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

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Date:	Wed, 9 Jul 2014 08:08:23 -0400
From:	Jason Cooper <jason@...edaemon.net>
To:	Amit Shah <amit.shah@...hat.com>
Cc:	Kees Cook <keescook@...omium.org>,
	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>,
	"# 3.4.x" <stable@...r.kernel.org>
Subject: Re: [PATCH v2 2/2] virtio: rng: ensure reads happen after
 successful probe

On Mon, Jul 07, 2014 at 12:04:09PM +0530, Amit Shah wrote:
> 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.

Yes, but this is a critical case.  There are three common scenarios
where long term keys are generated in entropy-deprived states:

  - boot to a Linux Install CD, encrypt system during install
  - first boot of a Linux-based embedded router, need SSL keys
  - first boot of a Linux VM, need SSH host keys

We have the opportunity to make the third option suck less if we can get
this right.

> 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).

Well, I'm confused.  virtio-rng has no init function defined.  So
hwrng_init() will just return zero.

I think the basic question is: Where in the virtio-rng driver does it
execute init-style code?  And why isn't that in an init function?

Should we create a small init function that simply checks this DRIVER_OK
bit?

thx,

Jason.
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