Re: [PATCH v4 0/2] vfio/mdev: Device namespace protection

Cornelia Huck
2018-05-23T08:56:41Z

On Tue, 22 May 2018 12:38:29 -0600
Alex Williamson <alex.williamson@redhat.com> wrote:

> On Tue, 22 May 2018 19:17:07 +0200
> Halil Pasic <pasic@linux.ibm.com> wrote:
>
> > From vfio-ccw perspective I join Connie's assessment: vfio-ccw should
> > be fine with these changes. I'm however not too deeply involved with
> > the mdev framework, thus I don't feel comfortable r-b-ing. That results
> > in
> > Acked-by: Halil Pasic <pasic@linux.ibm.com>
> > for both patches.
> >
> > While at it I have would like to ask about the semantics and intended
> > use of the mdev interfaces.
> >
> > static int vfio_ccw_sch_probe(struct subchannel *sch)
> > {
> >
> > /* HALIL: 8< Not so interesting stuff happens here. >8 */
>
> This was interesting:
>
> private->state = VFIO_CCW_STATE_NOT_OPER;
>
> > ret = vfio_ccw_mdev_reg(sch);
> > if (ret)
> > goto out_disable;
> > /*
> > * HALIL:
> > * This might be racy. Somewhere in vfio_ccw_mdev_reg() the create attribute
> > * is made available (it calls mdev_register_device()). For instance create will
> > * attempt to decrement private->avail which is initialized below. I fail to
> > * understand how is this well synchronized.
> > */
> > INIT_WORK(&private->io_work, vfio_ccw_sch_io_todo);
> > atomic_set(&private->avail, 1);
> > private->state = VFIO_CCW_STATE_STANDBY;
> >
> > return 0;
> >
> > out_disable:
> > cio_disable_subchannel(sch);
> > out_free:
> > dev_set_drvdata(&sch->dev, NULL);
> > kfree(private);
> > return ret;
> > }
> >
> > Should not initialization of go before mdev_register_device(), and then rolled
> > back if necessary if mdev_register_device() fails?
> >
> > In practice it does not seem very likely that userspace can trigger
> > mdev_device_create() before vfio_ccw_sch_probe() finishes so it should
> > not be a practical problem. But I would like to understand how synchronization
> > is supposed to work.
> >
> > [Added Dong Jia, maybe he is also able to answer my question.]
>
> vfio_ccw_mdev_create() requires that private->state is not
> VFIO_CCW_STATE_NOT_OPER but vfio_ccw_sch_probe() explicitly sets state
> to this value before calling vfio_ccw_mdev_reg(), so a create should
> return -ENODEV if racing with parent registration. Is there something
> else that I'm missing? Thanks,
>
> Alex

No, I think your understanding is correct. We move the state from
NOT_OPER to STANDBY only after we're set up completely, so our create
callback will simply fail early with -ENODEV. This looks fine to me.