 instead to really give more isolation and
+ * privacy, this only consumes 1 KB of kernel memory.
+ */
+#define INET_TABLE_PERTURB_SHIFT 8
+static u32 table_perturb[1 << INET_TABLE_PERTURB_SHIFT];
+
 int __inet_hash_connect(struct inet_timewait_death_row *death_row,
 		struct sock *sk, u32 port_offset,
 		int (*check_established)(struct inet_timewait_death_row *,
@@ -722,8 +733,8 @@ int __inet_hash_connect(struct inet_timewait_death_row *death_row,
 	struct inet_bind_bucket *tb;
 	u32 remaining, offset;
 	int ret, i, low, high;
-	static u32 hint;
 	int l3mdev;
+	u32 index;
 
 	if (port) {
 		head = &hinfo->bhash[inet_bhashfn(net, port,
@@ -750,7 +761,10 @@ int __inet_hash_connect(struct inet_timewait_death_row *death_row,
 	if (likely(remaining > 1))
 		remaining &= ~1U;
 
-	offset = (hint + port_offset) % remaining;
+	net_get_random_once(table_perturb, sizeof(table_perturb));
+	index = hash_32(port_offset, INET_TABLE_PERTURB_SHIFT);
+
+	offset = (READ_ONCE(table_perturb[index]) + port_offset) % remaining;
 	/* In first pass we try ports of @low parity.
 	 * inet_csk_get_port() does the opposite choice.
 	 */
@@ -804,7 +818,12 @@ int __inet_hash_connect(struct inet_timewait_death_row *death_row,
 	return -EADDRNOTAVAIL;
 
 ok:
-	hint += i + 2;
+	/* If our first attempt found a candidate, skip next candidate
+	 * in 1/16 of cases to add some noise.
+	 */
+	if (!i && !(prandom_u32() % 16))
+		i = 2;
+	WRITE_ONCE(table_perturb[index], READ_ONCE(table_perturb[index]) + i + 2);
 
 	/* Head lock still held and bh's disabled */
 	inet_bind_hash(sk, tb, port);