From 76d3468b1af6622651f89401634bff6e40da18c1 Mon Sep 17 00:00:00 2001 From: Eric Dumazet Date: Tue, 9 Feb 2021 11:20:28 -0800 Subject: [PATCH] tcp: add some entropy in __inet_hash_connect() commit c579bd1b4021c42ae247108f1e6f73dd3f08600c upstream. 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 Cc: David Dworken Cc: Willem de Bruijn Signed-off-by: David S. Miller Signed-off-by: Ben Hutchings Signed-off-by: Greg Kroah-Hartman --- net/ipv4/inet_hashtables.c | 5 +++++ 1 file changed, 5 insertions(+) diff --git a/net/ipv4/inet_hashtables.c b/net/ipv4/inet_hashtables.c index 1ebad5a024a7d..455a0a1aae380 100644 --- a/net/ipv4/inet_hashtables.c +++ b/net/ipv4/inet_hashtables.c @@ -694,6 +694,11 @@ 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 */