linux-kernel - Re: [PATCH v2 2/2] virtio: rng: ensure reads happen after successful probe

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Date:	Mon, 7 Jul 2014 12:04:09 +0530
From:	Amit Shah <amit.shah@...hat.com>
To:	Kees Cook <keescook@...omium.org>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Virtualization List <virtualization@...ts.linux-foundation.org>,
	Rusty Russell <rusty@...tcorp.com.au>,
	Herbert Xu <herbert@...dor.apana.org.au>,
	Jason Cooper <jason@...edaemon.net>,
	"# 3.4.x" <stable@...r.kernel.org>
Subject: Re: [PATCH v2 2/2] virtio: rng: ensure reads happen after successful
 probe

On (Sun) 06 Jul 2014 [23:09:49], Kees Cook wrote:
> On Sun, Jul 6, 2014 at 10:51 PM, Amit Shah <amit.shah@...hat.com> wrote:
> > On (Sun) 06 Jul 2014 [21:38:36], Kees Cook wrote:
> >> On Fri, Jul 4, 2014 at 10:34 PM, Amit Shah <amit.shah@...hat.com> wrote:
> >> > The hwrng core asks for random data in the hwrng_register() call itself
> >> > from commit d9e7972619.  This doesn't play well with virtio -- the
> >> > DRIVER_OK bit is only set by virtio core on a successful probe, and
> >> > we're not yet out of our probe routine when this call is made.  This
> >> > causes the host to not acknowledge any requests we put in the virtqueue,
> >> > and the insmod or kernel boot process just waits for data to arrive from
> >> > the host, which never happens.
> >>
> >> Doesn't this mean that virtio-rng won't ever contribute entropy to the system?
> >
> > The initial randomness?  Yes.  But it'll start contributing entropy as
> > soon as it's used as the current source.
> 
> How does that happen? I don't see an init function defined for it?

I mean the regular usage; not the initial randomness patch that you
added.

Initial randomness from virtio-rng currently won't be sourced.  That's
no different from the way things were before your patch; and I can't
think of a way to make that happen for now.

virtio's probe() has to finish before communication with the host can
start.  If a virtio-rng device is the only hwrng in the system (very
likely in a guest), it's almost guaranteed that hwrng_init() won't be
called after hwrng_register() completes (as it would have already been
called and the virtio-rng device will have become the current_rng).

If the VM admin configured two virtio-rng sources, though, or
hotplugged one later, the early randomness will be obtained for the
second virtio-rng device.  But this is a scenario I don't expect to
ever arise -- it's just a theoretical possibility.

		Amit
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