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

[Cornelia Huck <cohuck@redhat.com>]

On Tue,  9 Feb 2021 14:48:30 -0500
Tony Krowiak <akrowiak@linux.ibm.com> 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@vger.kernel.org
> Signed-off-by: Tony Krowiak <akrowiak@linux.ibm.com>
> ---
>  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?

> +		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);
> +	}
>  }