[45/99] virtio: order used ring after used index read

[Greg KH]

2.6.31-stable review patch.  If anyone has any objections, please let us know.

------------------
From: Michael S. Tsirkin <mst@xxxxxxxxxx>

commit 2d61ba95034f1abbdec7729d52c740870a5eddb6 upstream.

On SMP guests, reads from the ring might bypass used index reads. This
causes guest crashes because host writes to used index to signal ring
data readiness.  Fix this by inserting rmb before used ring reads.

Signed-off-by: Michael S. Tsirkin <mst@xxxxxxxxxx>
Signed-off-by: Rusty Russell <rusty@xxxxxxxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxx>

---
 drivers/virtio/virtio_ring.c |    3 +++
 1 file changed, 3 insertions(+)

--- a/drivers/virtio/virtio_ring.c
+++ b/drivers/virtio/virtio_ring.c
@@ -281,6 +281,9 @@ static void *vring_get_buf(struct virtqu
 		return NULL;
 	}
 
+	/* Only get used array entries after they have been exposed by host. */
+	rmb();
+
 	i = vq->vring.used->ring[vq->last_used_idx%vq->vring.num].id;
 	*len = vq->vring.used->ring[vq->last_used_idx%vq->vring.num].len;
 


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