Re: [PATCH 1/2] hwrng: don't fetch rng from sources without init

[Amit Shah <amit.shah@redhat.com>]

On (Wed) 02 Jul 2014 [07:58:23], Jason Cooper wrote:
> On Wed, Jul 02, 2014 at 03:58:15PM +0530, Amit Shah wrote:
> > Commit d9e7972619334 "hwrng: add randomness to system from rng sources"
> > added a call to rng_get_data() from the hwrng_register() function.
> > However, some rng devices need initialization before data can be read
> > from them.
> > 
> > Also, the virtio-rng device does not behave properly when this call is
> > made in its probe() routine - the virtio core sets the DRIVER_OK status
> > bit only on a successful probe, which means the host ignores all
> > communication from the guest, and the guest insmod or boot process just
> > sits there doing nothing.
> > 
> > This commit makes the call to rng_get_data() depend on no init fn
> > pointer being registered by the device.  If an init function is
> > registered, this call isn't made.
> > 
> > CC: Kees Cook <keescook@chromium.org>
> > CC: Jason Cooper <jason@lakedaemon.net>
> > CC: Herbert Xu <herbert@gondor.apana.org.au>
> > CC: <stable@vger.kernel.org> # For 3.15 only
> 
> # v3.15+ should be fine here.
> 
> > Signed-off-by: Amit Shah <amit.shah@redhat.com>
> > ---
> >  drivers/char/hw_random/core.c | 8 +++++---
> >  1 file changed, 5 insertions(+), 3 deletions(-)
> > 
> > diff --git a/drivers/char/hw_random/core.c b/drivers/char/hw_random/core.c
> > index 334601c..3f3941d 100644
> > --- a/drivers/char/hw_random/core.c
> > +++ b/drivers/char/hw_random/core.c
> > @@ -347,9 +347,11 @@ int hwrng_register(struct hwrng *rng)
> >  	INIT_LIST_HEAD(&rng->list);
> >  	list_add_tail(&rng->list, &rng_list);
> >  
> > -	bytes_read = rng_get_data(rng, bytes, sizeof(bytes), 1);
> > -	if (bytes_read > 0)
> > -		add_device_randomness(bytes, bytes_read);
> > +	if (!rng->init) {
> > +		bytes_read = rng_get_data(rng, bytes, sizeof(bytes), 1);
> > +		if (bytes_read > 0)
> > +			add_device_randomness(bytes, bytes_read);
> > +	}
> 
> afaict, this is redundant at initialization time.  current_rng shouldn't
> be set yet, so hwrng_init(rng) will get called at line 333.  Or, am I
> missing something?

You're right -- the device will have been initialized if it's the
first hwrng device to be registered.  And in that case, this device
won't contribute to the initial pool (and that's the only case).

Can someone think of a better way to handle this for that case, then?


		Amit