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Re: [Qemu-devel] RFC: virtio-rng and /dev/urandom

From: Daniel P. Berrange
Subject: Re: [Qemu-devel] RFC: virtio-rng and /dev/urandom
Date: Mon, 18 Apr 2016 10:28:42 +0100
User-agent: Mutt/1.5.24 (2015-08-30)

On Fri, Apr 15, 2016 at 08:56:59AM -0700, H. Peter Anvin wrote:
> On April 15, 2016 3:41:34 AM PDT, Cole Robinson <address@hidden> wrote:
> >Libvirt currently rejects using host /dev/urandom as an input source
> >for a
> >virtio-rng device. The only accepted sources are /dev/random and
> >/dev/hwrng.
> >This is the result of discussions on qemu-devel around when the feature
> >was
> >first added (2013). Examples:
> >
> >http://lists.gnu.org/archive/html/qemu-devel/2012-09/msg02387.html
> >https://lists.gnu.org/archive/html/qemu-devel/2013-03/threads.html#00023
> >
> >libvirt's rejection of /dev/urandom has generated some complaints from
> >users:
> >
> >https://bugzilla.redhat.com/show_bug.cgi?id=1074464
> >* cited: http://www.2uo.de/myths-about-urandom/
> >http://www.redhat.com/archives/libvir-list/2016-March/msg01062.html
> >http://www.redhat.com/archives/libvir-list/2016-April/msg00186.html
> >
> >I think it's worth having another discussion about this, at least with
> >a
> >recent argument in one place so we can put it to bed. I'm CCing a bunch
> >of
> >people. I think the questions are:
> >
> >1) is the original recommendation to never use virtio-rng+/dev/urandom
> >correct?
> >
> >2) regardless of #1, should we continue to reject that config in
> >libvirt?
> >
> >Thanks,
> >Cole
> Using /dev/urandom for virtio-rng, *except* perhaps for a small seed,
> it a complete waste of cycles.  There is absolutely no reason to have
> one prng feed another.

Regardless of the performance aspect, the key question we need the
answer to is whether it *cryptographically safe* to use /dev/urandom
on the host to feed virtio-rng. The original discussion said it was
/unsafe/ to use /dev/urandom, hence why we do not allow it.  If the
only downside is wasted performance, then it is reasonable to allow
the user to use /dev/urandom if they so wish.

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