Re: [Qemu-devel] [PATCH 1/2] KVM: page track: add a new notifier type: track_flush_slot

[Paolo Bonzini <pbonzini@redhat.com>]

On 19/10/2016 07:45, Xiao Guangrong wrote:
> 
> 
> On 10/19/2016 10:32 AM, Jike Song wrote:
> +EXPORT_SYMBOL_GPL(vfio_group_set_usrdata);
>>>> +
>>>> +void *vfio_group_get_usrdata(struct vfio_group *group)
>>>> +{
>>>> +    return group->usrdata;
>>>> +}
>>>> +EXPORT_SYMBOL_GPL(vfio_group_get_usrdata);
>>>> +
>>>> +void *vfio_group_get_usrdata_by_device(struct device *dev)
>>>> +{
>>>> +    struct vfio_group *vfio_group;
>>>> +
>>>> +    vfio_group = __vfio_group_get_from_iommu(dev->iommu_group);
>>>
>>> We actually need to use iommu_group_get() here.  Kirti adds a
>>> vfio_group_get_from_dev() in v9 03/12 that does this properly.
>>>
>>>> +    if (!vfio_group)
>>>> +        return NULL;
>>>> +
>>>> +    return vfio_group_get_usrdata(vfio_group);
> 
> I am worrying if the kvm instance got from group->usrdata is safe
> enough? What happens if you get the instance after kvm released
> kvm-vfio device?

It shouldn't happen if you use kvm_get_kvm and kvm_put_kvm properly.  It
is almost okay in the patch, just:

> @@ -200,6 +216,8 @@ static int kvm_vfio_set_group(struct kvm_device *dev, long attr, u64 arg)
>  
>  		kvm_vfio_update_coherency(dev);
>  
> +		kvm_put_kvm(dev->kvm);
> +
>  		return ret;
>  	}

... please add a new function kvm_vfio_group_clear_kvm(vfio_group) here,
that does vfio_group_set_usrdata(vfio_group, NULL) and kvm_put_kvm.
This should avoid use-after-free.

Paolo