[PATCH v2 1/2] mm/page_reporting: use system_freezable_wq to fix UAF during suspend

During PM freeze (e.g. S3 suspend or S4 hibernation), device drivers like
virtio_balloon reset their underlying virtio devices and delete their
virtqueues via vdev->config->del_vqs().

However, page reporting work (page_reporting_process) was scheduled on
the global system_wq. Because system_wq lacks the WQ_FREEZABLE flag, the
PM freezer skips it, leaving page_reporting_process active during
suspend. If pages are freed into the buddy allocator while suspending,
page reporting invokes virtballoon_free_page_report() on deleted
virtqueues:

    [  196.795226] general protection fault, probably for non-canonical address 0xaa1436fe70dae6df: 0000 [#1] SMP NOPTI
    [  196.825967] Workqueue: events page_reporting_process
    [  196.831038] RIP: 0010:virtqueue_add_split+0x233/0x4c0 [virtio_ring]
    [  196.927073] virtballoon_free_page_report+0x3a/0xe0 [virtio_balloon]
    [  196.946943] page_reporting_process+0x370/0x4f0

Fix this by switching page reporting work to system_freezable_wq. This
ensures that the PM freezer pauses page_reporting_process before device
drivers destroy their reporting virtqueues.

This aligns with the driver's existing design. The comment in
virtballoon_freeze() states:
    /*
     * The workqueue is already frozen by the PM core before this
     * function is called.
     */

Suggested-by: David Hildenbrand <[email protected]>
Suggested-by: Michael S. Tsirkin <[email protected]>
Acked-by: David Rientjes <[email protected]>
Fixes: 924a663f75e2 ("virtio-balloon: Reporting free page reservations")
Cc: [email protected]
Signed-off-by: Link Lin <[email protected]>
---
 mm/page_reporting.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/mm/page_reporting.c b/mm/page_reporting.c
index 7418f2e500..4dc6f4b852 100644
--- a/mm/page_reporting.c
+++ b/mm/page_reporting.c
@@ -80,7 +80,8 @@ __page_reporting_request(struct page_reporting_dev_info *prdev)
 	 * now we are limiting this to running no more than once every
 	 * couple of seconds.
 	 */
-	schedule_delayed_work(&prdev->work, PAGE_REPORTING_DELAY);
+	queue_delayed_work(system_freezable_wq, &prdev->work,
+			   PAGE_REPORTING_DELAY);
 }
 
 /* notify prdev of free page reporting request */
@@ -343,7 +344,8 @@ static void page_reporting_process(struct work_struct *work)
 	 */
 	state = atomic_cmpxchg(&prdev->state, state, PAGE_REPORTING_IDLE);
 	if (state == PAGE_REPORTING_REQUESTED)
-		schedule_delayed_work(&prdev->work, PAGE_REPORTING_DELAY);
+		queue_delayed_work(system_freezable_wq, &prdev->work,
+				   PAGE_REPORTING_DELAY);
 }
 
 static DEFINE_MUTEX(page_reporting_mutex);
-- 
2.55.0.229.g6434b31f56-goog