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Re: [Xen-devel] [RFC] Hypervisor RNG and enumeration

On Wed, Oct 29, 2014 at 9:29 AM, Jake Oshins <jakeo@xxxxxxxxxxxxx> wrote:
>>I have no objection to specifying that these reads may be quite slow.
>>Guests should only use them at boot and if they have some reason to
>>distrust their RNG pool.
>>The latter can legitimately happen after various types of suspend or
>>after migration (detected by VM Generation ID, for example).
> Just as a point of clarification, the VM Generation ID changes (at least in 
> the Hyper-V implementation) only when the VM may have observed a different 
> future, as when a VM backup is restored, a checkpoint is applied, etc.  It 
> does not change during migration, when the VM is suspended or when it is 
> rebooted.  I've heard anecdotes from application vendors saying that there is 
> some other hypervisor that actually does change the ID at these moments and 
> they wanted us to us to fix that, until I explained that I only control 
> Hyper-V.

Fair enough.

If the VM may indeed have observed a different future, then I would
argue that reseeding the RNG is very important -- more so than after a
normal migration.

If the VM trusts that its other history hasn't been compromised, then
merely mixing in a unique value would get most of the benefit.


> -- Jake Oshins

Andy Lutomirski
AMA Capital Management, LLC

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