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

[Tian, Kevin]

> From: Alex Williamson <alex.williamson@xxxxxxxxxx>
> Sent: Wednesday, September 21, 2022 3:17 AM
> 
> On Fri,  9 Sep 2022 18:22:38 +0800
> Kevin Tian <kevin.tian@xxxxxxxxx> wrote:
> 
> > From: Yi Liu <yi.l.liu@xxxxxxxxx>
> >
> > and manage available ports inside @init/@release.
> >
> > Signed-off-by: Yi Liu <yi.l.liu@xxxxxxxxx>
> > Signed-off-by: Kevin Tian <kevin.tian@xxxxxxxxx>
> > Reviewed-by: Jason Gunthorpe <jgg@xxxxxxxxxx>
> > ---
> >  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?

Yes, it's a use-after-free indeed. Thanks for catching it!

> 
> 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,
> 

Yes. I'll scrutinize it again. To be consistent I'll make sure the free
is the last line in all .release callbacks though not required for
some (e.g. mdpy and vfio-ap).