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

Richard W.M. Jones rjones at redhat.com
Fri Apr 15 11:46:46 UTC 2016

On Fri, Apr 15, 2016 at 06:41:34AM -0400, 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?
> 2) regardless of #1, should we continue to reject that config in libvirt?

There was a lot of internal-to-Red Hat discussion on this which I
can't reproduce here unfortunately.  However the crux of it was that
it's quite safe to read enormous amounts from /dev/urandom, even
without adding any entropy at all, and use those numbers for
cryptographic purposes.

Steve: can we disclose the research that was done into this?  If so
can you summarise the results for us?


Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones
Read my programming and virtualization blog: http://rwmj.wordpress.com
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