Re: [PATCH] virtio-blk: handle block_device_operations callbacks after hot unplug

[Christoph Hellwig]

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;

If this ever hits you have a nasty race condition and this is not
going to fix it, as it could be removed just after this check as well.