netns: provide pure entropy for net_hash_mix()

net_hash_mix() currently uses kernel address of a struct net,
and is used in many places that could be used to reveal this
address to a patient attacker, thus defeating KASLR, for
the typical case (initial net namespace, &init_net is
not dynamically allocated)

I believe the original implementation tried to avoid spending
too many cycles in this function, but security comes first.

Also provide entropy regardless of CONFIG_NET_NS.

Fixes: 0b4419162aa6 ("netns: introduce the net_hash_mix "salt" for hashes")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Reported-by: Amit Klein <aksecurity@gmail.com>
Reported-by: Benny Pinkas <benny@pinkas.net>
Cc: Pavel Emelyanov <xemul@openvz.org>
Signed-off-by: David S. Miller <davem@davemloft.net>
int peernet2id_alloc(struct net *net, struct net *peer)
{
	bool alloc = false, alive = false;
	int id;

	if (refcount_read(&net->count) == 0)
		return NETNSA_NSID_NOT_ASSIGNED;
	spin_lock_bh(&net->nsid_lock);
	/*
	 * When peer is obtained from RCU lists, we may race with
	 * its cleanup. Check whether it's alive, and this guarantees
	 * we never hash a peer back to net->netns_ids, after it has
	 * just been idr_remove()'d from there in cleanup_net().
	 */
	if (maybe_get_net(peer))
		alive = alloc = true;
	id = __peernet2id_alloc(net, peer, &alloc);
	spin_unlock_bh(&net->nsid_lock);
	if (alloc && id >= 0)
		rtnl_net_notifyid(net, RTM_NEWNSID, id);
	if (alive)
		put_net(peer);
	return id;
}

int peernet2id_alloc(struct net *net, struct net *peer)
{
	bool alloc = false, alive = false;
	int id;

	if (refcount_read(&net->count) == 0)
		return NETNSA_NSID_NOT_ASSIGNED;
	spin_lock_bh(&net->nsid_lock);
	/*
	 * When peer is obtained from RCU lists, we may race with
	 * its cleanup. Check whether it's alive, and this guarantees
	 * we never hash a peer back to net->netns_ids, after it has
	 * just been idr_remove()'d from there in cleanup_net().
	 */
	if (maybe_get_net(peer))
		alive = alloc = true;
	id = __peernet2id_alloc(net, peer, &alloc);
	spin_unlock_bh(&net->nsid_lock);
	if (alloc && id >= 0)
		rtnl_net_notifyid(net, RTM_NEWNSID, id);
	if (alive)
		put_net(peer);
	return id;
}
