From: Steve Grubb Subject: Re: [PATCH] random: add blocking facility to urandom Date: Wed, 7 Sep 2011 16:30:14 -0400 Message-ID: <201109071630.15261.sgrubb@redhat.com> References: <1314974248-1511-1-git-send-email-jarod@redhat.com> <201109071602.24519.sgrubb@redhat.com> <1315426993.3576.38.camel@lappy> Mime-Version: 1.0 Content-Type: Text/Plain; charset="iso-8859-1" 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: Sasha Levin Return-path: Received: from mx1.redhat.com ([209.132.183.28]:43816 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1756323Ab1IGUaa (ORCPT ); Wed, 7 Sep 2011 16:30:30 -0400 In-Reply-To: <1315426993.3576.38.camel@lappy> Sender: linux-crypto-owner@vger.kernel.org List-ID: 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. > 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. -Steve