Linux Virtualization - Jul 2014 - [PATCH v2 1/2] hwrng: fetch randomness only after device init

On Fri, Jul 4, 2014 at 10:34 PM, Amit Shah <amit.shah at redhat.com>
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.
>
> 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 is made after device init.
Thanks, this seems pretty reasonable. One side-effect is that cycling
between hwrngs via sysfs (when they have init functions) will cause
them to add more entropy. I don't think this is a problem, but it is
kind of a weird side-effect.

Reviewed-by: Kees Cook <keescook at chromium.org>

-Kees
>
> CC: Kees Cook <keescook at chromium.org>
> CC: Jason Cooper <jason at lakedaemon.net>
> CC: Herbert Xu <herbert at gondor.apana.org.au>
> CC: <stable at vger.kernel.org> # For v3.15+
> Signed-off-by: Amit Shah <amit.shah at redhat.com>
> ---
>  drivers/char/hw_random/core.c | 31 +++++++++++++++++++++++++------
>  1 file changed, 25 insertions(+), 6 deletions(-)
>
> diff --git a/drivers/char/hw_random/core.c b/drivers/char/hw_random/core.c
> index 334601c..df95e2f 100644
> --- a/drivers/char/hw_random/core.c
> +++ b/drivers/char/hw_random/core.c
> @@ -55,16 +55,37 @@ static DEFINE_MUTEX(rng_mutex);
>  static int data_avail;
>  static u8 *rng_buffer;
>
> +static inline int rng_get_data(struct hwrng *rng, u8 *buffer, size_t size,
> +                              int wait);
> +
>  static size_t rng_buffer_size(void)
>  {
>         return SMP_CACHE_BYTES < 32 ? 32 : SMP_CACHE_BYTES;
>  }
>
> +static void add_early_randomness(struct hwrng *rng)
> +{
> +       unsigned char bytes[16];
> +       int bytes_read;
> +
> +       bytes_read = rng_get_data(rng, bytes, sizeof(bytes), 1);
> +       if (bytes_read > 0)
> +               add_device_randomness(bytes, bytes_read);
> +}
> +
>  static inline int hwrng_init(struct hwrng *rng)
>  {
> +       int ret;
> +
>         if (!rng->init)
>                 return 0;
> -       return rng->init(rng);
> +
> +       ret =  rng->init(rng);
> +       if (ret)
> +               return ret;
> +
> +       add_early_randomness(rng);
> +       return ret;
>  }
>
>  static inline void hwrng_cleanup(struct hwrng *rng)
> @@ -304,8 +325,6 @@ int hwrng_register(struct hwrng *rng)
>  {
>         int err = -EINVAL;
>         struct hwrng *old_rng, *tmp;
> -       unsigned char bytes[16];
> -       int bytes_read;
>
>         if (rng->name == NULL ||
>             (rng->data_read == NULL && rng->read == NULL))
> @@ -347,9 +366,9 @@ 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)
> +               add_early_randomness(rng);
> +
>  out_unlock:
>         mutex_unlock(&rng_mutex);
>  out:
> --
> 1.9.3
>


-- 
Kees Cook
Chrome OS Security

On (Sun) 06 Jul 2014 [21:41:47], Kees Cook wrote:
> On Fri, Jul 4, 2014 at 10:34 PM, Amit Shah <amit.shah at redhat.com>
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.
> >
> > 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 is made after device init.
> 
> Thanks, this seems pretty reasonable. One side-effect is that cycling
> between hwrngs via sysfs (when they have init functions) will cause
> them to add more entropy. I don't think this is a problem, but it is
> kind of a weird side-effect.
Yes, agreed.  Having a per-device bool that indicates whether the
initial randomness is obtained is quite a heavy solution for this
side-effect.  But I can put this in the commit log so it's clear..
> Reviewed-by: Kees Cook <keescook at chromium.org>
Thanks!

		Amit

On (Mon) 07 Jul 2014 [11:23:52], Amit Shah wrote:
> On (Sun) 06 Jul 2014 [21:41:47], Kees Cook wrote:
> > On Fri, Jul 4, 2014 at 10:34 PM, Amit Shah <amit.shah at
redhat.com> 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.
> > >
> > > 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 is made after device init.
> > 
> > Thanks, this seems pretty reasonable. One side-effect is that cycling
> > between hwrngs via sysfs (when they have init functions) will cause
> > them to add more entropy. I don't think this is a problem, but it
is
> > kind of a weird side-effect.
> 
> Yes, agreed.  Having a per-device bool that indicates whether the
> initial randomness is obtained is quite a heavy solution for this
> side-effect.  But I can put this in the commit log so it's clear..
Well actually this feels a little weird, so how about making this the
default behaviour for all devices, whether they have ->init() or not?

This also means that if a virtio-rng device is switched, it can
contribute as its probe would have been over.

		Amit