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Date:   Thu, 27 Jun 2019 08:59:59 -0400
From:   Tony Krowiak <>
To:     Cornelia Huck <>
Subject: Re: [PATCH v4 3/7] s390: zcrypt: driver callback to indicate resource
 in use

On 6/27/19 3:25 AM, Cornelia Huck wrote:
> On Wed, 26 Jun 2019 17:13:50 -0400
> Tony Krowiak <> wrote:
>> On 6/19/19 9:04 AM, Tony Krowiak wrote:
>>> On 6/18/19 12:25 PM, Cornelia Huck wrote:
>>>> On Thu, 13 Jun 2019 15:39:36 -0400
>>>> Tony Krowiak <> 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 <>
>>>>> ---
>>>>>    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?
>>> You recall correctly; however, after many internal crypto team
>>> discussions, it was decided that this feature was important
>>> and should be kept.
>>> Allow me to first address your fear that a bad actor can hog
>>> resources that can't be removed by root. With this enhancement,
>>> there is nothing preventing a root user from taking resources
>>> from a matrix mdev, it simply forces him/her to follow the
>>> proper procedure. The resources to be removed must first be
>>> unassigned from the matrix mdev to which they are assigned.
>>> The AP bus's /sys/bus/ap/apmask and /sys/bus/ap/aqmask
>>> sysfs attributes can then be edited to transfer ownership
>>> of the resources to zcrypt.
>>> The rationale for keeping this feature is:
>>> * It is a bad idea to steal an adapter in use from a guest. In the worst
>>>     case, the guest could end up without access to any crypto adapters
>>>     without knowing why. This could lead to performance issues on guests
>>>     that rely heavily on crypto such as guests used for blockchain
>>>     transactions.
>>> * There are plenty of examples in linux of the kernel preventing a root
>>>     user from performing a task. For example, a module can't be removed
>>>     if references are still held for it. Another example would be trying
>>>     to bind a CEX4 adapter to a device driver not registered for CEX4;
>>>     this action will also be rejected.
>>> * The semantics are much cleaner and the logic is far less complicated.
>>> * It forces the use of the proper procedure to change ownership of AP
>>>     queues.
>> Any feedback on this?
> Had not yet time to look at this, sorry.

No problem, just wanted to make sure it didn't get lost in the shuffle.

>> Tony K

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