CVE-2026-80893: mm/hugetlb: fix swap entry corruption when clearing uffd-wp at fork()

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

mm/hugetlb: fix swap entry corruption when clearing uffd-wp at fork()

copyhugetlbpagerange() clears the uffd-wp bit of migration and hwpoison entries with hugepteclearuffdwp(), which operates on the present-PTE bit position. Swap entries keep the uffd-wp state elsewhere -- the migration branch reads and sets it with pteswpuffdwp() and pteswpmkuffdwp() -- and the present-PTE position falls into the swap payload. On x86-64 it lands in the inverted swap offset, where a naturally-aligned hugetlb PFN always has the affected bit set, so the clear advances the encoded PFN by two pages.

No userfaultfd needs to be involved: the clear is guarded only by the child VMA not being uffd-wp registered, so a plain fork() with an in-flight hugetlb migration entry (or a poisoned hugetlb page) corrupts the entry copied into the child. Instrumenting the clear and forking after MADVHWPOISON on a 2MB anon hugetlb page shows:

offset before=120e00 offset after =120e02

The fallout is mostly latent: rmap walks match migration entries by folio range and removemigrationpte() rebuilds the PTE from the folio, so a within-folio PFN skew heals once migration completes. But any path that re-encodes the corrupted offset -- e.g. hugetlbchangeprotection() rewriting a writable migration entry via makereadablemigrationentry(swpoffset(entry)) -- propagates it.

Migration entries legitimately carry uffd-wp, so clear it with pteswpclearuffdwp(), matching copynonpresentpte() and movehugepte().

A hwpoison entry, on the other hand, never carries the uffd-wp bit: it is installed fresh by makehwpoisonentry() (trytounmapone() does not preserve uffd-wp on the hwpoison path) and hugetlbchangeprotection() leaves hwpoison entries untouched. There was nothing to clear there, only the corruption, so drop the clear entirely.