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 ;-).
>
> 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).

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) {