Virtualization - Jul 2022 - [PATCH v11 21/40] virtio_ring: packed: introduce virtqueue_resize_packed()

? 2022/6/29 14:56, Xuan Zhuo ??:
> virtio ring packed supports resize.
>
> Only after the new vring is successfully allocated based on the new num,
> we will release the old vring. In any case, an error is returned,
> indicating that the vring still points to the old vring.
>
> In the case of an error, re-initialize(by virtqueue_reinit_packed()) the
> virtqueue to ensure that the vring can be used.
>
> Signed-off-by: Xuan Zhuo <xuanzhuo at linux.alibaba.com>
> ---
>   drivers/virtio/virtio_ring.c | 29 +++++++++++++++++++++++++++++
>   1 file changed, 29 insertions(+)
>
> diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
> index 650f701a5480..4860787286db 100644
> --- a/drivers/virtio/virtio_ring.c
> +++ b/drivers/virtio/virtio_ring.c
> @@ -2042,6 +2042,35 @@ static struct virtqueue
*vring_create_virtqueue_packed(
>   	return NULL;
>   }
>   
> +static int virtqueue_resize_packed(struct virtqueue *_vq, u32 num)
> +{
> +	struct vring_virtqueue_packed vring = {};
> +	struct vring_virtqueue *vq = to_vvq(_vq);
> +	struct virtio_device *vdev = _vq->vdev;
> +	int err;
> +
> +	if (vring_alloc_queue_packed(&vring, vdev, num))
> +		goto err_ring;
> +
> +	err = vring_alloc_state_extra_packed(&vring);
> +	if (err)
> +		goto err_state_extra;
> +
> +	vring_free(&vq->vq);
> +
> +	virtqueue_init(vq, vring.vring.num);
> +	virtqueue_vring_attach_packed(vq, &vring);
> +	virtqueue_vring_init_packed(vq);
> +
> +	return 0;
> +
> +err_state_extra:
> +	vring_free_packed(&vring, vdev);
> +err_ring:
> +	virtqueue_reinit_packed(vq);

So desc_state and desc_extra has been freed vring_free_packed() when 
vring_alloc_state_extra_packed() fails. We might get use-after-free here?

Actually, I think for resize we need

1) detach old
2) allocate new
3) if 2) succeed, attach new otherwise attach old

This seems more clearer than the current logic?

Thanks

> +	return -ENOMEM;
> +}
> +
>   
>   /*
>    * Generic functions and exported symbols.

On Fri, 1 Jul 2022 17:27:48 +0800, Jason Wang <jasowang at redhat.com>
wrote:
>
> ? 2022/6/29 14:56, Xuan Zhuo ??:
> > virtio ring packed supports resize.
> >
> > Only after the new vring is successfully allocated based on the new
num,
> > we will release the old vring. In any case, an error is returned,
> > indicating that the vring still points to the old vring.
> >
> > In the case of an error, re-initialize(by virtqueue_reinit_packed())
the
> > virtqueue to ensure that the vring can be used.
> >
> > Signed-off-by: Xuan Zhuo <xuanzhuo at linux.alibaba.com>
> > ---
> >   drivers/virtio/virtio_ring.c | 29 +++++++++++++++++++++++++++++
> >   1 file changed, 29 insertions(+)
> >
> > diff --git a/drivers/virtio/virtio_ring.c
b/drivers/virtio/virtio_ring.c
> > index 650f701a5480..4860787286db 100644
> > --- a/drivers/virtio/virtio_ring.c
> > +++ b/drivers/virtio/virtio_ring.c
> > @@ -2042,6 +2042,35 @@ static struct virtqueue
*vring_create_virtqueue_packed(
> >   	return NULL;
> >   }
> >
> > +static int virtqueue_resize_packed(struct virtqueue *_vq, u32 num)
> > +{
> > +	struct vring_virtqueue_packed vring = {};
> > +	struct vring_virtqueue *vq = to_vvq(_vq);
> > +	struct virtio_device *vdev = _vq->vdev;
> > +	int err;
> > +
> > +	if (vring_alloc_queue_packed(&vring, vdev, num))
> > +		goto err_ring;
> > +
> > +	err = vring_alloc_state_extra_packed(&vring);
> > +	if (err)
> > +		goto err_state_extra;
> > +
> > +	vring_free(&vq->vq);
> > +
> > +	virtqueue_init(vq, vring.vring.num);
> > +	virtqueue_vring_attach_packed(vq, &vring);
> > +	virtqueue_vring_init_packed(vq);
> > +
> > +	return 0;
> > +
> > +err_state_extra:
> > +	vring_free_packed(&vring, vdev);
> > +err_ring:
> > +	virtqueue_reinit_packed(vq);
>
>
> So desc_state and desc_extra has been freed vring_free_packed() when
> vring_alloc_state_extra_packed() fails. We might get use-after-free here?
vring_free_packed() frees the temporary structure vring. It does not affect
desc_state and desc_extra of vq. So it is safe.
>
> Actually, I think for resize we need
>
> 1) detach old
> 2) allocate new
> 3) if 2) succeed, attach new otherwise attach old

The implementation is now:

1. allocate new
2. free old (detach old)
3. attach new

error:
1. free temporary
2. reinit old

Do you think this is ok? We need to add a new variable to save the old vring in
the process you mentioned, there is not much difference in other.

Thanks.

>
> This seems more clearer than the current logic?
>
> Thanks
>
>
> > +	return -ENOMEM;
> > +}
> > +
> >
> >   /*
> >    * Generic functions and exported symbols.
>