On Wed, 6 Mar 2019 at 11:55, Dr. David Alan Gilbert (git)
<[email protected]> wrote:
>
> From: Wei Wang <[email protected]>
>
> 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 <[email protected]>
> CC: Michael S. Tsirkin <[email protected]>
> CC: Dr. David Alan Gilbert <[email protected]>
> CC: Juan Quintela <[email protected]>
> CC: Peter Xu <[email protected]>
> Message-Id: <[email protected]>
> Reviewed-by: Michael S. Tsirkin <[email protected]>
> Signed-off-by: Dr. David Alan Gilbert <[email protected]>
> 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
