tcp: add some entropy in __inet_hash_connect()

Even when implementing RFC 6056 3.3.4 (Algorithm 4: Double-Hash Port Selection Algorithm), a patient attacker could still be able to collect enough state from an otherwise idle host. Idea of this patch is to inject some noise, in the cases __inet_hash_connect() found a candidate in the first attempt. This noise should not significantly reduce the collision avoidance, and should be zero if connection table is already well used. Note that this is not implementing RFC 6056 3.3.5 because we think Algorithm 5 could hurt typical workloads. Signed-off-by: Eric Dumazet <[email protected]> Cc: David Dworken <[email protected]> Cc: Willem de Bruijn <[email protected]> Signed-off-by: David S. Miller <[email protected]>
josf · Feb 11, 2021 · c579bd1 · c579bd1
1 parent 190cc82
commit c579bd1
Showing 1 changed file with 5 additions and 0 deletions.
diff --git a/net/ipv4/inet_hashtables.c b/net/ipv4/inet_hashtables.c
@@ -818,6 +818,11 @@ int __inet_hash_connect(struct inet_timewait_death_row *death_row,
 	return -EADDRNOTAVAIL;
 
 ok:
+	/* 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 */