CVE-2026-68163: mm/page_vma_mapped: fix device-private PMD handling

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

mm/pagevmamapped: fix device-private PMD handling

Commit 65edfda6f3f2 ("mm/rmap: extend rmap and migration support device-private entries") introduced the concept of device-private PMD entries, but did not correctly update the rmap walk code to account for them.

As a result, when pagevmamappedwalk() encounters device-private PMD entries, it takes no action other than to acquire the PMD lock and exit.

However this is highly problematic for two reasons - firstly, device private entries possess a PFN so checkpmd() needs to be called to ensure an overlapping PFN range.

Secondly, and more importantly, if PVMWMIGRATION is set the caller assumes the returned entry is a migration entry, resulting in memory corruption when the caller tries to interpret the device private entry as such.

In addition, commit 146287290023 ("mm/hugememory: implement device-private THP splitting") allowed device private PMDs to be split like THP mappings, but again did not update this code path.

As a result, we might race a PMD split prior to acquiring the PMD lock.

This patch addresses all of these issues by invoking checkpmd(), ensuring PMVWMIGRATION is not set and checks whether a split raced us we do for PMD THP and migration entries.

Instead of checking for a subset of the cases after taking the pmdlock(), put device-private along with pmdtranshuge() and pmdismigrationentry(). Also remove thpmigrationsupported() as it is already guarded by pmdismigrationentry().

[akpm@linux-foundation.org: fix Raspberry Pi 1 build, per David]