Messages in this thread | | | Subject | Re: [PATCH v4 3/7] s390: zcrypt: driver callback to indicate resource in use | From | Tony Krowiak <> | 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
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