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

Rusty Russell
2013-02-27T01:52:09Z

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));
}