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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 14:45:29 -0700
User-agent: K-9 Mail for Android

On April 18, 2016 4:26:24 AM PDT, "Daniel P. Berrange" <address@hidden> wrote:
>On Mon, Apr 18, 2016 at 01:07:40PM +0200, Hubert Kario wrote:
>> On Monday 18 April 2016 02:46:19 H. Peter Anvin wrote:
>> > Another thing that really needs to be addressed, but is a separate
>> > issue: invalidating and reseeding the entropy pool after a snapshot
>> > event.
>> definitely agreed
>> though just reseeding would be sufficient - the goal is to make the 
>> output unpredictable and unique between multiple machines starting
>> the same snapshot, feeding enough random data to make the entropy
>> unique again is sufficient to achieve that
>If you're spawning multiple machines from the same base snapshot,
>the seeding of RNG is just one of many many things that need
>dealing with. eg new /etc/machine-id, new ssh host keys, changing
>MAC address of NICs with corresponding guest config file changes,
>many other application specific identifiers / keys intended to
>be unique per machine.  As such, libvirt explicitly tries to
>prevent you spawning multiple machines from the same snapshot.
>That all said, Microsoft HyperV has defined a concept of a
>"Virtual Machine Generation ID" and specified various hypervisor
>operations which should result in this value changing[1]. For example
>restoring from a snapshot should always change the genid, as would
>restoring from backup, or cloned from another image, or failed over
>during disaster recovery.
>This vm genid is exposed to the guest via ACPI and there's an
>notification whenever it changes.
>There are patches for QEMU[2] to support this feature in a manner that
>is compatible with the hyperv spec, but they are sadly still not
>merged :-(
>So it would be possible for the Linux kernel to re-initialize its
>RNG after snapshot by hooking into the vm-genid ACPI notification.
>[2] https://lists.gnu.org/archive/html/qemu-devel/2016-01/msg05599.html

There are multiple machines, and there are snapshots restored.
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