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

[Tony Krowiak <akrowiak@linux.ibm.com>]

On 5/6/21 6:45 AM, Cornelia Huck wrote:
> On Thu, 6 May 2021 12:22:45 +0200
> Cornelia Huck <cohuck@redhat.com> wrote:
>
>> 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?)
> 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?

If you stare at it too long, you'll go blind:) As I stated in my response
to your previous review comment, I'm going to remove the function
above and call the vfio_ap_mdev_unset_kvm() function from the
remove callback.

>
>>> +
>>>   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);