Re: [PATCH v12 07/17] s390/vfio-ap: implement in-use callback for vfio_ap driver

[Halil Pasic <pasic@linux.ibm.com>]

On Tue, 24 Nov 2020 16:40:06 -0500
Tony Krowiak <akrowiak@linux.ibm.com> wrote:

> Let's implement the callback to indicate when an APQN
> is in use by the vfio_ap device driver. The callback is
> invoked whenever a change to the apmask or aqmask would
> result in one or more queue devices being removed from the driver. The
> vfio_ap device driver will indicate a resource is in use
> if the APQN of any of the queue devices to be removed are assigned to
> any of the matrix mdevs under the driver's control.
> 
> There is potential for a deadlock condition between the matrix_dev->lock
> used to lock the matrix device during assignment of adapters and domains
> and the ap_perms_mutex locked by the AP bus when changes are made to the
> sysfs apmask/aqmask attributes.
> 
> Consider following scenario (courtesy of Halil Pasic):
> 1) apmask_store() takes ap_perms_mutex
> 2) assign_adapter_store() takes matrix_dev->lock
> 3) apmask_store() calls vfio_ap_mdev_resource_in_use() which tries
>    to take matrix_dev->lock
> 4) assign_adapter_store() calls ap_apqn_in_matrix_owned_by_def_drv
>    which tries to take ap_perms_mutex
> 
> BANG!
> 
> To resolve this issue, instead of using the mutex_lock(&matrix_dev->lock)
> function to lock the matrix device during assignment of an adapter or
> domain to a matrix_mdev as well as during the in_use callback, the
> mutex_trylock(&matrix_dev->lock) function will be used. If the lock is not
> obtained, then the assignment and in_use functions will terminate with
> -EBUSY.

Good news is: the final product is OK with regards to in_use(). Bad news
is: this patch does not do enough. At this stage we are still racy.

The problem is that the assign operations don't bother to take the
ap_perms_mutex lock under the matrix_dev->lock.

The scenario is the following:
1) apmask_store() takes ap_perms_mutex
2) apmask_store() calls vfio_ap_mdev_resource_in_use() which
     takes matrix_dev->lock
3) vfio_ap_mdev_resource_in_use() releases matrix_dev->lock
   and returns 0
4) assign_adapter_store() takes matrix_dev->lock does the
   assign (the queues are still bound to vfio_ap) and releases
   matrix_dev->lock 
5) apmask_store() carries on, does the update to apask and releases
   ap_perms_mutex
6) The queues get 'stolen' from vfio ap while used.

This gets fixed with "s390/vfio-ap: allow assignment of unavailable AP
queues to mdev device". Maybe we can reorder these patches. I didn't
look into that.

We could also just ignore the problem, because it is just for a couple
of commits, but I would prefer it gone.

Regards,
Halil