tcp: change source port randomizarion at connect() time

RFC 6056 (Recommendations for Transport-Protocol Port Randomization)
provides good summary of why source selection needs extra care.

David Dworken reminded us that linux implements Algorithm 3
as described in RFC 6056 3.3.3

Quoting David :
   In the context of the web, this creates an interesting info leak where
   websites can count how many TCP connections a user's computer is
   establishing over time. For example, this allows a website to count
   exactly how many subresources a third party website loaded.
   This also allows:
   - Distinguishing between different users behind a VPN based on
       distinct source port ranges.
   - Tracking users over time across multiple networks.
   - Covert communication channels between different browsers/browser
       profiles running on the same computer
   - Tracking what applications are running on a computer based on
       the pattern of how fast source ports are getting incremented.

Section 3.3.4 describes an enhancement, that reduces
attackers ability to use the basic information currently
stored into the shared 'u32 hint'.

This change also decreases collision rate when
multiple applications need to connect() to
different destinations.
Signed-off-by: Eric Dumazet <edumazet@google.com>
Reported-by: David Dworken <ddworken@google.com>
Cc: Willem de Bruijn <willemb@google.com>
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 +818,7 @@ int __inet_hash_connect(struct inet_timewait_death_row *death_row,
 	return -EADDRNOTAVAIL;
 
 ok:
-	hint += 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);