Re: [PATCH 3/3] caif_virtio: fix the race between reset and netdev unregister

[Jason Wang]

On Mon, Jun 20, 2022 at 5:09 PM Michael S. Tsirkin <mst@xxxxxxxxxx> wrote:
>
> On Mon, Jun 20, 2022 at 01:11:15PM +0800, Jason Wang wrote:
> > We use to do the following steps during .remove():
>
> We currently do
>
>
> > static void cfv_remove(struct virtio_device *vdev)
> > {
> >       struct cfv_info *cfv = vdev->priv;
> >
> >       rtnl_lock();
> >       dev_close(cfv->ndev);
> >       rtnl_unlock();
> >
> >       tasklet_kill(&cfv->tx_release_tasklet);
> >       debugfs_remove_recursive(cfv->debugfs);
> >
> >       vringh_kiov_cleanup(&cfv->ctx.riov);
> >       virtio_reset_device(vdev);
> >       vdev->vringh_config->del_vrhs(cfv->vdev);
> >       cfv->vr_rx = NULL;
> >       vdev->config->del_vqs(cfv->vdev);
> >       unregister_netdev(cfv->ndev);
> > }
> > This is racy since device could be re-opened after dev_close() but
> > before unregister_netdevice():
> >
> > 1) RX vringh is cleaned before resetting the device, rx callbacks that
> >    is called after the vringh_kiov_cleanup() will result a UAF
> > 2) Network stack can still try to use TX virtqueue even if it has been
> >    deleted after dev_vqs()
> >
> > Fixing this by unregistering the network device first to make sure not
> > device access from both TX and RX side.
> >
> > Fixes: 0d2e1a2926b18 ("caif_virtio: Introduce caif over virtio")
> > Signed-off-by: Jason Wang <jasowang@xxxxxxxxxx>
> > ---
> >  drivers/net/caif/caif_virtio.c | 6 ++----
> >  1 file changed, 2 insertions(+), 4 deletions(-)
> >
> > diff --git a/drivers/net/caif/caif_virtio.c b/drivers/net/caif/caif_virtio.c
> > index 66375bea2fcd..a29f9b2df5b1 100644
> > --- a/drivers/net/caif/caif_virtio.c
> > +++ b/drivers/net/caif/caif_virtio.c
> > @@ -752,9 +752,8 @@ static void cfv_remove(struct virtio_device *vdev)
> >  {
> >       struct cfv_info *cfv = vdev->priv;
> >
> > -     rtnl_lock();
> > -     dev_close(cfv->ndev);
> > -     rtnl_unlock();
> > +     /* Make sure NAPI/TX won't try to access the device */
> > +     unregister_netdev(cfv->ndev);
> >
> >       tasklet_kill(&cfv->tx_release_tasklet);
> >       debugfs_remove_recursive(cfv->debugfs);
> > @@ -764,7 +763,6 @@ static void cfv_remove(struct virtio_device *vdev)
> >       vdev->vringh_config->del_vrhs(cfv->vdev);
> >       cfv->vr_rx = NULL;
> >       vdev->config->del_vqs(cfv->vdev);
> > -     unregister_netdev(cfv->ndev);
> >  }
>
>
> This gives me pause, callbacks can now trigger after device
> has been unregistered. Are we sure this is safe?

It looks safe, for RX NAPI is disabled. For TX, tasklet is disabled
after tasklet_kill(). I can add a comment to explain this.

> Won't it be safer to just keep the rtnl_lock around
> the whole process?

It looks to me we rtnl_lock can't help in synchronizing with the
callbacks, anything I miss?

Thanks

>
> >  static struct virtio_device_id id_table[] = {
> > --
> > 2.25.1
>