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

[Cornelia Huck]

On Thu, 6 May 2021 12:22:45 +0200
Cornelia Huck <cohuck@xxxxxxxxxx> wrote:

> On Wed,  5 May 2021 13:28:26 -0400
> Tony Krowiak <akrowiak@xxxxxxxxxxxxx> 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@xxxxxxxxxxxxxxx
> > Signed-off-by: Tony Krowiak <akrowiak@xxxxxxxxxxx>
> > Signed-off-by: Tony Krowiak <akrowiak@xxxxxxxxxxxxx>
> > ---
> >  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?)

Staring at the code some more, the rules where you unset the kvm stuff
seem pretty confusing (at least to me). Does this partial unhooking in
the remove callback make sense?

> 
> > +
> >  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);  
>