Re: [PATCH 2/2] lguest: virtio-rng support

From: Rusty Russell
Date: Mon May 19 2008 - 05:28:49 EST

On Monday 19 May 2008 19:10:49 Jeremy Fitzhardinge wrote:
> Rusty Russell wrote:
> > On Saturday 17 May 2008 17:47:01 Jeremy Fitzhardinge wrote:
> >> But the host->guest protocol should include the number of bits estimated
> >> entropy along with the bits themselves.
> >
> > If we go down that path, we have to come up with a way of feeding that
> > information to guest userspace. Patches welcome. If we meanwhile assume
> > entropy == #bits, it fits the current /dev/hwrng model, and we can add an
> > entropy count later if we want to change that.
> Really? Wouldn't guest userspace just use /dev/random as usual, which I
> think does have an ioctl to give that kind of information? And the
> interfaces do allow things injecting entropy into the kernel pool to
> provide entropy estimates along with the actual bits themselves.

Unfortunately not. Hardware randomness devices export /dev/hwrng, and it's up
to userspace to feed that into /dev/random (or not). That's usually done by
rngd, which at least on my system, assumes 1 bit of entropy per bit of data
from /dev/hwrng by default.

I was a little surprised that this decision was exported to userspace, but if
you're not prepared to unconditionally trust hw rngs, it makes sense to palm
it off. We could write a boutique device for virtualization which *did* feed
directly, but that would be a little gauche.

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