linux-kernel - Re: [PATCH v10 11/26] s390: vfio-ap: implement mediated device open callback

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Message-Id: <bc642c76-09e2-586e-b512-55f1679beb0c@linux.ibm.com>
Date:   Tue, 18 Sep 2018 17:57:31 -0400
From:   Tony Krowiak <akrowiak@...ux.ibm.com>
To:     Halil Pasic <pasic@...ux.ibm.com>,
        Tony Krowiak <akrowiak@...ux.vnet.ibm.com>,
        linux-s390@...r.kernel.org, linux-kernel@...r.kernel.org,
        kvm@...r.kernel.org
Cc:     freude@...ibm.com, schwidefsky@...ibm.com,
        heiko.carstens@...ibm.com, borntraeger@...ibm.com,
        cohuck@...hat.com, kwankhede@...dia.com,
        bjsdjshi@...ux.vnet.ibm.com, pbonzini@...hat.com,
        alex.williamson@...hat.com, pmorel@...ux.vnet.ibm.com,
        alifm@...ux.vnet.ibm.com, mjrosato@...ux.vnet.ibm.com,
        jjherne@...ux.vnet.ibm.com, thuth@...hat.com,
        pasic@...ux.vnet.ibm.com, berrange@...hat.com,
        fiuczy@...ux.vnet.ibm.com, buendgen@...ibm.com,
        frankja@...ux.ibm.com
Subject: Re: [PATCH v10 11/26] s390: vfio-ap: implement mediated device open
 callback

On 09/18/2018 01:00 PM, Halil Pasic wrote:
>
> On 09/12/2018 09:43 PM, Tony Krowiak wrote:
>> +/**
>> + * vfio_ap_mdev_open_once
>> + *
>> + * @matrix_mdev: a mediated matrix device
>> + *
>> + * Return 0 if no other mediated matrix device has been opened for the
>> + * KVM guest assigned to @matrix_mdev; otherwise, returns an error.
>> + */
>> +static int vfio_ap_mdev_open_once(struct ap_matrix_mdev *matrix_mdev,
>> +				  struct kvm *kvm)
>> +{
>> +	struct ap_matrix_mdev *m;
>> +
>> +	mutex_lock(&matrix_dev->lock);
>> +
>> +	list_for_each_entry(m, &matrix_dev->mdev_list, node) {
>> +		if ((m != matrix_mdev) && (m->kvm == kvm)) {
>> +			mutex_unlock(&matrix_dev->lock);
>> +			return -EPERM;
>> +		}
>> +	}
>> +
>> +	mutex_unlock(&matrix_dev->lock);
>> +
>> +	return 0;
>> +}
>> +
>> +static int vfio_ap_mdev_group_notifier(struct notifier_block *nb,
>> +				       unsigned long action, void *data)
>> +{
>> +	int ret;
>> +	struct ap_matrix_mdev *matrix_mdev;
>> +
>> +	if (action != VFIO_GROUP_NOTIFY_SET_KVM)
>> +		return NOTIFY_OK;
>> +
>> +	matrix_mdev = container_of(nb, struct ap_matrix_mdev, group_notifier);
>> +
>> +	if (!data) {
>> +		matrix_mdev->kvm = NULL;
>> +		return NOTIFY_OK;
>> +	}
>> +
>> +	ret = vfio_ap_mdev_open_once(matrix_mdev, data);
> This could be racy. Two threads doing vfio_ap_mdev_group_notifier()
> can first get 0 here in a sense that there is no such kvm in the list,
> and then both set the very same kvm three lines below. Which would
> result in what we are trying to prevent.
>
> Also vfio_ap_mdev_open_once() does not seem like an appropriate name
> any more. If we were to do the matrix_mdev->kvm = kvm in there we could
> call it something like vfio_ap_mdev_set_kvm().

I'm moving the matrix-mdev->kvm = kvm inside the mutex lock in
vfio_ap_mdev_open_once() ... also renaming it to vfio_ap_mdev_set_kvm().

>
>> +	if (ret)
>> +		return NOTIFY_DONE;
>> +
>> +	matrix_mdev->kvm = data;
>> +
>> +	ret = kvm_ap_validate_crypto_setup(matrix_mdev->kvm);
>> +	if (ret)
>> +		return ret;
>> +
>> +	vfio_ap_mdev_copy_masks(matrix_mdev);
>> +
>> +	return NOTIFY_OK;
>> +}