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Subject: Re: [PATCH] s390/vfio-ap: fix memory leak in mdev remove callback
To:     Tony Krowiak <akrowiak@linux.ibm.com>, linux-s390@vger.kernel.org,
        linux-kernel@vger.kernel.org
Cc:     cohuck@redhat.com, pasic@linux.vnet.ibm.com, jjherne@linux.ibm.com,
        jgg@nvidia.com, alex.williamson@redhat.com, kwankhede@nvidia.com,
        stable@vger.kernel.org, Tony Krowiak <akrowiak@stny.rr.com>
References: <20210505172826.105304-1-akrowiak@linux.ibm.com>
From:   Christian Borntraeger <borntraeger@de.ibm.com>
Message-ID: <7b20afce-5782-53c6-beab-ae852ae69b40@de.ibm.com>
Date:   Wed, 5 May 2021 19:44:55 +0200
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On 05.05.21 19:28, Tony Krowiak wrote:
> The mdev remove callback for the vfio_ap device driver bails out with
> -EBUSY if the mdev is in use by a KVM guest. The intended purpose was
> to prevent the mdev from being removed while in use; however, returning a
> non-zero rc does not prevent removal. This could result in a memory leak
> of the resources allocated when the mdev was created. In addition, the
> KVM guest will still have access to the AP devices assigned to the mdev
> even though the mdev no longer exists.
> 
> To prevent this scenario, cleanup will be done - including unplugging the
> AP adapters, domains and control domains - regardless of whether the mdev
> is in use by a KVM guest or not.
[...]
>   static int vfio_ap_mdev_create(struct mdev_device *mdev)
>   {
>   	struct ap_matrix_mdev *matrix_mdev;
> @@ -366,16 +392,9 @@ static int vfio_ap_mdev_remove(struct mdev_device *mdev)
>   	struct ap_matrix_mdev *matrix_mdev = mdev_get_drvdata(mdev);
>   
>   	mutex_lock(&matrix_dev->lock);
> -
> -	/*
> -	 * If the KVM pointer is in flux or the guest is running, disallow
> -	 * un-assignment of control domain.
> -	 */
> -	if (matrix_mdev->kvm_busy || matrix_mdev->kvm) {
> -		mutex_unlock(&matrix_dev->lock);
> -		return -EBUSY;
> -	}
> -
> +	WARN(vfio_ap_mdev_has_crycb(matrix_mdev),
> +	     "Removing mdev leaves KVM guest without any crypto devices");
> +	vfio_ap_mdev_clear_apcb(matrix_mdev);

Triggering a kernel warning due to an administrative task is not good.
Can't you simply clear the crycb? Maybe do a printk, but not a WARN.

>   	vfio_ap_mdev_reset_queues(mdev);
>   	list_del(&matrix_mdev->node);
>   	kfree(matrix_mdev);
>