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

Aurelien Jarno
2013-02-27T16:36:42Z

On Wed, Feb 27, 2013 at 11:56:55AM +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 ;-).
> >
> > 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;
> >
> > This is basically the same issue than the following one:
> >
> > https://lists.linux-foundation.org/pipermail/virtualization/2008-May/010946.html
> >
> > but introduced in a more subtle way in this commit:
> >
> > commit bb347d98079a547e80bd4722dee1de61e4dca0e8
> > Author: Ian Molton <ian.molton@collabora.co.uk>
> > Date: Tue Dec 1 15:26:33 2009 +0800
>
> OK, I looked at doing a kmalloc and copy in virtio_rng, but it's very
> inelegant (we don't know what size of buffer to allocate).

On the other hand, the rng API allows to return less bytes than
requested, so it's possible to have a fixed buffer size of for example
64 or 128 bytes. But I agree it's better to do that in rng core.

> No driver other than virtio_rng cares about this issue, but it's still
> far easier to fix in the core.
>
> How's this? Works here...
>
> Subject: hw_random: make buffer usable in scatterlist.
>
> virtio_rng feeds the randomness buffer handed by the core directly
> into the scatterlist, since commit bb347d98079a547e80bd4722dee1de61e4dca0e8.
>
> However, if CONFIG_HW_RANDOM=m, the static buffer isn't a linear address
> (at least on most archs). We could fix this in virtio_rng, but it's actually
> far easier to just do it in the core as virtio_rng would have to allocate
> a buffer every time (it doesn't know how much the core will want to read).
>
> Reported-by: Aurelien Jarno <aurelien@aurel32.net>
> Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
>
> diff --git a/drivers/char/hw_random/core.c b/drivers/char/hw_random/core.c
> index 1bafb40..69ae597 100644
> --- a/drivers/char/hw_random/core.c
> +++ b/drivers/char/hw_random/core.c
> @@ -40,6 +40,7 @@
> #include <linux/init.h>
> #include <linux/miscdevice.h>
> #include <linux/delay.h>
> +#include <linux/slab.h>
> #include <asm/uaccess.h>
>
>
> @@ -52,8 +53,12 @@ static struct hwrng *current_rng;
> static LIST_HEAD(rng_list);
> static DEFINE_MUTEX(rng_mutex);
> static int data_avail;
> -static u8 rng_buffer[SMP_CACHE_BYTES < 32 ? 32 : SMP_CACHE_BYTES]
> - __cacheline_aligned;
> +static u8 *rng_buffer;
> +
> +static size_t rng_buffer_size(void)
> +{
> + return SMP_CACHE_BYTES < 32 ? 32 : SMP_CACHE_BYTES;
> +}
>
> static inline int hwrng_init(struct hwrng *rng)
> {
> @@ -116,7 +121,7 @@ static ssize_t rng_dev_read(struct file *filp, char __user *buf,
>
> if (!data_avail) {
> bytes_read = rng_get_data(current_rng, rng_buffer,
> - sizeof(rng_buffer),
> + rng_buffer_size(),
> !(filp->f_flags & O_NONBLOCK));
> if (bytes_read < 0) {
> err = bytes_read;
> @@ -307,6 +312,14 @@ int hwrng_register(struct hwrng *rng)
>
> mutex_lock(&rng_mutex);
>
> + /* kmalloc makes this safe for virt_to_page() in virtio_rng.c */
> + err = -ENOMEM;
> + if (!rng_buffer) {
> + rng_buffer = kmalloc(rng_buffer_size(), GFP_KERNEL);
> + if (!rng_buffer)
> + goto out_unlock;
> + }
> +
> /* Must not register two RNGs with the same name. */
> err = -EEXIST;
> list_for_each_entry(tmp, &rng_list, list) {
>

It works fine for me. Thanks for the patch.

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

--
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