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

Halil Pasic
2018-05-22T17:17:07Z

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 */
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.]

Regards,
Halil

On 05/18/2018 09:10 PM, Alex Williamson wrote:
> v4: Fix the 'create' racing 'remove' gap noted by Kirti by moving
> removal from mdev_list to mdev_device_release(). Fix missing
> mdev_put_parent() cases in mdev_device_create(), also noted
> by Kirti. Added documention update regarding serialization as
> noted by Cornelia. Added additional commit log comment about
> -EAGAIN vs -ENODEV for 'remove' racing 'create'. Added second
> patch to re-order sysfs attributes, with this my targeted
> scripts can no longer hit the gap where -EAGAIN is regurned.
> BTW, the gap where the current code returns -ENODEV in this
> race condition is about 50% easier to hit than it exists in
> this series with patch 1 alone.
>
> Thanks,
> Alex
>
> ---
>
> Alex Williamson (2):
> vfio/mdev: Check globally for duplicate devices
> vfio/mdev: Re-order sysfs attribute creation
>
>
> Documentation/vfio-mediated-device.txt | 5 ++
> drivers/vfio/mdev/mdev_core.c | 102 +++++++++++---------------------
> drivers/vfio/mdev/mdev_private.h | 2 -
> drivers/vfio/mdev/mdev_sysfs.c | 14 ++--
> 4 files changed, 49 insertions(+), 74 deletions(-)
>