[PATCH v7 00/15] s390/vfio-ap: dynamic configuration support

Tony Krowiak
2020-04-07T19:20:33Z

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 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. Allow a root user to hot plug/unplug AP adapters, domains and control
domains using the matrix mdev's 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.

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 v6-v7:
----------------
* Added callbacks to AP bus:
- on_config_changed: Notifies implementing drivers that
the AP configuration has changed since last AP device scan.
- on_scan_complete: Notifies implementing drivers that the device scan
has completed.
- implemented on_config_changed and on_scan_complete callbacks for
vfio_ap device driver.
- updated vfio_ap device driver's probe and remove callbacks to handle
dynamic changes to the AP device model.
* Added code to filter APQNs when assigning AP resources to a KVM guest's
CRYCB

Change log v5-v6:
----------------
* Fixed a bug in ap_bus.c introduced with patch 2/7 of the v5
series. Harald Freudenberer pointed out that the mutex lock
for ap_perms_mutex in the apmask_store and aqmask_store functions
was not being freed.

* Removed patch 6/7 which added logging to the vfio_ap driver
to expedite acceptance of this series. The logging will be introduced
with a separate patch series to allow more time to explore options
such as DBF logging vs. tracepoints.

* Added 3 patches related to ensuring that APQNs that do not reference
AP queue devices bound to the vfio_ap device driver are not assigned
to the guest CRYCB:

Patch 4: Filter CRYCB bits for unavailable queue devices
Patch 5: sysfs attribute to display the guest CRYCB
Patch 6: update guest CRYCB in vfio_ap probe and remove callbacks

* Added a patch (Patch 9) to version the vfio_ap module.

* Reshuffled patches to allow the in_use callback implementation to
invoke the vfio_ap_mdev_verify_no_sharing() function introduced in
patch 2.

Change log v4-v5:
----------------
* Added a patch to provide kernel s390dbf debug logs for VFIO AP

Change log v3->v4:
-----------------
* Restored patches preventing root user from changing ownership of
APQNs from zcrypt drivers to the vfio_ap driver if the APQN is
assigned to an mdev.

* No longer enforcing requirement restricting guest access to
queues represented by a queue device bound to the vfio_ap
device driver.

* Removed shadow CRYCB and now directly updating the guest CRYCB
from the matrix mdev's matrix.

* Rebased the patch series on top of 'vfio: ap: AP Queue Interrupt
Control' patches.

* Disabled bind/unbind sysfs interfaces for vfio_ap driver

Change log v2->v3:
-----------------
* Allow guest access to an AP queue only if the queue is bound to
the vfio_ap device driver.

* Removed the patch to test CRYCB masks before taking the vCPUs
out of SIE. Now checking the shadow CRYCB in the vfio_ap driver.

Change log v1->v2:
-----------------
* Removed patches preventing root user from unbinding AP queues from
the vfio_ap device driver
* Introduced a shadow CRYCB in the vfio_ap driver to manage dynamic
changes to the AP guest configuration due to root user interventions
or hardware anomalies.

Harald Freudenberger (1):
s390/zcrypt: Notify driver on config changed and scan complete
callbacks

Tony Krowiak (14):
s390/vfio-ap: store queue struct in hash table for quick access
s390/vfio-ap: manage link between queue struct and matrix mdev
s390/zcrypt: driver callback to indicate resource in use
s390/vfio-ap: implement in-use callback for vfio_ap driver
s390/vfio-ap: introduce shadow CRYCB
s390/vfio-ap: sysfs attribute to display the guest CRYCB
s390/vfio-ap: filter CRYCB bits for unavailable queue devices
s390/vfio_ap: add qlink from ap_matrix_mdev struct to vfio_ap_queue
struct
s390/vfio-ap: allow assignment of unavailable AP queues to mdev device
s390/vfio-ap: allow configuration of matrix mdev in use by a KVM guest
s390/vfio-ap: allow hot plug/unplug of AP resources using mdev device
s390/vfio-ap: handle host AP config change notification
s390/vfio-ap: handle AP bus scan completed notification
s390/vfio-ap: handle probe/remove not due to host AP config changes

drivers/s390/crypto/ap_bus.c | 301 ++++++-
drivers/s390/crypto/ap_bus.h | 17 +
drivers/s390/crypto/vfio_ap_drv.c | 35 +-
drivers/s390/crypto/vfio_ap_ops.c | 1103 +++++++++++++++++++------
drivers/s390/crypto/vfio_ap_private.h | 24 +-
5 files changed, 1167 insertions(+), 313 deletions(-)

--
2.21.1