From: Sasha Levin Subject: Re: [PATCH] random: add blocking facility to urandom Date: Thu, 08 Sep 2011 00:10:27 +0300 Message-ID: <1315429827.3576.61.camel@lappy> References: <1314974248-1511-1-git-send-email-jarod@redhat.com> <201109071630.15261.sgrubb@redhat.com> <1315427877.3576.46.camel@lappy> <201109071656.49758.sgrubb@redhat.com> Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Cc: Ted Ts'o , Jarod Wilson , linux-crypto@vger.kernel.org, Matt Mackall , Neil Horman , Herbert Xu , Stephan Mueller , lkml To: Steve Grubb Return-path: Received: from mail-ww0-f44.google.com ([74.125.82.44]:48541 "EHLO mail-ww0-f44.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1757011Ab1IGVKu (ORCPT ); Wed, 7 Sep 2011 17:10:50 -0400 In-Reply-To: <201109071656.49758.sgrubb@redhat.com> Sender: linux-crypto-owner@vger.kernel.org List-ID: On Wed, 2011-09-07 at 16:56 -0400, Steve Grubb wrote: > On Wednesday, September 07, 2011 04:37:57 PM Sasha Levin wrote: > > On Wed, 2011-09-07 at 16:30 -0400, Steve Grubb wrote: > > > On Wednesday, September 07, 2011 04:23:13 PM Sasha Levin wrote: > > > > On Wed, 2011-09-07 at 16:02 -0400, Steve Grubb wrote: > > > > > On Wednesday, September 07, 2011 03:27:37 PM Ted Ts'o wrote: > > > > > > On Wed, Sep 07, 2011 at 02:26:35PM -0400, Jarod Wilson wrote: > > > > > > > We're looking for a generic solution here that doesn't require > > > > > > > re-educating every single piece of userspace. And anything done > > > > > > > in userspace is going to be full of possible holes -- there > > > > > > > needs to be something in place that actually *enforces* the > > > > > > > policy, and centralized accounting/tracking, lest you wind up > > > > > > > with multiple processes racing to grab the entropy. > > > > > > > > > > > > Yeah, but there are userspace programs that depend on urandom not > > > > > > blocking... so your proposed change would break them. > > > > > > > > > > The only time this kicks in is when a system is under attack. If you > > > > > have set this and the system is running as normal, you will never > > > > > notice it even there. Almost all uses of urandom grab 4 bytes and > > > > > seed openssl or libgcrypt or nss. It then uses those libraries. > > > > > There are the odd cases where something uses urandom to generate a > > > > > key or otherwise grab a chunk of bytes, but these are still small > > > > > reads in the scheme of things. Can you think of any legitimate use > > > > > of urandom that grabs 100K or 1M from urandom? Even those numbers > > > > > still won't hit the sysctl on a normally function system. > > > > > > > > As far as I remember, several wipe utilities are using /dev/urandom to > > > > overwrite disks (possibly several times). > > > > > > Which should generate disk activity and feed entropy to urandom. > > > > I thought you need to feed random, not urandom. > > I think they draw from the same pool. There is a blocking and a non blocking pool. > > > Anyway, it won't happen fast enough to actually not block. > > > > Writing 1TB of urandom into a disk won't generate 1TB (or anything close > > to that) of randomness to cover for itself. > > We don't need a 1:1 mapping of RNG used to entropy acquired. Its more on the scale of > 8,000,000:1 or higher. I'm just saying that writing 1TB into a disk using urandom will start to block, it won't generate enough randomness by itself. > > > > > Something similar probably happens for getting junk on disks before > > > > creating an encrypted filesystem on top of them. > > > > > > During system install, this sysctl is not likely to be applied. > > > > It may happen at any time you need to create a new filesystem, which > > won't necessarily happen during system install. > > > > See for example the instructions on how to set up a LUKS filesystem: > > https://wiki.archlinux.org/index.php/System_Encryption_with_LUKS#Preparatio > > n_and_mapping > > Those instructions might need to be changed. That is one way of many to get random > numbers on the disk. Anyone really needing the security to have the sysctl on will > also probably accept that its doing its job and keeping the numbers random. Again, no > effect unless you turn it on. There are bunch of other places that would need to be changed in that case :) Why not implement it as a user mode CUSE driver that would wrap /dev/urandom and make it behave any way you want to? why push it into the kernel? -- Sasha.