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

[Tony Krowiak]

On 2/10/21 10:24 AM, Halil Pasic wrote:
> On Wed, 10 Feb 2021 11:53:34 +0100
> Cornelia Huck <cohuck@xxxxxxxxxx> wrote:
>
> > On Tue,  9 Feb 2021 14:48:30 -0500
> > Tony Krowiak <akrowiak@xxxxxxxxxxxxx> wrote:
> >
> > > This patch fixes a circular locking dependency in the CI introduced by
> > > commit f21916ec4826 ("s390/vfio-ap: clean up vfio_ap resources when KVM
> > > pointer invalidated"). The lockdep only occurs when starting a Secure
> > > Execution guest. Crypto virtualization (vfio_ap) is not yet supported for
> > > SE guests; however, in order to avoid CI errors, this fix is being
> > > provided.
> > >
> > > The circular lockdep was introduced when the masks in the guest's APCB
> > > were taken under the matrix_dev->lock. While the lock is definitely
> > > needed to protect the setting/unsetting of the KVM pointer, it is not
> > > necessarily critical for setting the masks, so this will not be done under
> > > protection of the matrix_dev->lock.
> > >
> > > Fixes: f21916ec4826 ("s390/vfio-ap: clean up vfio_ap resources when KVM pointer invalidated")
> > > Cc: stable@xxxxxxxxxxxxxxx
> > > Signed-off-by: Tony Krowiak <akrowiak@xxxxxxxxxxxxx>
> > > ---
> > >   drivers/s390/crypto/vfio_ap_ops.c | 75 ++++++++++++++++++-------------
> > >   1 file changed, 45 insertions(+), 30 deletions(-)
> > >    
> > >   static void vfio_ap_mdev_unset_kvm(struct ap_matrix_mdev *matrix_mdev)
> > >   {
> > > -	kvm_arch_crypto_clear_masks(matrix_mdev->kvm);
> > > -	matrix_mdev->kvm->arch.crypto.pqap_hook = NULL;
> > > -	vfio_ap_mdev_reset_queues(matrix_mdev->mdev);
> > > -	kvm_put_kvm(matrix_mdev->kvm);
> > > -	matrix_mdev->kvm = NULL;
> > > +	if (matrix_mdev->kvm) {
> > If you're doing setting/unsetting under matrix_dev->lock, is it
> > possible that matrix_mdev->kvm gets unset between here and the next
> > line, as you don't hold the lock?
> >
> > Maybe you could
> > - grab a reference to kvm while holding the lock
> > - call the mask handling functions with that kvm reference
> > - lock again, drop the reference, and do the rest of the processing?
> I agree, matrix_mdev->kvm can go NULL any time and we are risking
> a null pointer dereference here.
>
> Another idea would be to do
>
>
> static void vfio_ap_mdev_unset_kvm(struct ap_matrix_mdev *matrix_mdev)
> {
>          struct kvm *kvm;
>                                                          
>          mutex_lock(&matrix_dev->lock);
>          if (matrix_mdev->kvm) {
>                  kvm = matrix_mdev->kvm;
>                  matrix_mdev->kvm = NULL;
>                  mutex_unlock(&matrix_dev->lock);
>                  kvm_arch_crypto_clear_masks(kvm);
>                  mutex_lock(&matrix_dev->lock);
>                  matrix_mdev->kvm->arch.crypto.pqap_hook = NULL;
>                  vfio_ap_mdev_reset_queues(matrix_mdev->mdev);
>                  kvm_put_kvm(kvm);
>          }
>          mutex_unlock(&matrix_dev->lock);
> }
>
> That way only one unset would actually do the unset and cleanup
> and every other invocation would bail out with only checking
> matrix_mdev->kvm.

How ironic, that is exactly what I did after agreeing with Connie.

>   
> > > +		kvm_arch_crypto_clear_masks(matrix_mdev->kvm);
> > > +		mutex_lock(&matrix_dev->lock);
> > > +		matrix_mdev->kvm->arch.crypto.pqap_hook = NULL;
> > > +		vfio_ap_mdev_reset_queues(matrix_mdev->mdev);
> > > +		kvm_put_kvm(matrix_mdev->kvm);
> > > +		matrix_mdev->kvm = NULL;
> > > +		mutex_unlock(&matrix_dev->lock);
> > > +	}
> > >   }