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

["Richard W.M. Jones" <rjones@redhat.com>]

On Fri, Apr 15, 2016 at 12:46:46PM +0100, Richard W.M. Jones wrote:
> 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.

I should clarify I mean: *after* the pool has been initialized with
sufficient entropy in the first place (which happens very early in
boot), you can then read almost indefinitely.

Rich.

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