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Date:   Tue, 3 May 2022 13:39:21 -0400
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Subject: Re: [PATCH v19 00/20] s390/vfio-ap: dynamic configuration support
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From:   Tony Krowiak <akrowiak@linux.ibm.com>
To:     linux-s390@vger.kernel.org, linux-kernel@vger.kernel.org,
        kvm@vger.kernel.org
Cc:     jjherne@linux.ibm.com, freude@linux.ibm.com,
        borntraeger@de.ibm.com, cohuck@redhat.com, mjrosato@linux.ibm.com,
        pasic@linux.ibm.com, alex.williamson@redhat.com,
        kwankhede@nvidia.com, fiuczy@linux.ibm.com
References: <20220404221039.1272245-1-akrowiak@linux.ibm.com>
In-Reply-To: <20220404221039.1272245-1-akrowiak@linux.ibm.com>
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Hello, anybody home?

On 4/4/22 6:10 PM, Tony Krowiak wrote:
> 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 v18-v19:
> ------------------
> * Changed name of vfio_ap_mdev_hotplug_apcb (vfio_ap_ops.c) to
>    vfio_ap_mdev_update_guest_apcb
>    (Suggested by Jason: review of patch 10/18)
>
> * Replace call to kvm_arch_crypto_set_masks in vfio_ap_mdev_set_kvm with
>    call to vfio_ap_mdev_update_guest_apcb
>    (Suggested by Jason: review of patch 10/18)
>
> * Moved changes related to new locking scheme into its own set of
>    patches (Suggested by Jason: review of patch 10/18)
>
> * Consolidated some of the lock acquisition code into macros called by the
>    functions that update a KVM guest's APCB.
>
> * Refactored vfio_ap_mdev_unlink_adapter() and
>    vfio_ap_unlink_apqn_fr_mdev() functions according to Jason's sample
>    code. (Suggested by Jason: review of patch 12/18)
>
> * Require callers of the AP bus ap_apqn_in_matrix_owned_by_def_drv and
>    ap_owned_by_def_drv - only called by the vfio_ap driver - to take the
>    ap_perms_mutex lock. The adapter/domain assignment interfaces will take
>    the ap_perms_mutex lock prior to other required locks to maintain a
>    proper locking order and avoid circular locking dependencies when the
>    vfio_ap device driver's in_use callback is invoked simultaneously with
>    the adapter/domain assignment interfaces. (Suggested by Jason)
>
> * Refactored patch 15/18: handle config changed and scan complete
>    notification (Suggested by Jason)
>
> * Refactored filtering of the matrix to reduce redundant processing of
>    APQNs:
>    - Inspect only the new APIDs or APQIs assigned to the matrix mdev or
>      added to the host's AP configuration
>    - Automatically removing APIDs or APQIs unassigned from the matrix mdev
>      or removed from the host's AP configuration.
>    (Suggested by Halil)
>
> 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 |  389 +++++++
>   Documentation/s390/vfio-ap.rst         |  492 ++++++---
>   MAINTAINERS                            |    6 +-
>   drivers/s390/crypto/ap_bus.c           |   31 +-
>   drivers/s390/crypto/vfio_ap_drv.c      |   69 +-
>   drivers/s390/crypto/vfio_ap_ops.c      | 1321 ++++++++++++++++++------
>   drivers/s390/crypto/vfio_ap_private.h  |   47 +-
>   7 files changed, 1820 insertions(+), 535 deletions(-)
>   create mode 100644 Documentation/s390/vfio-ap-locking.rst
>