CVE-2026-74481: mm/page_reporting: use system_freezable_wq to fix UAF during suspend

In the Linux kernel, the following vulnerability has been resolved:

mm/pagereporting: use systemfreezablewq to fix UAF during suspend

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

However, page reporting work (pagereportingprocess) was scheduled on the global systemwq. Because systemwq lacks the WQFREEZABLE flag, the PM freezer skips it, leaving pagereportingprocess active during suspend.

If pages are freed into the buddy allocator while suspending (for example, when core MM invokes the balloon shrinker during S4 hibernation image saving), page reporting triggers virtballoonfreepagereport() on deleted virtqueues, resulting in a Use-After-Free / General Protection Fault:

[ 196.795226] general protection fault, probably for non-canonical address 0xaa1436fe70dae6df: 0000 [#1] SMP NOPTI [ 196.825967] Workqueue: events pagereportingprocess [ 196.831038] RIP: 0010:virtqueueaddsplit+0x233/0x4c0 [virtioring] [ 196.927073] virtballoonfreepagereport+0x3a/0xe0 [virtioballoon] [ 196.946943] pagereportingprocess+0x370/0x4f0

Fix this by switching page reporting work to systemfreezablewq. This ensures that the PM freezer pauses pagereportingprocess before device drivers destroy their reporting virtqueues. Because the reporting worker is frozen, memory reclamation/freeing (e.g. via shrinker execution) can safely return pages to MM during freeze without triggering unfrozen reporting work on deleted virtqueues.

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

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 pagereportingorder 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.