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Subject[PATCH RFC] virtio-balloon: fix add/get API use
In virtio balloon virtqueue_get_buf might now run concurrently with
virtqueue_kick. I audited both and this seems safe in practice but
this is not guaranteed by the API.
Additionally, a spurious interrupt might in theory make
virtqueue_get_buf run in parallel with virtqueue_add_buf, which is racy.

While we might try to protect against spurious callbacks it's
easier to fix the driver: balloon seems to be the only one
(mis)using the API like this, so let's just fix balloon.

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

---
Warning: completely untested.

diff --git a/drivers/virtio/virtio_balloon.c b/drivers/virtio/virtio_balloon.c
index bfbc15c..a26eb4f 100644
--- a/drivers/virtio/virtio_balloon.c
+++ b/drivers/virtio/virtio_balloon.c
@@ -47,7 +47,7 @@ struct virtio_balloon
struct task_struct *thread;

/* Waiting for host to ack the pages we released. */
- struct completion acked;
+ wait_queue_head_t acked;

/* Number of balloon pages we've told the Host we're not using. */
unsigned int num_pages;
@@ -89,29 +89,26 @@ static struct page *balloon_pfn_to_page(u32 pfn)

static void balloon_ack(struct virtqueue *vq)
{
- struct virtio_balloon *vb;
- unsigned int len;
+ struct virtio_balloon *vb = vq->vdev->priv;

- vb = virtqueue_get_buf(vq, &len);
- if (vb)
- complete(&vb->acked);
+ wake_up(&vb->acked);
}

static void tell_host(struct virtio_balloon *vb, struct virtqueue *vq)
{
struct scatterlist sg;
+ unsigned int len;
+ void *buf;

sg_init_one(&sg, vb->pfns, sizeof(vb->pfns[0]) * vb->num_pfns);

- init_completion(&vb->acked);
-
/* We should always be able to add one buffer to an empty queue. */
if (virtqueue_add_buf(vq, &sg, 1, 0, vb, GFP_KERNEL) < 0)
BUG();
virtqueue_kick(vq);

/* When host has read buffer, this completes via balloon_ack */
- wait_for_completion(&vb->acked);
+ wait_event(vb->acked, virtqueue_get_buf(vq, &len));
}

static void set_page_pfns(u32 pfns[], struct page *page)
@@ -231,12 +228,8 @@ static void update_balloon_stats(struct virtio_balloon *vb)
*/
static void stats_request(struct virtqueue *vq)
{
- struct virtio_balloon *vb;
- unsigned int len;
+ struct virtio_balloon *vb = vq->vdev->priv;

- vb = virtqueue_get_buf(vq, &len);
- if (!vb)
- return;
vb->need_stats_update = 1;
wake_up(&vb->config_change);
}
@@ -245,11 +238,14 @@ static void stats_handle_request(struct virtio_balloon *vb)
{
struct virtqueue *vq;
struct scatterlist sg;
+ unsigned int len;

vb->need_stats_update = 0;
update_balloon_stats(vb);

vq = vb->stats_vq;
+ if (!virtqueue_get_buf(vq, &len))
+ return;
sg_init_one(&sg, vb->stats, sizeof(vb->stats));
if (virtqueue_add_buf(vq, &sg, 1, 0, vb, GFP_KERNEL) < 0)
BUG();
@@ -358,6 +354,7 @@ static int virtballoon_probe(struct virtio_device *vdev)
INIT_LIST_HEAD(&vb->pages);
vb->num_pages = 0;
init_waitqueue_head(&vb->config_change);
+ init_waitqueue_head(&vb->acked);
vb->vdev = vdev;
vb->need_stats_update = 0;


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