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(-)
> 
> diff --git a/drivers/s390/crypto/ap_bus.c b/drivers/s390/crypto/ap_bus.c
> index 5256e3ce84e5..af15c095e76a 100644
> --- a/drivers/s390/crypto/ap_bus.c
> +++ b/drivers/s390/crypto/ap_bus.c
> @@ -35,6 +35,7 @@
>  #include <linux/mod_devicetable.h>
>  #include <linux/debugfs.h>
>  #include <linux/ctype.h>
> +#include <linux/module.h>
>  
>  #include "ap_bus.h"
>  #include "ap_debug.h"
> @@ -995,9 +996,11 @@ int ap_parse_mask_str(const char *str,
>  	newmap = kmalloc(size, GFP_KERNEL);
>  	if (!newmap)
>  		return -ENOMEM;
> -	if (mutex_lock_interruptible(lock)) {
> -		kfree(newmap);
> -		return -ERESTARTSYS;
> +	if (lock) {
> +		if (mutex_lock_interruptible(lock)) {
> +			kfree(newmap);
> +			return -ERESTARTSYS;
> +		}

This whole function is a bit odd. It seems all masks we want to
manipulate are always guarded by the ap_perms_mutex, and the need for
allowing lock == NULL comes from wanting to call this function with the
ap_perms_mutex already held.

That would argue for a locked/unlocked version of this function... but
looking at it, why do we lock the way we do? The one thing this
function (prior to this patch) does outside of the holding of the mutex
is the allocation and freeing of newmap. But with this patch, we do the
allocation and freeing of newmap while holding the mutex. Something
seems a bit weird here.

>  	}
>  
>  	if (*str == '+' || *str == '-') {