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

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

On 2/11/21 11:47 AM, Halil Pasic wrote:
> On Thu, 11 Feb 2021 09:21:26 -0500
> Tony Krowiak <akrowiak@linux.ibm.com> wrote:
>
>> 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);
> The problem with this one is that as soon as we drop
> the lock here, another thread can in theory execute
> the critical section below, which drops our reference
> to kvm via kvm_put_kvm(kvm). Thus when we enter
> kvm_arch_crypto_clear_mask(), even if we are guaranteed
> to have a non-null pointer, the pointee is not guaranteed
> to be around. So like Connie suggested, you better take
> another reference to kvm in the first critical section.

Sure.

>
> Regards,
> Halil
>
>>           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);
>> }