Re: [PATCH v6 00/10] s390: vfio-ap: dynamic configuration support

[Tony Krowiak]

On 10/8/19 6:48 AM, Halil Pasic wrote:
> On Fri, 13 Sep 2019 17:26:48 -0400
> Tony Krowiak <akrowiak@xxxxxxxxxxxxx> wrote:
>
> > The current design for AP pass-through does not support making dynamic
> > changes to the AP matrix of a running guest resulting in three deficiencies
> > this patch series is intended to mitigate:
> >
> > 1. Adapters, domains and control domains can not be added to or removed
> >     from a running guest. In order to modify a guest's AP configuration,
> >     the guest must be terminated; only then can AP resources be assigned
> >     to or unassigned from the guest's matrix mdev. The new AP configuration
> >     becomes available to the guest when it is subsequently restarted.
> >
> > 2. The AP bus's /sys/bus/ap/apmask and /sys/bus/ap/aqmask interfaces can
> >     be modified by a root user without any restrictions. A change to either
> >     mask can result in AP queue devices being unbound from the vfio_ap
> >     device driver and bound to a zcrypt device driver even if a guest is
> >     using the queues, thus giving the host access to the guest's private
> >     crypto data and vice versa.
> >
> > 3. The APQNs derived from the Cartesian product of the APIDs of the
> >     adapters and APQIs of the domains assigned to a matrix mdev must
> >     reference an AP queue device bound to the vfio_ap device driver.
> >
> > This patch series introduces the following changes to the current design
> > to alleviate the shortcomings described above as well as to implement more
> > of the AP architecture:
> >
> > 1. A root user will be prevented from making changes to the AP bus's
> >     /sys/bus/ap/apmask or /sys/bus/ap/aqmask if the ownership of an APQN
> >     changes from the vfio_ap device driver to a zcrypt driver when the APQN
> >     is assigned to a matrix mdev.
> >
> > 2. The sysfs bind/unbind interfaces will be disabled for the vfio_ap
> >     device driver.
>
> Doesn't this have the potential of breaking some userspace stuff that
> might be out there?

I have decided to leave these interfaces enabled.

> > 3. Allow AP resources to be assigned to or removed from a matrix mdev
> >     while a guest is using it and hot plug the resource into or hot unplug
> >     the resource from the running guest.
>
> This looks like a natural extension of the interface -- i.e. should not
> break any userspace.

We agree

> > 4. Allow assignment of an AP adapter or domain to a matrix mdev even if it
> >     results in assignment of an APQN that does not reference an AP queue
> >     device bound to the vfio_ap device driver, as long as the APQN is owned
> >     by the vfio_ap driver. Allowing over-provisioning of AP resources
> >     better models the architecture which does not preclude assigning AP
> >     resources that are not yet available in the system. If/when the queue
> >     becomes available to the host, it will immediately also become available
> >     to the guest.
>
> Same here -- I don't think this change breaks any userspace.

We agree here also

> > 1. Rationale for changes to AP bus's apmask/aqmask interfaces:
> > ----------------------------------------------------------
> > Due to the extremely sensitive nature of cryptographic data, it is
> > imperative that great care be taken to ensure that such data is secured.
> > Allowing a root user, either inadvertently or maliciously, to configure
> > these masks such that a queue is shared between the host and a guest is
> > not only avoidable, it is advisable. It was suggested that this scenario
> > is better handled in user space with management software, but that does
> > not preclude a malicious administrator from using the sysfs interfaces
> > to gain access to a guest's crypto data. It was also suggested that this
> > scenario could be avoided by taking access to the adapter away from the
> > guest and zeroing out the queues prior to the vfio_ap driver releasing the
> > device; however, stealing an adapter in use from a guest as a by-product
> > of an operation is bad and will likely cause problems for the guest
> > unnecessarily. It was decided that the most effective solution with the
> > least number of negative side effects is to prevent the situation at the
> > source.
> >
> > 2. Rationale for disabling bind/unbind interfaces for vfio_ap driver:
> > -----------------------------------------------------------------
> > By disabling the bind/unbind interfaces for the vfio_ap device driver,
> > the user is forced to use the AP bus's apmask/aqmask interfaces to control
> > the probing and removing of AP queues. There are two primary reasons for
> > disabling the bind/unbind interfaces for the vfio_ap device driver:
> >
> > * The device architecture does not provide a means to prevent unbinding
> >    a device from a device driver, so an AP queue device can be unbound
> >    from the vfio_ap driver even when the queue is in use by a guest. By
> >    disabling the unbind interface, the user is forced to use the AP bus's
> >    apmask/aqmask interfaces which will prevent this.
>
> Isn't this fixed by your filtering (if implemented correctly)? BTW I believe
> it solves the problem regardless whether the unbind was triggered by the
> drivers unbind attribute or by a[pq]mask.

IMHO, it would be better if we didn't rely on the filtering because when
an unbind is done, the filtering will remove access to the entire
adapter. My goal was to limit the need for filtering to unbinds
triggered by AP deconfiguration via the SE or SCLP command over which we
have no control. We can control all other scenarios except for when an
adapter goes away for some other reason such as a failure.

> > * Binding of AP queues is controlled by the AP bus /sys/bus/ap/apmask and
> >    /sys/bus/ap/aqmask interfaces. If the masks indicate that an APQN is
> >    owned by zcrypt, trying to bind it to the vfio_ap device driver will
> >    fail; therefore, the bind interface is somewhat redundant and certainly
> >    unnecessary.
> >    
>
> [..]
>
> > Tony Krowiak (10):
> >    s390: vfio-ap: Refactor vfio_ap driver probe and remove callbacks
> >    s390: vfio-ap: allow assignment of unavailable AP resources to mdev
> >      device
> >    s390: vfio-ap: allow hot plug/unplug of AP resources using mdev device
> >    s390: vfio-ap: filter CRYCB bits for unavailable queue devices
> >    s390: vfio-ap: sysfs attribute to display the guest CRYCB
> >    s390: vfio-ap: update guest CRYCB in vfio_ap probe and remove
> >      callbacks
> >    s390: zcrypt: driver callback to indicate resource in use
> >    s390: vfio-ap: implement in-use callback for vfio_ap driver
> >    s390: vfio-ap: added versioning to vfio_ap module
> >    s390: vfio-ap: update documentation
>
> I believe it would be worthwhile to reorder the patches (fixes and
> re-factoring first, the features).

Suggestions?

> Regards,
> Halil
>
> >   Documentation/s390/vfio-ap.rst        | 899 +++++++++++++++++++++++++---------
> >   drivers/s390/crypto/ap_bus.c          | 144 +++++-
> >   drivers/s390/crypto/ap_bus.h          |   4 +
> >   drivers/s390/crypto/vfio_ap_drv.c     |  27 +-
> >   drivers/s390/crypto/vfio_ap_ops.c     | 610 ++++++++++++++---------
> >   drivers/s390/crypto/vfio_ap_private.h |  12 +-
> >   6 files changed, 1200 insertions(+), 496 deletions(-)