Re: [PATCH v5 1/1] s390/vfio-ap: fix circular lockdep when setting/clearing crypto masks

[Tony Krowiak]

On 3/25/21 4:32 PM, Halil Pasic wrote:
> On Thu, 25 Mar 2021 08:46:40 -0400
> Tony Krowiak <akrowiak@xxxxxxxxxxxxx> wrote:
>
> > This patch fixes a lockdep splat introduced by commit f21916ec4826
> > ("s390/vfio-ap: clean up vfio_ap resources when KVM pointer invalidated").
> > The lockdep splat only occurs when starting a Secure Execution guest.
> > Crypto virtualization (vfio_ap) is not yet supported for SE guests;
> > however, in order to avoid this problem when support becomes available,
> > this fix is being provided.
> >
> > The circular locking dependency was introduced when the setting of the
> > masks in the guest's APCB was executed while holding the matrix_dev->lock.
> > While the lock is definitely needed to protect the setting/unsetting of the
> > matrix_mdev->kvm pointer, it is not necessarily critical for setting the
> > masks; so, the matrix_dev->lock will be released while the masks are being
> > set or cleared.
> >
> > Keep in mind, however, that another process that takes the matrix_dev->lock
> > can get control while the masks in the guest's APCB are being set or
> > cleared as a result of the driver being notified that the KVM pointer
> > has been set or unset. This could result in invalid access to the
> > matrix_mdev->kvm pointer by the intervening process. To avoid this
> > scenario, two new fields are being added to the ap_matrix_mdev struct:
> >
> > struct ap_matrix_mdev {
> > 	...
> > 	bool kvm_busy;
> > 	wait_queue_head_t wait_for_kvm;
> >     ...
> > };
> >
> > The functions that handle notification that the KVM pointer value has
> > been set or cleared will set the kvm_busy flag to true until they are done
> > processing at which time they will set it to false and wake up the tasks on
> > the matrix_mdev->wait_for_kvm wait queue. Functions that require
> > access to matrix_mdev->kvm will sleep on the wait queue until they are
> > awakened at which time they can safely access the matrix_mdev->kvm
> > field.
> >
> > Fixes: f21916ec4826 ("s390/vfio-ap: clean up vfio_ap resources when KVM pointer invalidated")
> > Cc: stable@xxxxxxxxxxxxxxx
> > Signed-off-by: Tony Krowiak <akrowiak@xxxxxxxxxxxxx>
> Reviewed-by: Halil Pasic <pasic@xxxxxxxxxxxxx>
>
> I intend to give a couple of work-days to others, and if nobody objects
> merge this. (I will wait till Tuesday.)

Thanks Halil.

> I've tested it and it does silence the lockdep splat.
>
> Regards,
> Halil