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Message-ID: <14631333-995b-e6f8-71ea-f6a65855f3f3@linux.ibm.com>
Date: Wed, 22 Jun 2022 08:40:03 -0400
From: Tony Krowiak <akrowiak@...ux.ibm.com>
To: Christian Borntraeger <borntraeger@...ibm.com>,
linux-s390@...r.kernel.org, linux-kernel@...r.kernel.org,
kvm@...r.kernel.org
Cc: jjherne@...ux.ibm.com, freude@...ux.ibm.com, cohuck@...hat.com,
mjrosato@...ux.ibm.com, pasic@...ux.ibm.com,
alex.williamson@...hat.com, kwankhede@...dia.com,
fiuczy@...ux.ibm.com
Subject: Re: [PATCH v20 00/20] s390/vfio-ap: dynamic configuration support
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.
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