Re: [Qemu-devel] [PATCH 0/6] add paravirtualization hwrng support

[H. Peter Anvin]

On 10/29/2012 01:45 AM, Paolo Bonzini wrote:
> Il 26/10/2012 22:29, H. Peter Anvin ha scritto:
> > > > This is surreal.  Output from /dev/hwrng turns into output for /dev/random... 
> > > > it us guaranteed worse; period, end of story.
> > > >
> > > > Isn't that exactly what happens in bare-metal?  hwrng -> rngd -> random.  
> > > > Instead here
> > > > we'd have, host hwrng -> virtio-rng-pci -> guest hwrng -> guest rngd -> guest 
> > > > random.
> > > >
> > > > The only difference is that you paravirtualize access to the host hwrng to a) 
> > > > distribute
> > > > entropy to multiple guests; b) support migration across hosts with different 
> > > > CPUs and
> > > > hardware.
> > First, hwrng is only one of the sources used by rngd.  It can also
> > (currently) use RDRAND or TPM; additional sources are likely to be added
> > in the future.
> >
> > Second, the harvesting of environmental noise -- timings -- is not as
> > good in a VM as on plain hardware, so for the no-hwrng case it is better
> > for this to be done in the host than in the VM.
>
> Neither of these make /dev/random with virtio-rng-pci worse than without
> (as would be the case if you fed /dev/urandom).  And migration works.
> This, and avoiding denial of service for the host's /dev/random, is all
> I care about at this time.
>
> There is always time to change defaults to something better.

Let me be more specific.

First of all, feeding /dev/urandom to the guest is dangerous -- you are 
feeding it PRNG contents but telling it that it is real entropy.  This 
is a security hole.

Second of all, you're doing something pointless: you are still 
exhausting the entropy pool on the host at the same rate, and all you 
end up with is something that isn't what you want.  You still have the 
same DoS on the host /dev/random that you're worried about.

Third, you're doing something inefficient: you're running a PRNG in the 
host which could be run more efficiently in guest space.

From an Intel perspective I guess I should be happy, as it functionally 
would mean that unless you have RDRAND in the host you're insecure, but 
I'd much rather see the Right Thing done.

        -hpa

--
H. Peter Anvin, Intel Open Source Technology Center
I work for Intel.  I don't speak on their behalf.