Messages in this thread | | | Subject | Re: [PATCH v4 3/7] s390: zcrypt: driver callback to indicate resource in use | From | Tony Krowiak <> | Date | Thu, 27 Jun 2019 08:59:59 -0400 |
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On 6/27/19 3:25 AM, Cornelia Huck wrote: > On Wed, 26 Jun 2019 17:13:50 -0400 > Tony Krowiak <akrowiak@linux.ibm.com> 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 <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? > > 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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