On Thu, Apr 23, 2020 at 01:37:17PM +0100, Stefan Hajnoczi wrote:
> A virtio_blk block device can still be referenced after hot unplug by
> userspace processes that hold the file descriptor. In this case
> virtblk_getgeo() can be invoked after virtblk_remove() was called. For
> example, a program that has /dev/vdb open can call ioctl(HDIO_GETGEO)
> after hot unplug.
>
> Fix this by clearing vblk->disk->private_data and checking that the
> virtio_blk driver instance is still around in virtblk_getgeo().
>
> Note that the virtblk_getgeo() function itself is guaranteed to remain
> in memory after hot unplug because the virtio_blk module refcount is
> still held while a block device reference exists.
>
> Originally-by: Lance Digby <ldigby@xxxxxxxxxx>
> Signed-off-by: Stefan Hajnoczi <stefanha@xxxxxxxxxx>
> ---
> drivers/block/virtio_blk.c | 5 +++++
> 1 file changed, 5 insertions(+)
>
> diff --git a/drivers/block/virtio_blk.c b/drivers/block/virtio_blk.c
> index 93468b7c6701..b50cdf37a6f7 100644
> --- a/drivers/block/virtio_blk.c
> +++ b/drivers/block/virtio_blk.c
> @@ -300,6 +300,10 @@ static int virtblk_getgeo(struct block_device *bd, struct hd_geometry *geo)
> {
> struct virtio_blk *vblk = bd->bd_disk->private_data;
>
> + /* Driver instance has been removed */
> + if (!vblk)
> + return -ENOTTY;
> +
> /* see if the host passed in geometry config */
> if (virtio_has_feature(vblk->vdev, VIRTIO_BLK_F_GEOMETRY)) {
> virtio_cread(vblk->vdev, struct virtio_blk_config,
> @@ -835,6 +839,7 @@ static void virtblk_remove(struct virtio_device *vdev)
> vdev->config->reset(vdev);
>
> refc = kref_read(&disk_to_dev(vblk->disk)->kobj.kref);
> + vblk->disk->private_data = NULL;
> put_disk(vblk->disk);
> vdev->config->del_vqs(vdev);
> kfree(vblk->vqs);
As pointed out, can be a race. We had a very similar issue in
virtio-vsock, and we solved using RCU to assign and get the pointer [1],
maybe the same solution can work here.
Cheers,
Stefano
[1] 0deab087b16a vsock/virtio: use RCU to avoid use-after-free on the_virtio_vsock
