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 7:23 AM, Cornelia Huck wrote:
> On Wed, 10 Feb 2021 15:34:24 -0500
> Tony Krowiak <akrowiak@linux.ibm.com> wrote:
>
>> 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.
> Yes, if userspace is able to use the interfaces in the certain way, we
> should always make sure that nothing bad happens if it does so, even if
> known userspace applications are well-behaved.
>
> [Can we make an 'evil userspace' test program, maybe? The hardware
> dependency makes this hard to run, though.]

Of course it is possible to create such a test program, but off the
top of my head, I can't come up with an algorithm that would
result in the scenario you have laid out. I haven't dabbled in the QEMU
space in quite some time; so, there would also be a bit of a re-learning
curve. I'm not sure it would be worth the effort to take this on given
how unlikely it is this scenario can happen, but I will take it into
consideration as it is a good idea.

>
>>> 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);
>>>> +	}
>>>>    }