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Re: [Qemu-devel] RFC: virtio-rng and /dev/urandom
From: |
H. Peter Anvin |
Subject: |
Re: [Qemu-devel] RFC: virtio-rng and /dev/urandom |
Date: |
Mon, 18 Apr 2016 02:46:19 -0700 |
User-agent: |
K-9 Mail for Android |
On April 18, 2016 2:28:42 AM PDT, "Daniel P. Berrange" <address@hidden> wrote:
>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.
>
>Regards,
>Daniel
Perhaps. What we do know is that it at least used to be a fairly common
misconfiguration; up there with people who would feed /dev/urandom to rngd!
Probably there ought to be a backend which knows to use urandom for a seed and
then fall back to random, rather than simply relying on a file name.
Another thing that really needs to be addressed, but is a separate issue:
invalidating and reseeding the entropy pool after a snapshot event.
--
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- Re: [Qemu-devel] RFC: virtio-rng and /dev/urandom, (continued)
- Re: [Qemu-devel] RFC: virtio-rng and /dev/urandom, H. Peter Anvin, 2016/04/15
- Re: [Qemu-devel] RFC: virtio-rng and /dev/urandom, Paolo Bonzini, 2016/04/16
- Re: [Qemu-devel] RFC: virtio-rng and /dev/urandom, H. Peter Anvin, 2016/04/17
- Re: [Qemu-devel] RFC: virtio-rng and /dev/urandom, H. Peter Anvin, 2016/04/17
- Re: [Qemu-devel] RFC: virtio-rng and /dev/urandom, Hubert Kario, 2016/04/18
- Re: [Qemu-devel] RFC: virtio-rng and /dev/urandom, Hubert Kario, 2016/04/18
Re: [Qemu-devel] [libvirt] RFC: virtio-rng and /dev/urandom, Yaniv Kaul, 2016/04/19
Re: [Qemu-devel] RFC: virtio-rng and /dev/urandom, H. Peter Anvin, 2016/04/15
Re: [Qemu-devel] RFC: virtio-rng and /dev/urandom, Cole Robinson, 2016/04/20