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

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

On 2/10/21 5:53 AM, Cornelia Huck wrote:
> 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?

That is highly unlikely because the only place the matrix_mdev->kvm
pointer is cleared is in this function which is called from only two
places: the notifier that handles the VFIO_GROUP_NOTIFY_SET_KVM
notification when the KVM pointer is cleared; the vfio_ap_mdev_release()
function which is called when the mdev fd is closed (i.e., when the guest
is shut down). The fact is, with the only end-to-end implementation
currently available, the notifier callback is never invoked to clear
the KVM pointer because the vfio_ap_mdev_release callback is
invoked first and it unregisters the notifier callback.

Having said that, I suppose there is no guarantee that there will not
be different userspace clients in the future that do things in a
different order. At the very least, it wouldn't hurt to protect against
that as you suggest below.

>
> 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);
>> +	}
>>   }