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

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Date:   Wed, 23 May 2018 10:56:41 +0200
From:   Cornelia Huck <cohuck@...hat.com>
To:     Alex Williamson <alex.williamson@...hat.com>
Cc:     Halil Pasic <pasic@...ux.ibm.com>, kwankhede@...dia.com,
        Dong Jia <bjsdjshi@...ux.vnet.ibm.com>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4 0/2] vfio/mdev: Device namespace protection

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

> On Tue, 22 May 2018 19:17:07 +0200
> Halil Pasic <pasic@...ux.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@...ux.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.