[PATCH v2 0/2] virtio_balloon: fix Use-After-Free bugs during PM freeze

[Link Lin <linkl@google.com>]

This patch series addresses a class of Use-After-Free vulnerabilities
in the virtio_balloon driver that can occur during system power management
freeze (e.g., suspend or hibernation).

The core issue is a lifecycle mismatch: when the system enters freeze,
virtballoon_freeze() resets the virtio device and deletes its virtqueues.
However, several asynchronous or event-driven components of the driver and
related subsystems (Free Page Reporting and Shrinker) remain registered
and active. If they trigger during the freeze/suspend process, they
attempt to access the now-deleted virtqueues, causing kernel crashes
(General Protection Faults or UAF).

To fix this:
1. Migrate Free Page Reporting to use the system_freezable_wq, ensuring
   its worker is frozen before driver freeze callbacks run. (Suggested
   by David Hildenbrand).
2. Avoid shrinker execution while the device is suspended by adding a
   suspended flag to struct virtio_balloon. Wrap lockless reads of this
   flag in READ_ONCE() and writes in WRITE_ONCE() to prevent data races and
   KCSAN warnings.

Note: The OOM notifier (virtio_balloon_oom_notify) is also registered, but
since the PM core disables the OOM killer (via oom_killer_disable()) during
freeze_processes() before device drivers are frozen, it cannot trigger
while the device is frozen. Thus, it does not require a fix.

Testing:
I have verified these fixes using Google’s virtualization infrastructure by
running continuous suspend/resume iterations (40+ cycles) while 
churning memory using stress-ng (stress-ng --vm 4 --vm-bytes 60% --timeout 1)
to constantly create free pages for the buddy allocator. We also set the
page_reporting_order parameter to 0 to make the page reporting worker
highly sensitive, forcing it to pick up any 4K free pages. This 
confirmed that the UAF crashes are no longer reproducible.

RFC: https://lore.kernel.org/r/20260709224330.946683-1-linkl@google.com
---
RFC -> v2:
- Switched page reporting fix from unregister/re-register in driver to
  using system_freezable_wq in mm/page_reporting.c, which avoids complex
  restore rollback logic for that component.
- Switched shrinker fix from unregister/re-register to using a simple
  suspended boolean flag to avoid complex rollback logic. Wrap its reads
  and writes with READ_ONCE()/WRITE_ONCE() to prevent KCSAN data race
  warnings.
- Dropped OOM notifier fix since it's already protected by the PM core's
  oom_killer_disable().

Link Lin (2):
  mm/page_reporting: use system_freezable_wq to fix UAF during suspend
  virtio_balloon: avoid shrinker execution during PM suspend

 drivers/virtio/virtio_balloon.c | 49 +++++++++++++++++++++++++++------
 mm/page_reporting.c             |  6 ++--
 2 files changed, 45 insertions(+), 10 deletions(-)
-- 
2.55.0.229.g6434b31f56-goog