From mboxrd@z Thu Jan 1 00:00:00 1970 From: Hannes Frederic Sowa Subject: Re: [PATCH 01/12] netfilter: avoid get_random_bytes calls Date: Mon, 6 Jan 2014 00:41:57 +0100 Message-ID: <20140105234157.GB29910@order.stressinduktion.org> References: <1388963586-5049-1-git-send-email-pablo@netfilter.org> <1388963586-5049-2-git-send-email-pablo@netfilter.org> Mime-Version: 1.0 Content-Type: text/plain; charset=utf-8 Cc: netfilter-devel@vger.kernel.org, davem@davemloft.net, netdev@vger.kernel.org, fw@strlen.de To: Pablo Neira Ayuso Return-path: Content-Disposition: inline In-Reply-To: <1388963586-5049-2-git-send-email-pablo@netfilter.org> Sender: netfilter-devel-owner@vger.kernel.org List-Id: netdev.vger.kernel.org On Mon, Jan 06, 2014 at 12:12:55AM +0100, Pablo Neira Ayuso wrote: > From: Florian Westphal > > 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