Re: [RFC] virtio_balloon: fix Use-After-Free in page reporting during PM freeze

[David Rientjes <rientjes@google.com>]

On Thu, 9 Jul 2026, Link Lin wrote:

> > > Fix this by:
> > > 1. Unregistering page reporting in virtballoon_freeze() prior to calling
> > >    remove_common(). This clears the RCU pr_dev_info pointer and flushes/
> > >    cancels prdev->work on system_wq via cancel_delayed_work_sync().
> > > 2. Re-registering page reporting in virtballoon_restore() after the
> > >    virtqueues are re-initialized and virtio_device_ready() has been called.
> > > 3. Unwinding virtqueue initialization via remove_common() in
> > >    virtballoon_restore() if page_reporting_register() fails.
> >
> > AI review thinks the patch didn't do the above:
> >         https://sashiko.dev/#/patchset/20260709224330.946683-1-linkl@google.com
> 
> The AI reviewer might not have parsed the entirety of the fix I proposed.
> The patch submitted definitely includes the changes to virtballoon_restore()
> for steps 2 and 3 (re-registering page reporting and unwinding init_vqs
> on failure). It seems the AI failed to parse the diff correctly. See:
> 
> +       if (virtio_has_feature(vb->vdev, VIRTIO_BALLOON_F_REPORTING)) {
> +               ret = page_reporting_register(&vb->pr_dev_info);
> +               if (ret)
> +                       goto out_remove_vqs;
> +       }
> +
>         if (towards_target(vb))
>                 virtballoon_changed(vdev);
>         update_balloon_size(vb);
>         return 0;
> +
> +out_remove_vqs:
> +       remove_common(vb);
> +       return ret;
>  }
> 
> > It also might have found a couple of pre-existing bugs in there.
> 
> Indeed. Regarding the first pre-existing bug found by the AI (leaving the
> OOM notifier registered during suspend, leading to a UAF if memory pressure
> spikes during S4 hibernation):
> 
> I actually addressed this in the commit message:
> 
>   "(Note: The OOM Notifier and Shrinker/Free Page Hinting features suffer
>   from an identical lifecycle flaw and are also vulnerable to UAFs during
>   S4 hibernation when memory pressure spikes. This patch focuses on Free
>   Page Reporting, which runs periodically, to ensure clean backports to
>   stable kernels)."
> 
> Regarding the second pre-existing bug the AI flagged (leaving uncancelled
> works on system_freezable_wq if virtballoon_restore fails on the cold path):
> the AI is correct that this asynchronous work cancellation failure exists.
> 
> Since these are separate, pre-existing lifecycle bugs, would you prefer I
> roll fixes for the OOM notifier, shrinker/free page hinting, and work
> cancellations into a v2 of this patch, or submit them as a separate patch
> series to keep the stable backports clean?
> 

I think it would be best to have separate patches for each fix; Andrew, 
please correct me if you'd prefer one patch to address everything.