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Subject: Re: [PATCH v4 3/7] s390: zcrypt: driver callback to indicate resource
 in use
From:   Tony Krowiak <akrowiak@linux.ibm.com>
To:     Cornelia Huck <cohuck@redhat.com>
Cc:     linux-s390@vger.kernel.org, linux-kernel@vger.kernel.org,
        kvm@vger.kernel.org, freude@linux.ibm.com, borntraeger@de.ibm.com,
        frankja@linux.ibm.com, david@redhat.com, mjrosato@linux.ibm.com,
        schwidefsky@de.ibm.com, heiko.carstens@de.ibm.com,
        pmorel@linux.ibm.com, pasic@linux.ibm.com,
        alex.williamson@redhat.com, kwankhede@nvidia.com
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 <20190618182558.7d7e025a.cohuck@redhat.com>
 <2366c6b6-fd9e-0c32-0e9d-018cd601a0ad@linux.ibm.com>
Date:   Wed, 26 Jun 2019 17:13:50 -0400
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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 <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?
> 
> 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?

Tony K

> 
>>
>