Re: [PATCH v4 3/7] s390: zcrypt: driver callback to indicate resource in use
From: Cornelia Huck
Date: Thu Jun 27 2019 - 03:25:31 EST
On Wed, 26 Jun 2019 17:13:50 -0400
Tony Krowiak <akrowiak@xxxxxxxxxxxxx> 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@xxxxxxxxxxxxx> 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@xxxxxxxxxxxxx>
> >>> ---
> >>> Â drivers/s390/crypto/ap_bus.c | 138
> >>> +++++++++++++++++++++++++++++++++++++++++--
> >>> Â drivers/s390/crypto/ap_bus.h |ÂÂ 3 +
> >>> Â 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
> > Â case, the guest could end up without access to any crypto adapters
> > Â without knowing why. This could lead to performance issues on guests
> > Â that rely heavily on crypto such as guests used for blockchain
> > Â transactions.
> > * There are plenty of examples in linux of the kernel preventing a root
> > Â user from performing a task. For example, a module can't be removed
> > Â if references are still held for it. Another example would be trying
> > Â to bind a CEX4 adapter to a device driver not registered for CEX4;
> > Â 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
> > Â queues.
> Any feedback on this?
Had not yet time to look at this, sorry.
> Tony K