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

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Date:   Wed, 10 Feb 2021 17:05:48 -0500
From:   Tony Krowiak <akrowiak@...ux.ibm.com>
To:     Halil Pasic <pasic@...ux.ibm.com>,
        Cornelia Huck <cohuck@...hat.com>
Cc:     linux-s390@...r.kernel.org, linux-kernel@...r.kernel.org,
        kvm@...r.kernel.org, stable@...r.kernel.org,
        borntraeger@...ibm.com, kwankhede@...dia.com, pbonzini@...hat.com,
        alex.williamson@...hat.com, pasic@...ux.vnet.ibm.com
Subject: Re: [PATCH 1/1] s390/vfio-ap: fix circular lockdep when
 setting/clearing crypto masks



On 2/10/21 10:32 AM, Halil Pasic wrote:
> On Wed, 10 Feb 2021 16:24:29 +0100
> Halil Pasic <pasic@...ux.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.