Re: virtio-rng only returns zeros with CONFIG_HW_RANDOM=m

[Aurelien Jarno <aurelien@aurel32.net>]

On Wed, Feb 27, 2013 at 10:43:37AM +1030, Rusty Russell wrote:
> Aurelien Jarno <aurelien@aurel32.net> writes:
> > Hi,
> >
> > I have noticed that virtio-rng only returns zero for kernels >= 2.6.33
> > built with CONFIG_HW_RANDOM=m. This is a bit much too predictable for a
> > random generator ;-).
> 
> Wow.  Fortunately, all of SLES, RHEL, Ubuntu or Fedora set
> CONFIG_HW_RANDOM=y.  What do they know that we don't?
> 
> Oops, looks like Debian testing: config-3.2.0-4-amd64:CONFIG_HW_RANDOM=m
> 
> > The reason for that is virtio expects guest real addresses, while
> > rng_core.ko (ie when built as a module) is passing a vmalloced buffer 
> > to the virtio-rng read function, declared as such:
> >
> >   static u8 rng_buffer[SMP_CACHE_BYTES < 32 ? 32 : SMP_CACHE_BYTES]
> >           __cacheline_aligned;
> 
> Yuck...  It would be nice if this has oopsed.  Jens, what about this patch?
> 
> Cheers,
> Rusty.
> 
> Subject: scatterlist: sg_set_buf() argument must be in linear mapping.
> 
> Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
> 
> diff --git a/include/linux/scatterlist.h b/include/linux/scatterlist.h
> index 4bd6c06..9365375 100644
> --- a/include/linux/scatterlist.h
> +++ b/include/linux/scatterlist.h
> @@ -111,6 +111,9 @@ static inline struct page *sg_page(struct scatterlist *sg)
>  static inline void sg_set_buf(struct scatterlist *sg, const void *buf,
>  			      unsigned int buflen)
>  {
> +#ifdef CONFIG_DEBUG_SG
> +	BUG_ON(!virt_addr_valid(buf));
> +#endif
>  	sg_set_page(sg, virt_to_page(buf), buflen, offset_in_page(buf));
>  }
>  

I confirm this patch catches the issue. Thanks.

Tested-by: Aurelien Jarno <aurelien@aurel32.net>

-- 
Aurelien Jarno                          GPG: 1024D/F1BCDB73
aurelien@aurel32.net                 http://www.aurel32.net