[PATCH] hwrng: virtio - Fix race on data_avail and actual data

[Herbert Xu]

On Fri, Apr 21, 2023 at 04:52:13PM +0200, Dmitry Vyukov wrote:
>
> Here this:
> 
> size = min_t(unsigned int, size, vi->data_avail);
> memcpy(buf, vi->data + vi->data_idx, size);
> vi->data_idx += size;
> vi->data_avail -= size;
> 
> runs concurrently with:
> 
> if (!virtqueue_get_buf(vi->vq, &vi->data_avail))
>     return;
> vi->data_idx = 0;
> 
> I did not fully grasp how/where vi->data is populated, but it looks
> like it can lead to use of uninit/stale random data, or even to out of
> bounds access, say if vi->data_avail is already updated, but
> vi->data_idx is not yet reset to 0. Then concurrent reading will read
> not where it's supposed to read.

Yes this is a real race.  This bug appears to have been around
forever.

---8<---
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.

Reported-by: syzbot+726dc8c62c3536431ceb@xxxxxxxxxxxxxxxxxxxxxxxxx
Fixes: f7f510ec1957 ("virtio: An entropy device, as suggested by hpa.")
Signed-off-by: Herbert Xu <herbert@xxxxxxxxxxxxxxxxxxx>

diff --git a/drivers/char/hw_random/virtio-rng.c b/drivers/char/hw_random/virtio-rng.c
index f7690e0f92ed..e41a84e6b4b5 100644
--- a/drivers/char/hw_random/virtio-rng.c
+++ b/drivers/char/hw_random/virtio-rng.c
@@ -4,6 +4,7 @@
  *  Copyright (C) 2007, 2008 Rusty Russell IBM Corporation
  */
 
+#include <asm/barrier.h>
 #include <linux/err.h>
 #include <linux/hw_random.h>
 #include <linux/scatterlist.h>
@@ -37,13 +38,13 @@ 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);
 }
 
@@ -52,7 +53,6 @@ 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));
@@ -88,7 +88,7 @@ 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;
-- 
Email: Herbert Xu <herbert@xxxxxxxxxxxxxxxxxxx>
Home Page: http://gondor.apana.org.au/~herbert/
PGP Key: http://gondor.apana.org.au/~herbert/pubkey.txt