Re: [PATCH v20 00/20] s390/vfio-ap: dynamic configuration support

Tony Krowiak
2022-06-22T12:40:48Z

On 6/22/22 2:48 AM, Christian Borntraeger wrote:
> Am 21.06.22 um 17:51 schrieb Tony Krowiak:
>> The current design for AP pass-through does not support making dynamic
>> changes to the AP matrix of a running guest resulting in a few
>> 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.
>> The
>>      AP architecture allows assignment of AP resources that are not
>>      available to the system, so this artificial restriction is not
>>      compliant with the architecture.
>>
>> 4. The AP configuration profile can be dynamically changed for the linux
>>      host after a KVM guest is started. For example, a new domain can be
>>      dynamically added to the configuration profile via the SE or an HMC
>>      connected to a DPM enabled lpar. Likewise, AP adapters can be
>>      dynamically configured (online state) and deconfigured (standby
>> state)
>>      using the SE, an SCLP command or an HMC connected to a DPM enabled
>>      lpar. This can result in inadvertent sharing of AP queues
>> between the
>>      guest and host.
>>
>> 5. A root user can manually unbind an AP queue device representing a
>>      queue in use by a KVM guest via the vfio_ap device driver's sysfs
>>      unbind attribute. In this case, the guest will be using a queue
>> that
>>      is not bound to the driver which violates the device model.
>>
>> 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 edits to the AP bus's
>>      /sys/bus/ap/apmask or /sys/bus/ap/aqmask if the change would
>> transfer
>>      ownership of an APQN from the vfio_ap device driver to a zcrypt
>> driver
>>      while the APQN is assigned to a matrix mdev.
>>
>> 2. Allow a root user to hot plug/unplug AP adapters, domains and control
>>      domains for a KVM guest using the matrix mdev via its sysfs
>>      assign/unassign attributes.
>>
>> 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 not reserved for use by the default zcrypt drivers (also
>> known as
>>      over-provisioning of AP resources). 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.
>> Such
>>      APQNs, however, will not be assigned to the guest using the matrix
>>      mdev; only APQNs referencing AP queue devices bound to the vfio_ap
>>      device driver will actually get assigned to the guest.
>>
>> 5. Handle dynamic changes to the AP device model.
>>
>> 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 hot plug/unplug using matrix mdev sysfs interfaces:
>> ----------------------------------------------------------------
>> Allowing a user to hot plug/unplug AP resources using the matrix mdev
>> sysfs interfaces circumvents the need to terminate the guest in order to
>> modify its AP configuration. Allowing dynamic configuration makes
>> reconfiguring a guest's AP matrix much less disruptive.
>>
>> 3. Rationale for allowing over-provisioning of AP resources:
>> -----------------------------------------------------------
>> Allowing assignment of AP resources to a matrix mdev and ultimately to a
>> guest better models the AP architecture. The architecture does not
>> preclude assignment of unavailable AP resources. If a queue subsequently
>> becomes available while a guest using the matrix mdev to which its APQN
>> is assigned, the guest will be given access to it. If an APQN
>> is dynamically unassigned from the underlying host system, it will
>> automatically become unavailable to the guest.
>>
>> Change log v19-v20:
>> ------------------
>> * Fixed patch 02/20: failed to move creation of status attribute
>>    for a queue device to the vfio_ap_mdev_probe_queue function in
>>    drivers/s390/crypto/vfio_ap_ops.c. (Jason)
>>
>> * Fixed signature of get_update_locks_for_queue macro
>>
>> * Take lock in get_update_locks_for_queue macro before
>>    accessing q->matrix_mdev
>>
>> * Renamed vfio_ap_mdev_get_update_locks_for_apqn function to
>>    get_update_locks_for_apqn (Jason)
>>
>> * Fix comments in function implementing the AP bus's in_use callback
>> (Jason)
>>
>> * Fix function name in prologue for ap_owned_by_def_drv function
>>
>> Tony Krowiak (20):
>>    s390/vfio-ap: use new AP bus interface to search for queue devices
>>    s390/vfio-ap: move probe and remove callbacks to vfio_ap_ops.c
>>    s390/vfio-ap: manage link between queue struct and matrix mdev
>>    s390/vfio-ap: introduce shadow APCB
>>    s390/vfio-ap: refresh guest's APCB by filtering AP resources assigned
>>      to mdev
>>    s390/vfio-ap: allow assignment of unavailable AP queues to mdev
>> device
>>    s390/vfio-ap: rename matrix_dev->lock mutex to matrix_dev->mdevs_lock
>>    s390/vfio-ap: introduce new mutex to control access to the KVM
>> pointer
>>    s390/vfio-ap: use proper locking order when setting/clearing KVM
>>      pointer
>>    s390/vfio-ap: prepare for dynamic update of guest's APCB on
>>      assign/unassign
>>    s390/vfio-ap: prepare for dynamic update of guest's APCB on queue
>>      probe/remove
>>    s390/vfio-ap: allow hot plug/unplug of AP devices when
>>      assigned/unassigned
>>    s390/vfio-ap: hot plug/unplug of AP devices when probed/removed
>>    s390/vfio-ap: reset queues after adapter/domain unassignment
>>    s390/vfio-ap: implement in-use callback for vfio_ap driver
>>    s390/vfio-ap: sysfs attribute to display the guest's matrix
>>    s390/vfio-ap: handle config changed and scan complete notification
>>    s390/vfio-ap: update docs to include dynamic config support
>>    s390/Docs: new doc describing lock usage by the vfio_ap device driver
>>    MAINTAINERS: pick up all vfio_ap docs for VFIO AP maintainers
>>
>>   Documentation/s390/vfio-ap-locking.rst |  105 ++
>>   Documentation/s390/vfio-ap.rst         |  492 +++++---
>>   MAINTAINERS                            |    2 +-
>>   drivers/s390/crypto/ap_bus.c           |   35 +-
>>   drivers/s390/crypto/vfio_ap_drv.c      |  124 +-
>>   drivers/s390/crypto/vfio_ap_ops.c      | 1436 ++++++++++++++++++------
>>   drivers/s390/crypto/vfio_ap_private.h  |   47 +-
>>   7 files changed, 1648 insertions(+), 593 deletions(-)
>>   create mode 100644 Documentation/s390/vfio-ap-locking.rst
>
> Unless somebody disagrees, I think we will carry these patches via the
> s390 tree.

Fine by me.