On 7/5/26 21:12, Michael S. Tsirkin wrote:
> The virtio spec requires the driver not to kick the device before
> DRIVER_OK is set. init_vqs() primes the stats virtqueue with a buffer
> and kicks the device before virtio_device_ready() is called in
> virtballoon_probe(), violating this requirement.
>
> Further, if the device responds to the early kick by processing the
> buffer before DRIVER_OK, stats_request() fires and queues
> update_balloon_stats_work. Should probe then fail and free vb, the work
> runs against freed memory.
>
> To fix, move buffer setup to after DRIVER_OK. Be careful to
> disable update_balloon_stats_work while this is going on,
> to make sure it does not race with the setup.
>
> setup_vqs() warns but does not fail probe or restore if
> virtqueue_add_outbuf() fails; the call never actually fails in these
> contexts since the queue is freshly initialized and empty.
>
> Testing: tested that stats still work after the change.
>
> Fixes: 9564e138b1f6 ("virtio: Add memory statistics reporting to the balloon
> driver (V4)")
> Reported-by: Sashiko:gemini-3.1-pro-preview
> Cc: David Hildenbrand <[email protected]>
> Assisted-by: Claude:claude-sonnet-4-6
> Signed-off-by: Michael S. Tsirkin <[email protected]>
> ---
>
> changes from v1:
> check that work enable/disable is balanced
> explain how add buf never fails in probe/restore
>
> drivers/virtio/virtio_balloon.c | 50 +++++++++++++++++++++------------
> 1 file changed, 32 insertions(+), 18 deletions(-)
>
> diff --git a/drivers/virtio/virtio_balloon.c b/drivers/virtio/virtio_balloon.c
> index 088b3a0e6ce6..bc0a2b19ca7d 100644
> --- a/drivers/virtio/virtio_balloon.c
> +++ b/drivers/virtio/virtio_balloon.c
> @@ -611,25 +611,8 @@ static int init_vqs(struct virtio_balloon *vb)
> vb->inflate_vq = vqs[VIRTIO_BALLOON_VQ_INFLATE];
> vb->deflate_vq = vqs[VIRTIO_BALLOON_VQ_DEFLATE];
> if (virtio_has_feature(vb->vdev, VIRTIO_BALLOON_F_STATS_VQ)) {
> - struct scatterlist sg;
> - unsigned int num_stats;
> vb->stats_vq = vqs[VIRTIO_BALLOON_VQ_STATS];
> -
> - /*
> - * Prime this virtqueue with one buffer so the hypervisor can
> - * use it to signal us later (it can't be broken yet!).
> - */
> - num_stats = update_balloon_stats(vb);
> -
> - sg_init_one(&sg, vb->stats, sizeof(vb->stats[0]) * num_stats);
> - err = virtqueue_add_outbuf(vb->stats_vq, &sg, 1, vb,
> - GFP_KERNEL);
> - if (err) {
> - dev_warn(&vb->vdev->dev, "%s: add stat_vq failed\n",
> - __func__);
> - return err;
> - }
> - virtqueue_kick(vb->stats_vq);
> + disable_work(&vb->update_balloon_stats_work);
That's to stop the stats queue triggering stats_request() I assume? Is that
valid before we actually added+kicked ourselves?
Also, can't we simply handle that in stats_request(), just ignoring it there?
Then we wouldn't have to go through the siable + enable.
> }
>
> if (virtio_has_feature(vb->vdev, VIRTIO_BALLOON_F_FREE_PAGE_HINT))
> @@ -916,6 +899,33 @@ static int virtio_balloon_register_shrinker(struct
> virtio_balloon *vb)
> return 0;
> }
>
> +static void setup_vqs(struct virtio_balloon *vb)
> +{
> + struct scatterlist sg;
> + unsigned int num_stats;
> + bool ret;
> +
> + if (!virtio_has_feature(vb->vdev, VIRTIO_BALLOON_F_STATS_VQ))
> + return;
> +
> + /*
> + * Prime this virtqueue with one buffer so the hypervisor can
> + * use it to signal us later (it can't be broken yet!).
> + */
> + num_stats = update_balloon_stats(vb);
> + sg_init_one(&sg, vb->stats, sizeof(vb->stats[0]) * num_stats);
> + if (virtqueue_add_outbuf(vb->stats_vq, &sg, 1, vb, GFP_KERNEL)) {
> + dev_warn(&vb->vdev->dev, "%s: add stat_vq failed\n", __func__);
> + return;
> + }
> + virtqueue_kick(vb->stats_vq);
> +
> + ret = enable_and_queue_work(system_freezable_wq,
> + &vb->update_balloon_stats_work);
> + /* Make sure we balanced enable/disable, or we won't report stats. */
> + BUG_ON(!ret);
A WARN_ON_ONCE() should be good enough here, no need to crash the kernel (no new
BUG_ON's).
--
Cheers,
David
