Re: [PATCH v7 03/15] s390/zcrypt: driver callback to indicate resource in use

[Cornelia Huck <cohuck@redhat.com>]

On Tue,  7 Apr 2020 15:20:03 -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. The intent of
> this callback is to provide a driver with the means to prevent a root user
> from inadvertently taking a queue away from a guest and giving it to the
> host while the guest is still using it. The callback will
> be invoked whenever a change to the AP bus's sysfs apmask or aqmask
> attributes would 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 | 144 +++++++++++++++++++++++++++++++++--
>  drivers/s390/crypto/ap_bus.h |   4 +
>  2 files changed, 142 insertions(+), 6 deletions(-)

(...)

> @@ -1196,12 +1202,75 @@ static ssize_t apmask_show(struct bus_type *bus, char *buf)
>  	return rc;
>  }
>  
> +int __verify_card_reservations(struct device_driver *drv, void *data)
> +{
> +	int rc = 0;
> +	struct ap_driver *ap_drv = to_ap_drv(drv);
> +	unsigned long *newapm = (unsigned long *)data;
> +
> +	/*
> +	 * If the reserved bits do not identify cards reserved for use by the
> +	 * non-default driver, there is no need to verify the driver is using
> +	 * the queues.

I had to read that one several times... what about

"No need to verify whether the driver is using the queues if it is the
default driver."

?

> +	 */
> +	if (ap_drv->flags & AP_DRIVER_FLAG_DEFAULT)
> +		return 0;
> +
> +	/* The non-default driver's module must be loaded */
> +	if (!try_module_get(drv->owner))
> +		return 0;

Is that really needed? I would have thought that the driver core's
klist usage would make sure that the callback would not be invoked for
drivers that are not registered anymore. Or am I missing a window?

> +
> +	if (ap_drv->in_use)
> +		if (ap_drv->in_use(newapm, ap_perms.aqm))

Can we log the offending apm somewhere, preferably with additional info
that allows the admin to figure out why an error was returned?

> +			rc = -EADDRINUSE;
> +
> +	module_put(drv->owner);
> +
> +	return rc;
> +}

(Same comments for the other changes further along in this patch.)