This is a note to let you know that I've just added the patch titled
hwrng: virtio - Fix race on data_avail and actual data
to the 4.19-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary
The filename of the patch is:
hwrng-virtio-fix-race-on-data_avail-and-actual-data.patch
and it can be found in the queue-4.19 subdirectory.
If you, or anyone else, feels it should not be added to the stable tree,
please let <stable@xxxxxxxxxxxxxxx> know about it.
commit 939a58b0fd48531e7170994e9836b43eb6a96c4e
Author: Herbert Xu <herbert@xxxxxxxxxxxxxxxxxxx>
Date: Thu May 4 11:59:32 2023 +0800
hwrng: virtio - Fix race on data_avail and actual data
[ Upstream commit ac52578d6e8d300dd50f790f29a24169b1edd26c ]
The virtio rng device kicks off a new entropy request whenever the
data available reaches zero. When a new request occurs at the end
of a read operation, that is, when the result of that request is
only needed by the next reader, then there is a race between the
writing of the new data and the next reader.
This is because there is no synchronisation whatsoever between the
writer and the reader.
Fix this by writing data_avail with smp_store_release and reading
it with smp_load_acquire when we first enter read. The subsequent
reads are safe because they're either protected by the first load
acquire, or by the completion mechanism.
Also remove the redundant zeroing of data_idx in random_recv_done
(data_idx must already be zero at this point) and data_avail in
request_entropy (ditto).
Reported-by: syzbot+726dc8c62c3536431ceb@xxxxxxxxxxxxxxxxxxxxxxxxx
Fixes: f7f510ec1957 ("virtio: An entropy device, as suggested by hpa.")
Signed-off-by: Herbert Xu <herbert@xxxxxxxxxxxxxxxxxxx>
Acked-by: Michael S. Tsirkin <mst@xxxxxxxxxx>
Signed-off-by: Herbert Xu <herbert@xxxxxxxxxxxxxxxxxxx>
Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>
diff --git a/drivers/char/hw_random/virtio-rng.c b/drivers/char/hw_random/virtio-rng.c
index a84248c26fd7f..58884d8752011 100644
--- a/drivers/char/hw_random/virtio-rng.c
+++ b/drivers/char/hw_random/virtio-rng.c
@@ -17,6 +17,7 @@
* Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA
*/
+#include <asm/barrier.h>
#include <linux/err.h>
#include <linux/hw_random.h>
#include <linux/scatterlist.h>
@@ -49,13 +50,13 @@ struct virtrng_info {
static void random_recv_done(struct virtqueue *vq)
{
struct virtrng_info *vi = vq->vdev->priv;
+ unsigned int len;
/* We can get spurious callbacks, e.g. shared IRQs + virtio_pci. */
- if (!virtqueue_get_buf(vi->vq, &vi->data_avail))
+ if (!virtqueue_get_buf(vi->vq, &len))
return;
- vi->data_idx = 0;
-
+ smp_store_release(&vi->data_avail, len);
complete(&vi->have_data);
}
@@ -64,7 +65,6 @@ static void request_entropy(struct virtrng_info *vi)
struct scatterlist sg;
reinit_completion(&vi->have_data);
- vi->data_avail = 0;
vi->data_idx = 0;
sg_init_one(&sg, vi->data, sizeof(vi->data));
@@ -100,7 +100,7 @@ static int virtio_read(struct hwrng *rng, void *buf, size_t size, bool wait)
read = 0;
/* copy available data */
- if (vi->data_avail) {
+ if (smp_load_acquire(&vi->data_avail)) {
chunk = copy_data(vi, buf, size);
size -= chunk;
read += chunk;
