Re: [PATCH v4 3/7] s390: zcrypt: driver callback to indicate resource in use

[Tony Krowiak]

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?

Tony K

> >