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

[Pierre Morel]

On 10/8/19 12:48 PM, 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?
>
> > 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.
>
> > 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.
>
> > 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.

Just curious: how is it possible to do such a configuration?


> >   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.


Stealing an adapter in use by a guest, insn't it what is done if we 
allow to unassign an AP/Domain using the unassign sysfs interface when 
the mediated device is in use by the guest?


> > 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
>
> > * 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).
>
> 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(-)
--
Pierre Morel
IBM Lab Boeblingen