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.
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.
Metrics
Affected Vendors & Products
No data.
No data.
OpenCVE Enrichment is a feature of OpenCVE that uses AI to automatically link vendors and products to CVEs. Learn more on GitHub.
No data.
Advisories
No advisories yet.
Fixes
Solution
No solution given by the vendor.
Workaround
No workaround given by the vendor.
References
History
Fri, 04 Sep 2026 20:00:00 +0000
| Type | Values Removed | Values Added |
|---|---|---|
| Weaknesses | CWE-119 |
Fri, 04 Sep 2026 17:30:00 +0000
| Type | Values Removed | Values Added |
|---|---|---|
| Description | 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. | |
| Title | mm/hugetlb: fix swap entry corruption when clearing uffd-wp at fork() | |
| First Time appeared |
Linux
Linux linux Kernel |
|
| CPEs | cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:* | |
| Vendors & Products |
Linux
Linux linux Kernel |
|
| References |
|
|
Projects
Sign in to view the affected projects.
MITRE
Status: PUBLISHED
Assigner: Linux
Published:
Updated: 2026-09-04T17:11:09.199Z
Reserved: 2026-08-26T14:34:25.800Z
Link: CVE-2026-80893
Vulnrichment
No data.
NVD
Status : Received
Published: 2026-09-04T18:17:57.467
Modified: 2026-09-04T18:17:57.467
Link: CVE-2026-80893
Redhat
No data.
OpenCVE Enrichment
Updated: 2026-09-04T19:45:03Z
Weaknesses