CVE-2026-97525: x86/mm/pat: Allocate split page tables as kernel page tables

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

x86/mm/pat: Allocate split page tables as kernel page tables

A PTE is allocated directly without going through the standard page table allocation routines (such as ptealloconekernel()) when the CPA code splits a large page (splitlargepage()).

This means the page table constructor is never called nor is the page table marked as a kernel page table.

The former results in the folio associated with the page table not being marked as a page table (pagetablector() is never called thus neither is foliosetpgtable()) nor are statistics updated to reflect it (lruvecstataddfolio() is never called).

The latter issue of failing to mark the page table as a kernel page table (ptdescsetkernel() is never called) is far more problematic.

Since commit:

5ba2f0a15564 ("mm: introduce deferred freeing for kernel page tables")

kernel page table freeing has been batched and since the subsequent commit:

e37d5a2d60a3 ("iommu/sva: invalidate stale IOTLB entries for kernel address space")

IOTLB cache entries for kernel page tables have been invalidated upon being freed.

Since split page tables are freed without this invalidation, the IOTLB can contain stale entries for them.

Resolve the issue by using the ordinary PTE allocation API at split time.

This results in these kernel page tables invoking a page table constructor, and thus requires a page table destructor.

Destructors are not always present, like for early allocated direct map page tables). Conditionally call pagetabledtorfree() if the PGtable folio flag for the ptdesc is set, otherwise we free the page table via pagetablefree().

Regardless of which path is taken page tables marked as kernel page tables, which now includes split page tables, take the correct route through pagetablefreekernel().

There is a user-visible side effect in that split page tables will appear in nrpagetablepages in /proc/vmstat (as do other kernel page tables allocated after early boot), however this is a positive change.

This issue started being markedly problematic after commit:

5ba2f0a15564 ("mm: introduce deferred freeing for kernel page tables")

so choose this as the Fixes target.

[ dhansen: rephrase in imperative mood ]