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tcp: add some entropy in __inet_hash_connect()

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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 <edumazet@google.com>
Cc: David Dworken <ddworken@google.com>
Cc: Willem de Bruijn <willemb@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
This commit is contained in:
Eric Dumazet 2021-02-09 11:20:28 -08:00 committed by David S. Miller
parent 190cc82489
commit c579bd1b40
1 changed files with 5 additions and 0 deletions
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5
net/ipv4/inet_hashtables.c
View file

@ -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 */