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

From: Yaniv Kaul
Subject: Re: [Qemu-devel] [libvirt] RFC: virtio-rng and /dev/urandom
Date: Tue, 19 Apr 2016 14:30:36 +0300

On Fri, Apr 15, 2016 at 6:47 PM, Eric Blake <address@hidden> wrote:
On 04/15/2016 04:41 AM, Cole Robinson 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?

That I'm not sure about - and the answer may be context-dependent (for
example a FIPS user may care more than an ordinary user)

> 2) regardless of #1, should we continue to reject that config in libvirt?

This one, I have a pretty strong opinion: libvirt should NOT enforce
policy.  If someone has a valid use case for doing it, we should permit
them to do it, even if it lets someone else shoot themselves in the
foot.  So I think we should relax libvirt to allow users that source
their virtio-rng from /dev/urandom.

I'd personally be happy (for some specific test-dev use case) with /dev/zero - I don't care about the security, but I want the entropy collection to be done as fast as possible.

Eric Blake   eblake redhat com    +1-919-301-3266
Libvirt virtualization library http://libvirt.org

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