Re: [PATCH v4 3/7] s390: zcrypt: driver callback to indicate resource in use

[Cornelia Huck <cohuck@redhat.com>]

On Thu, 13 Jun 2019 15:39:36 -0400
Tony Krowiak <akrowiak@linux.ibm.com> wrote:

> Introduces a new driver callback to prevent a root user from unbinding
> an AP queue from its device driver if the queue is in use. This prevents
> a root user from inadvertently taking a queue away from a guest and
> giving it to the host, or vice versa. The callback will be invoked
> whenever a change to the AP bus's apmask or aqmask sysfs interfaces may
> result in one or more AP queues being removed from its driver. If the
> callback responds in the affirmative for any driver queried, the change
> to the apmask or aqmask will be rejected with a device in use error.
> 
> For this patch, only non-default drivers will be queried. Currently,
> there is only one non-default driver, the vfio_ap device driver. The
> vfio_ap device driver manages AP queues passed through to one or more
> guests and we don't want to unexpectedly take AP resources away from
> guests which are most likely independently administered.
> 
> Signed-off-by: Tony Krowiak <akrowiak@linux.ibm.com>
> ---
>  drivers/s390/crypto/ap_bus.c | 138 +++++++++++++++++++++++++++++++++++++++++--
>  drivers/s390/crypto/ap_bus.h |   3 +
>  2 files changed, 135 insertions(+), 6 deletions(-)

Hm... I recall objecting to this patch before, fearing that it makes it
possible for a bad actor to hog resources that can't be removed by
root, even forcefully. (I have not had time to look at the intervening
versions, so I might be missing something.)

Is there a way for root to forcefully override this?