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

["Michael S. Tsirkin" <mst@redhat.com>]

On Tue, Jul 14, 2026 at 03:17:42PM +0200, David Hildenbrand (Arm) wrote:
> On 7/10/26 00:43, Link Lin wrote:
> > During system power management freeze (e.g. ACPI S3 suspend or S4
> > hibernation), virtballoon_freeze() calls remove_common() to reset the
> > virtio device and delete all virtqueues via vdev->config->del_vqs().
> > However, unlike virtballoon_remove(), virtballoon_freeze() fails to call
> > page_reporting_unregister(&vb->pr_dev_info).
> > 
> > The comment in virtballoon_freeze() states:
> >     /*
> >      * The workqueue is already frozen by the PM core before this
> >      * function is called.
> >      */
> > 
> > While this comment was accurate in 2011 for balloon-internal workqueues
> > (such as balloon_wq, which was created with WQ_FREEZABLE and is paused
> > by the PM freezer), it is invalid for Free Page Reporting.
> > 
> > Free Page Reporting (mm/page_reporting.c) schedules its delayed work
> > (prdev->work) on the global system_wq. Because system_wq lacks the
> > WQ_FREEZABLE flag, the PM freezer (freeze_workqueues_busy()) explicitly
> > skips it. Consequently, page_reporting_process() on system_wq remains
> > active and unfrozen throughout device suspend.
> > 
> > If memory is freed into the buddy allocator or a delayed work timer
> > expires while the device is being frozen, page_reporting_process() fires
> > on system_wq and calls virtballoon_free_page_report(). This function
> > passes vb->reporting_vq into virtqueue_add_inbuf() / virtqueue_add_split().
> > Because the virtqueues were already destroyed by del_vqs(), this results
> > in a Use-After-Free / General Protection Fault:
> > 
> >     [  250.709271] general protection fault, probably for non-canonical address 0x7f728084daf08d5e: 0000 [#1] SMP PTI
> >     [  250.732967] CPU: 2 PID: 38 Comm: kworker/2:1 Not tainted 5.10.0-44-cloud-amd64 #1 Debian 5.10.257-1
> >     [  250.751575] Workqueue: events page_reporting_process
> >     [  250.756665] RIP: 0010:virtqueue_add_split+0x233/0x4c0 [virtio_ring]
> >     ...
> >     [  250.867678] virtballoon_free_page_report+0x3a/0xe0 [virtio_balloon]
> >     [  250.883446] page_reporting_process+0x225/0x4f0
> > 
> > (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).
> > 
> > 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.
> > 
> > Fixes: 924a663f75e2 ("virtio-balloon: Reporting free page reservations")
> > Cc: stable@vger.kernel.org
> > Cc: jasowang@redhat.com
> > Cc: xuanzhuo@linux.alibaba.com
> > Cc: Ammar Faizi <ammarfaizi2@openresty.com>
> > Cc: jiaqiyan@google.com
> > Cc: ahwilkins@google.com
> > Cc: Greg Thelen <gthelen@google.com>
> > Cc: Alexander Duyck <alexander.duyck@gmail.com>
> > Signed-off-by: Link Lin <linkl@google.com>
> > ---
> >  drivers/virtio/virtio_balloon.c | 11 +++++++++++
> >  1 file changed, 11 insertions(+)
> > 
> > diff --git a/drivers/virtio/virtio_balloon.c b/drivers/virtio/virtio_balloon.c
> > index a1b2c3d4e5f6..45a90fb3abf8 100640
> > --- a/drivers/virtio/virtio_balloon.c
> > +++ b/drivers/virtio/virtio_balloon.c
> > @@ -1055,6 +1055,9 @@ static int virtballoon_freeze(struct virtio_device *vdev)
> >  	 * The workqueue is already frozen by the PM core before this
> >  	 * function is called.
> >  	 */
> > +	if (virtio_has_feature(vb->vdev, VIRTIO_BALLOON_F_REPORTING))
> > +		page_reporting_unregister(&vb->pr_dev_info);
> > +
> >  	remove_common(vb);
> >  	return 0;
> >  }
> >  
> >  static int virtballoon_restore(struct virtio_device *vdev)
> >  {
> >  	struct virtio_balloon *vb = vdev->priv;
> >  	int ret;
> >  
> >  	ret = init_vqs(vdev->priv);
> >  	if (ret)
> >  		return ret;
> >  
> >  	virtio_device_ready(vdev);
> >  
> > +	if (virtio_has_feature(vb->vdev, VIRTIO_BALLOON_F_REPORTING)) {
> > +		ret = page_reporting_register(&vb->pr_dev_info);
> > +		if (ret)
> > +			goto out_remove_vqs;
> > +	}
> 
> Hm, that failure handling is rather nasty.
> 
> 
> In virtballoon_freeze() we document:
> 
> "The workqueue is already frozen by the PM core before this function is called"
> 
> Your report states:
> 
> "Workqueue: events page_reporting_process"
> 
> 
> I assume that workqueue is not frozen yet because ... it's not freezable :)
> 
> So could we queue to system_freezable_wq instead, or define our own freezable
> workqueue there? Then a driver doesn't have to worry about that.
> 
> -- 
> Cheers,
> 
> David

+1.  Just system_freezable_wq will do the trick.

-- 
MST