linux-kernel - Re: [PATCH v3] virtio-blk: Add validation for block size in config space

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Date:   Tue, 22 Jun 2021 11:11:06 +0100
From:   Stefan Hajnoczi <stefanha@...hat.com>
To:     Xie Yongji <xieyongji@...edance.com>
Cc:     mst@...hat.com, jasowang@...hat.com, axboe@...nel.dk,
        virtualization@...ts.linux-foundation.org,
        linux-block@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3] virtio-blk: Add validation for block size in config
 space

On Thu, Jun 17, 2021 at 01:10:04PM +0800, Xie Yongji wrote:
> This ensures that we will not use an invalid block size
> in config space (might come from an untrusted device).
> 
> Signed-off-by: Xie Yongji <xieyongji@...edance.com>
> ---
>  drivers/block/virtio_blk.c | 29 +++++++++++++++++++++++------
>  1 file changed, 23 insertions(+), 6 deletions(-)
> 
> diff --git a/drivers/block/virtio_blk.c b/drivers/block/virtio_blk.c
> index b9fa3ef5b57c..bbdae989f1ea 100644
> --- a/drivers/block/virtio_blk.c
> +++ b/drivers/block/virtio_blk.c
> @@ -696,6 +696,28 @@ static const struct blk_mq_ops virtio_mq_ops = {
>  static unsigned int virtblk_queue_depth;
>  module_param_named(queue_depth, virtblk_queue_depth, uint, 0444);
>  
> +static int virtblk_validate(struct virtio_device *vdev)
> +{
> +	u32 blk_size;
> +
> +	if (!vdev->config->get) {
> +		dev_err(&vdev->dev, "%s failure: config access disabled\n",
> +			__func__);
> +		return -EINVAL;
> +	}
> +
> +	if (!virtio_has_feature(vdev, VIRTIO_BLK_F_BLK_SIZE))
> +		return 0;
> +
> +	blk_size = virtio_cread32(vdev,
> +			offsetof(struct virtio_blk_config, blk_size));
> +
> +	if (blk_size < SECTOR_SIZE || blk_size > PAGE_SIZE)
> +		__virtio_clear_bit(vdev, VIRTIO_BLK_F_BLK_SIZE);
> +
> +	return 0;
> +}

I saw Michael asked for .validate() in v2. I would prefer to keep
everything in virtblk_probe() instead of adding .validate() because:

- There is a race condition that an untrusted device can exploit since
  virtblk_probe() fetches the value again.

- It's more complex now that .validate() takes a first shot at blk_size
  and then virtblk_probe() deals with it again later on.

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