Re: [PATCH v3 06/15] vfio/mtty: Use the new device life cycle helpers

Alex Williamson
2022-09-20T19:17:34Z

On Fri, 9 Sep 2022 18:22:38 +0800
Kevin Tian <kevin.tian@intel.com> wrote:

> From: Yi Liu <yi.l.liu@intel.com>
>
> and manage available ports inside @init/@release.
>
> Signed-off-by: Yi Liu <yi.l.liu@intel.com>
> Signed-off-by: Kevin Tian <kevin.tian@intel.com>
> Reviewed-by: Jason Gunthorpe <jgg@nvidia.com>
> ---
> samples/vfio-mdev/mtty.c | 67 +++++++++++++++++++++++-----------------
> 1 file changed, 39 insertions(+), 28 deletions(-)
>
> diff --git a/samples/vfio-mdev/mtty.c b/samples/vfio-mdev/mtty.c
> index f42a59ed2e3f..41301d50b247 100644
> --- a/samples/vfio-mdev/mtty.c
> +++ b/samples/vfio-mdev/mtty.c
...
> +static int mtty_probe(struct mdev_device *mdev)
> +{
> + struct mdev_state *mdev_state;
> + int ret;
> +
> + mdev_state = vfio_alloc_device(mdev_state, vdev, &mdev->dev,
> + &mtty_dev_ops);
> + if (IS_ERR(mdev_state))
> + return PTR_ERR(mdev_state);
>
> ret = vfio_register_emulated_iommu_dev(&mdev_state->vdev);
> if (ret)
> - goto err_vconfig;
> + goto err_put_vdev;
> dev_set_drvdata(&mdev->dev, mdev_state);
> return 0;
>
> -err_vconfig:
> - kfree(mdev_state->vconfig);
> -err_state:
> - vfio_uninit_group_dev(&mdev_state->vdev);
> - kfree(mdev_state);
> -err_nr_ports:
> - atomic_add(nr_ports, &mdev_avail_ports);
> +err_put_vdev:
> + vfio_put_device(&mdev_state->vdev);
> return ret;
> }
>
> +static void mtty_release_dev(struct vfio_device *vdev)
> +{
> + struct mdev_state *mdev_state =
> + container_of(vdev, struct mdev_state, vdev);
> +
> + kfree(mdev_state->vconfig);
> + vfio_free_device(vdev);
> + atomic_add(mdev_state->nr_ports, &mdev_avail_ports);

I must be missing something, isn't this a use-after-free?

mdev_state is allocated via vfio_alloc_device(), where vdev is the
first entry in that structure, so this is equivalent to
kvfree(mdev_state). mbochs has the same issue. mdpy and vfio-ap
adjust global counters after vfio_free_device(), which I think muddies
the situation. Shouldn't we look suspiciously at any .release callback
where vfio_free_device() isn't the last thing executed? Thanks,

Alex