Re: [PATCH] s390/vfio-ap: fix memory leak in mdev remove callback

[Cornelia Huck <cohuck@redhat.com>]

On Wed,  5 May 2021 13:28:26 -0400
Tony Krowiak <akrowiak@linux.ibm.com> wrote:

> The mdev remove callback for the vfio_ap device driver bails out with
> -EBUSY if the mdev is in use by a KVM guest. The intended purpose was
> to prevent the mdev from being removed while in use; however, returning a
> non-zero rc does not prevent removal. This could result in a memory leak
> of the resources allocated when the mdev was created. In addition, the
> KVM guest will still have access to the AP devices assigned to the mdev
> even though the mdev no longer exists.
> 
> To prevent this scenario, cleanup will be done - including unplugging the
> AP adapters, domains and control domains - regardless of whether the mdev
> is in use by a KVM guest or not.
> 
> Fixes: 258287c994de ("s390: vfio-ap: implement mediated device open callback")
> Cc: stable@vger.kernel.org
> Signed-off-by: Tony Krowiak <akrowiak@stny.rr.com>
> Signed-off-by: Tony Krowiak <akrowiak@linux.ibm.com>
> ---
>  drivers/s390/crypto/vfio_ap_ops.c | 39 +++++++++++++++++++++++--------
>  1 file changed, 29 insertions(+), 10 deletions(-)
> 
> diff --git a/drivers/s390/crypto/vfio_ap_ops.c b/drivers/s390/crypto/vfio_ap_ops.c
> index b2c7e10dfdcd..757166da947e 100644
> --- a/drivers/s390/crypto/vfio_ap_ops.c
> +++ b/drivers/s390/crypto/vfio_ap_ops.c
> @@ -335,6 +335,32 @@ static void vfio_ap_matrix_init(struct ap_config_info *info,
>  	matrix->adm_max = info->apxa ? info->Nd : 15;
>  }
>  
> +static bool vfio_ap_mdev_has_crycb(struct ap_matrix_mdev *matrix_mdev)
> +{
> +	return (matrix_mdev->kvm && matrix_mdev->kvm->arch.crypto.crycbd);
> +}
> +
> +static void vfio_ap_mdev_clear_apcb(struct ap_matrix_mdev *matrix_mdev)
> +{
> +	/*
> +	 * If the KVM pointer is in the process of being set, wait until the
> +	 * process has completed.
> +	 */
> +	wait_event_cmd(matrix_mdev->wait_for_kvm,
> +		       !matrix_mdev->kvm_busy,
> +		       mutex_unlock(&matrix_dev->lock),
> +		       mutex_lock(&matrix_dev->lock));
> +
> +	if (vfio_ap_mdev_has_crycb(matrix_mdev)) {
> +		matrix_mdev->kvm_busy = true;
> +		mutex_unlock(&matrix_dev->lock);
> +		kvm_arch_crypto_clear_masks(matrix_mdev->kvm);
> +		mutex_lock(&matrix_dev->lock);
> +		matrix_mdev->kvm_busy = false;
> +		wake_up_all(&matrix_mdev->wait_for_kvm);
> +	}
> +}

Looking at vfio_ap_mdev_unset_kvm(), do you need to unhook the kvm here
as well?

(Or can you maybe even combine the two functions into one?)

> +
>  static int vfio_ap_mdev_create(struct mdev_device *mdev)
>  {
>  	struct ap_matrix_mdev *matrix_mdev;
> @@ -366,16 +392,9 @@ static int vfio_ap_mdev_remove(struct mdev_device *mdev)
>  	struct ap_matrix_mdev *matrix_mdev = mdev_get_drvdata(mdev);
>  
>  	mutex_lock(&matrix_dev->lock);
> -
> -	/*
> -	 * If the KVM pointer is in flux or the guest is running, disallow
> -	 * un-assignment of control domain.
> -	 */
> -	if (matrix_mdev->kvm_busy || matrix_mdev->kvm) {
> -		mutex_unlock(&matrix_dev->lock);
> -		return -EBUSY;
> -	}
> -
> +	WARN(vfio_ap_mdev_has_crycb(matrix_mdev),
> +	     "Removing mdev leaves KVM guest without any crypto devices");
> +	vfio_ap_mdev_clear_apcb(matrix_mdev);
>  	vfio_ap_mdev_reset_queues(mdev);
>  	list_del(&matrix_mdev->node);
>  	kfree(matrix_mdev);