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

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Message-Id: <200805191928.13043.rusty@rustcorp.com.au>
Date:	Mon, 19 May 2008 19:28:12 +1000
From:	Rusty Russell <rusty@...tcorp.com.au>
To:	Jeremy Fitzhardinge <jeremy@...p.org>
Cc:	"H. Peter Anvin" <hpa@...or.com>, Theodore Tso <tytso@....edu>,
	Herbert Xu <herbert@...dor.apana.org.au>,
	Jeff Garzik <jeff@...zik.org>,
	LKML <linux-kernel@...r.kernel.org>,
	virtualization@...ts.linux-foundation.org,
	Christian Borntraeger <borntraeger@...ibm.com>,
	Matt Mackall <mpm@...enic.com>,
	Johannes Berg <johannes@...solutions.net>
Subject: Re: [PATCH 2/2] lguest: virtio-rng support

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.

Cheers,
Rusty.
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