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

Overview

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

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

copy_hugetlb_page_range() clears the uffd-wp bit of migration and hwpoison entries with huge_pte_clear_uffd_wp(), which operates on the present-PTE bit position. Swap entries keep the uffd-wp state elsewhere -- the migration branch reads and sets it with pte_swp_uffd_wp() and pte_swp_mkuffd_wp() -- 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 MADV_HWPOISON 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 remove_migration_pte() 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. hugetlb_change_protection() rewriting a writable migration entry via make_readable_migration_entry(swp_offset(entry)) -- propagates it.

Migration entries legitimately carry uffd-wp, so clear it with pte_swp_clear_uffd_wp(), matching copy_nonpresent_pte() and move_huge_pte().

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

Affected

• Linux >= bc70fbf269fdff410b0b6d75c3770b9f59117b90 < f1b1311c0352873137768bac5a126e491271a747
• Linux >= bc70fbf269fdff410b0b6d75c3770b9f59117b90 < 69cb5825d9988c7944bc9f1dc08cb233655405a7
• Linux >= bc70fbf269fdff410b0b6d75c3770b9f59117b90 < 8b0de7005b148738d79d6c45594d566489948a68
• Linux >= bc70fbf269fdff410b0b6d75c3770b9f59117b90 < 2b9a07002c2f296aa6a9c591213933d3492e3089
• Linux >= bc70fbf269fdff410b0b6d75c3770b9f59117b90 < 2fa11c60c9c06bafc19cf4d9efdaa36a38079e87
• Linux >= bc70fbf269fdff410b0b6d75c3770b9f59117b90 < 83abe2fd5b3aeb3123b5408a5a91709c5538fb23
• Linux 5.19

Remediation

Refer to the linked advisories for vendor-supplied fixes and affected version ranges.