From: Kees Cook Subject: Re: [PATCH][RESEND 3] hwrng: add randomness to system from rng sources Date: Wed, 5 Mar 2014 17:34:48 -0800 Message-ID: References: <20140303235148.GA7601@www.outflux.net> <20140304153841.GN1872@titan.lakedaemon.net> <20140304195356.GS1872@titan.lakedaemon.net> <1393972797.8344.190.camel@calx> <20140305211145.GV1872@titan.lakedaemon.net> <1394067147.17842.45.camel@calx> Mime-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Cc: Jason Cooper , "Theodore Ts'o" , LKML , Herbert Xu , Rusty Russell , Satoru Takeuchi , linux-crypto , Andrew Morton To: Matt Mackall Return-path: Received: from mail-oa0-f53.google.com ([209.85.219.53]:39112 "EHLO mail-oa0-f53.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1756064AbaCFBes (ORCPT ); Wed, 5 Mar 2014 20:34:48 -0500 Received: by mail-oa0-f53.google.com with SMTP id j17so1929866oag.12 for ; Wed, 05 Mar 2014 17:34:48 -0800 (PST) In-Reply-To: <1394067147.17842.45.camel@calx> Sender: linux-crypto-owner@vger.kernel.org List-ID: On Wed, Mar 5, 2014 at 4:52 PM, Matt Mackall wrote: > On Wed, 2014-03-05 at 16:11 -0500, Jason Cooper wrote: >> > In other words, if there are 4096 bits of "unknownness" in X to start >> > with, and I can get those same 4096 bits of "unknownness" back by >> > unmixing X' and Y, then there must still be 4096 bits of "unknownness" >> > in X'. If X' is 4096 bits long, then we've just proven that >> > reversibility means the attacker can know nothing about the contents of >> > X' by his choice of Y. >> >> Well, this reinforces my comfortability with loadable modules. The pool >> is already initialized by the point at which the driver is loaded. >> >> Unfortunately, any of the drivers in hw_random can be built in. When >> built in, hwrng_register is going to be called during the kernel >> initialization process. In that case, the unknownness in X is not 4096 >> bits, but far less. Also, the items that may have seeded X (MAC addr, >> time, etc) are discoverable by a potential attacker. This is also well >> before random-seed has been fed in. > > To which I would respond.. so? > > If the pool is in an attacker-knowable state at early boot, adding > attacker-controlled data does not make the situation any worse. In fact, > if the attacker has less-than-perfect control of the inputs, mixing more > things in will make things exponentially harder for the attacker. > > Put another way: mixing can't ever removes unknownness from the pool, it > can only add more. So the only reason you should ever choose not to mix > something into the pool is performance. Excellent. So it sounds like you're okay with my original patch as-is? -Kees -- Kees Cook Chrome OS Security