Re: [Qemu-devel] [PULL 19/22] virtio-balloon: VIRTIO_BALLOON_F_FREE_PAGE_HINT

[Peter Maydell]

On Wed, 6 Mar 2019 at 11:55, Dr. David Alan Gilbert (git)
<address@hidden> wrote:
>
> From: Wei Wang <address@hidden>
>
> The new feature enables the virtio-balloon device to receive hints of
> guest free pages from the free page vq.
>
> A notifier is registered to the migration precopy notifier chain. The
> notifier calls free_page_start after the migration thread syncs the dirty
> bitmap, so that the free page optimization starts to clear bits of free
> pages from the bitmap. It calls the free_page_stop before the migration
> thread syncs the bitmap, which is the end of the current round of ram
> save. The free_page_stop is also called to stop the optimization in the
> case when there is an error occurred in the process of ram saving.
>
> Note: balloon will report pages which were free at the time of this call.
> As the reporting happens asynchronously, dirty bit logging must be
> enabled before this free_page_start call is made. Guest reporting must be
> disabled before the migration dirty bitmap is synchronized.
>
> Signed-off-by: Wei Wang <address@hidden>
> CC: Michael S. Tsirkin <address@hidden>
> CC: Dr. David Alan Gilbert <address@hidden>
> CC: Juan Quintela <address@hidden>
> CC: Peter Xu <address@hidden>
> Message-Id: <address@hidden>
> Reviewed-by: Michael S. Tsirkin <address@hidden>
> Signed-off-by: Dr. David Alan Gilbert <address@hidden>
>   dgilbert: Dropped kernel header update, fixed up CMD_ID_* name change
> ---

Hi -- Coverity points out a use-after-free here (CID 1399412):

> +static bool get_free_page_hints(VirtIOBalloon *dev)
> +{
> +    VirtQueueElement *elem;
> +    VirtIODevice *vdev = VIRTIO_DEVICE(dev);
> +    VirtQueue *vq = dev->free_page_vq;
> +
> +    while (dev->block_iothread) {
> +        qemu_cond_wait(&dev->free_page_cond, &dev->free_page_lock);
> +    }
> +
> +    elem = virtqueue_pop(vq, sizeof(VirtQueueElement));
> +    if (!elem) {
> +        return false;
> +    }
> +
> +    if (elem->out_num) {
> +        uint32_t id;
> +        size_t size = iov_to_buf(elem->out_sg, elem->out_num, 0,
> +                                 &id, sizeof(id));
> +        virtqueue_push(vq, elem, size);
> +        g_free(elem);

Here we free elem...

> +
> +        virtio_tswap32s(vdev, &id);
> +        if (unlikely(size != sizeof(id))) {
> +            virtio_error(vdev, "received an incorrect cmd id");
> +            return false;
> +        }
> +        if (id == dev->free_page_report_cmd_id) {
> +            dev->free_page_report_status = FREE_PAGE_REPORT_S_START;
> +        } else {
> +            /*
> +             * Stop the optimization only when it has started. This
> +             * avoids a stale stop sign for the previous command.
> +             */
> +            if (dev->free_page_report_status == FREE_PAGE_REPORT_S_START) {
> +                dev->free_page_report_status = FREE_PAGE_REPORT_S_STOP;
> +            }
> +        }
> +    }
> +
> +    if (elem->in_num) {

...but we can fall through here and try to dereference elem...

> +        if (dev->free_page_report_status == FREE_PAGE_REPORT_S_START) {
> +            qemu_guest_free_page_hint(elem->in_sg[0].iov_base,
> +                                      elem->in_sg[0].iov_len);
> +        }
> +        virtqueue_push(vq, elem, 1);
> +        g_free(elem);

...and then free it again.

> +    }
> +
> +    return true;
> +}

thanks
-- PMM