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

Tony Krowiak
2021-02-11T14:24:52Z

On 2/10/21 5:46 PM, Halil Pasic wrote:
> On Wed, 10 Feb 2021 17:05:48 -0500
> Tony Krowiak <akrowiak@linux.ibm.com> wrote:
>
>> On 2/10/21 10:32 AM, Halil Pasic wrote:
>>> On Wed, 10 Feb 2021 16:24:29 +0100
>>> Halil Pasic <pasic@linux.ibm.com> wrote:
>>>
>>>>> 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?
>>>> I agree, matrix_mdev->kvm can go NULL any time and we are risking
>>>> a null pointer dereference here.
>>>>
>>>> Another idea would be to do
>>>>
>>>>
>>>> static void vfio_ap_mdev_unset_kvm(struct ap_matrix_mdev *matrix_mdev)
>>>> {
>>>> struct kvm *kvm;
>>>>
>>>> mutex_lock(&matrix_dev->lock);
>>>> if (matrix_mdev->kvm) {
>>>> kvm = matrix_mdev->kvm;
>>>> matrix_mdev->kvm = NULL;
>>>> mutex_unlock(&matrix_dev->lock);
>>>> kvm_arch_crypto_clear_masks(kvm);
>>>> mutex_lock(&matrix_dev->lock);
>>>> matrix_mdev->kvm->arch.crypto.pqap_hook = NULL;
>>> s/matrix_mdev->kvm/kvm
>>>> vfio_ap_mdev_reset_queues(matrix_mdev->mdev);
>>>> kvm_put_kvm(kvm);
>>>> }
>>>> mutex_unlock(&matrix_dev->lock);
>>>> }
>>>>
>>>> That way only one unset would actually do the unset and cleanup
>>>> and every other invocation would bail out with only checking
>>>> matrix_mdev->kvm.
>>> But the problem with that is that we enable the the assign/unassign
>>> prematurely, which could interfere wit reset_queues(). Forget about
>>> it.
>> Not sure what you mean by this.
>>
>>
> I mean because above I first do
> (1) matrix_mdev->kvm = NULL;
> and then do
> (2) vfio_ap_mdev_reset_queues(matrix_mdev->mdev);
> another thread could do
> static ssize_t unassign_adapter_store(struct device *dev,
> struct device_attribute *attr,
> const char *buf, size_t count)
> {
> int ret;
> unsigned long apid;
> struct mdev_device *mdev = mdev_from_dev(dev);
> struct ap_matrix_mdev *matrix_mdev = mdev_get_drvdata(mdev);
>
> /* If the guest is running, disallow un-assignment of adapter */
> if (matrix_mdev->kvm)
> return -EBUSY;
> ...
> }
> between (1) and (2), and we would not bail out with -EBUSY because !!kvm
> because of (1). That means we would change matrix_mdev->matrix and we
> would not reset the queues that correspond to the apid that was just
> removed, because by the time we do the reset_queues, the queues are
> not in the matrix_mdev->matrix any more.
>
> Does that make sense?

Yes, it makes sense. I guess I didn't look closely at your
suggestion when I said it was exactly what I implemented
after agreeing with Connie. I had a slight difference in
my implementation:

static void vfio_ap_mdev_unset_kvm(struct ap_matrix_mdev *matrix_mdev)
{
struct kvm *kvm;

mutex_lock(&matrix_dev->lock);

if (matrix_mdev->kvm) {
kvm = matrix_mdev->kvm;
mutex_unlock(&matrix_dev->lock);
kvm_arch_crypto_clear_masks(kvm);
mutex_lock(&matrix_dev->lock);
kvm->arch.crypto.pqap_hook = NULL;
vfio_ap_mdev_reset_queues(matrix_mdev->mdev);
matrix_mdev->kvm = NULL;
kvm_put_kvm(kvm);
}

mutex_unlock(&matrix_dev->lock);
}

In your scenario, the unassignment would fail with -EBUSY because
the matrix_mdev->kvm pointer would not have yet been
cleared. The other problem with your implementation is that
IRQ resources would not get cleared after the reset because
the matrix_mdev->kvm pointer would be NULL at that time.