netdev - Re: [PATCH 01/12] netfilter: avoid get_random

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Date:	Mon, 6 Jan 2014 00:41:57 +0100
From:	Hannes Frederic Sowa <hannes@...essinduktion.org>
To:	Pablo Neira Ayuso <pablo@...filter.org>
Cc:	netfilter-devel@...r.kernel.org, davem@...emloft.net,
	netdev@...r.kernel.org, fw@...len.de
Subject: Re: [PATCH 01/12] netfilter: avoid get_random_bytes calls

On Mon, Jan 06, 2014 at 12:12:55AM +0100, Pablo Neira Ayuso wrote:
> From: Florian Westphal <fw@...len.de>
> 
> All these users need an initial seed value for jhash, prandom is
> perfectly fine.  This avoids draining the entropy pool where
> its not strictly required.

Secrets protecting hash tables should be rather strong.

prandom_u32() has two seeding points at boot-up. One is at late_initcall.
Thanks to parallel boot-up this gets executed fairly early. The other one is
when the RNG nonblocking pool is fully initialized. Only after this point we
can assume prandom_u32() returns truely random values. In between, only
get_random_bytes or net_get_random_once are safe for use.

To get the impression when prandom_u32 gets truely seeded, watch out
for the message "random: nonblocking pool is initialized" in dmesg. ;)

Hmm, some of them look like good candidates for net_get_random_once. I don't
see such a problem with draining entropy pool, especially as they don't run
that early and they don't request so many random bits.

Greetings,

  Hannes

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