* + mm-hugetlb-fix-swap-entry-corruption-when-clearing-uffd-wp-at-fork.patch added to mm-hotfixes-unstable branch
@ 2026-07-05 2:08 Andrew Morton
0 siblings, 0 replies; 2+ messages in thread
From: Andrew Morton @ 2026-07-05 2:08 UTC (permalink / raw)
To: mm-commits, stable, sashiko-bot, peterx, osalvador, muchun.song,
david, kas, akpm
The patch titled
Subject: mm/hugetlb: fix swap entry corruption when clearing uffd-wp at fork()
has been added to the -mm mm-hotfixes-unstable branch. Its filename is
mm-hugetlb-fix-swap-entry-corruption-when-clearing-uffd-wp-at-fork.patch
This patch will shortly appear at
https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches/mm-hugetlb-fix-swap-entry-corruption-when-clearing-uffd-wp-at-fork.patch
This patch will later appear in the mm-hotfixes-unstable branch at
git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm
Before you just go and hit "reply", please:
a) Consider who else should be cc'ed
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*** Remember to use Documentation/process/submit-checklist.rst when testing your code ***
The -mm tree is included into linux-next via various
branches at git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm
and is updated there most days
------------------------------------------------------
From: "Kiryl Shutsemau (Meta)" <kas@kernel.org>
Subject: mm/hugetlb: fix swap entry corruption when clearing uffd-wp at fork()
Date: Fri, 3 Jul 2026 17:18:33 +0100
copy_hugetlb_page_range() clears the uffd-wp bit of hwpoison and migration
entries with huge_pte_clear_uffd_wp(), which operates on the present-PTE
bit position. Swap entries keep the uffd-wp state elsewhere -- the same
branches read and set 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 hwpoison branch 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, and an
hwpoison entry misidentifies which page is poisoned.
Use pte_swp_clear_uffd_wp(), matching copy_nonpresent_pte() and
move_huge_pte().
Link: https://lore.kernel.org/20260703161833.57416-1-kirill@shutemov.name
Fixes: bc70fbf269fd ("mm/hugetlb: handle uffd-wp during fork()")
Signed-off-by: Kiryl Shutsemau <kas@kernel.org>
Assisted-by: Claude:claude-fable-5
Reported-by: Sashiko AI review <sashiko-bot@kernel.org>
Closes: https://lore.kernel.org/all/20260703140011.99E601F000E9@smtp.kernel.org/
Cc: David Hildenbrand <david@kernel.org>
Cc: Muchun Song <muchun.song@linux.dev>
Cc: Oscar Salvador <osalvador@suse.de>
Cc: Peter Xu <peterx@redhat.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---
mm/hugetlb.c | 4 ++--
1 file changed, 2 insertions(+), 2 deletions(-)
--- a/mm/hugetlb.c~mm-hugetlb-fix-swap-entry-corruption-when-clearing-uffd-wp-at-fork
+++ a/mm/hugetlb.c
@@ -4918,7 +4918,7 @@ again:
softleaf = softleaf_from_pte(entry);
if (unlikely(softleaf_is_hwpoison(softleaf))) {
if (!userfaultfd_wp(dst_vma))
- entry = huge_pte_clear_uffd_wp(entry);
+ entry = pte_swp_clear_uffd_wp(entry);
set_huge_pte_at(dst, addr, dst_pte, entry, sz);
} else if (unlikely(softleaf_is_migration(softleaf))) {
bool uffd_wp = pte_swp_uffd_wp(entry);
@@ -4936,7 +4936,7 @@ again:
set_huge_pte_at(src, addr, src_pte, entry, sz);
}
if (!userfaultfd_wp(dst_vma))
- entry = huge_pte_clear_uffd_wp(entry);
+ entry = pte_swp_clear_uffd_wp(entry);
set_huge_pte_at(dst, addr, dst_pte, entry, sz);
} else if (unlikely(pte_is_marker(entry))) {
const pte_marker marker = copy_pte_marker(softleaf, dst_vma);
_
Patches currently in -mm which might be from kas@kernel.org are
mm-hugetlb-fix-swap-entry-corruption-when-clearing-uffd-wp-at-fork.patch
^ permalink raw reply [flat|nested] 2+ messages in thread
* + mm-hugetlb-fix-swap-entry-corruption-when-clearing-uffd-wp-at-fork.patch added to mm-hotfixes-unstable branch
@ 2026-07-09 3:37 Andrew Morton
0 siblings, 0 replies; 2+ messages in thread
From: Andrew Morton @ 2026-07-09 3:37 UTC (permalink / raw)
To: mm-commits, stable, sashiko-bot, peterx, osalvador, muchun.song,
david, kas, akpm
The patch titled
Subject: mm/hugetlb: fix swap entry corruption when clearing uffd-wp at fork()
has been added to the -mm mm-hotfixes-unstable branch. Its filename is
mm-hugetlb-fix-swap-entry-corruption-when-clearing-uffd-wp-at-fork.patch
This patch will shortly appear at
https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches/mm-hugetlb-fix-swap-entry-corruption-when-clearing-uffd-wp-at-fork.patch
This patch will later appear in the mm-hotfixes-unstable branch at
git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm
Before you just go and hit "reply", please:
a) Consider who else should be cc'ed
b) Prefer to cc a suitable mailing list as well
c) Ideally: find the original patch on the mailing list and do a
reply-to-all to that, adding suitable additional cc's
*** Remember to use Documentation/process/submit-checklist.rst when testing your code ***
The -mm tree is included into linux-next via various
branches at git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm
and is updated there most days
------------------------------------------------------
From: "Kiryl Shutsemau (Meta)" <kas@kernel.org>
Subject: mm/hugetlb: fix swap entry corruption when clearing uffd-wp at fork()
Date: Wed, 8 Jul 2026 10:01:10 +0100
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.
Link: https://lore.kernel.org/20260708090110.136162-1-kirill@shutemov.name
Fixes: bc70fbf269fd ("mm/hugetlb: handle uffd-wp during fork()")
Signed-off-by: Kiryl Shutsemau <kas@kernel.org>
Reported-by: Sashiko AI review <sashiko-bot@kernel.org>
Closes: https://lore.kernel.org/all/20260703140011.99E601F000E9@smtp.kernel.org/
Suggested-by: David Hildenbrand <david@kernel.org>
Acked-by: David Hildenbrand (Arm) <david@kernel.org>
Assisted-by: Claude:claude-fable-5
Cc: Muchun Song <muchun.song@linux.dev>
Cc: Oscar Salvador <osalvador@suse.de>
Cc: Peter Xu <peterx@redhat.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---
mm/hugetlb.c | 10 +++++++---
1 file changed, 7 insertions(+), 3 deletions(-)
--- a/mm/hugetlb.c~mm-hugetlb-fix-swap-entry-corruption-when-clearing-uffd-wp-at-fork
+++ a/mm/hugetlb.c
@@ -4929,8 +4929,12 @@ again:
softleaf = softleaf_from_pte(entry);
if (unlikely(softleaf_is_hwpoison(softleaf))) {
- if (!userfaultfd_wp(dst_vma))
- entry = huge_pte_clear_uffd_wp(entry);
+ /*
+ * A hwpoison entry never carries the uffd-wp bit: it is
+ * installed fresh by make_hwpoison_entry() and
+ * hugetlb_change_protection() leaves it untouched, so
+ * there is nothing to clear for the child.
+ */
set_huge_pte_at(dst, addr, dst_pte, entry, sz);
} else if (unlikely(softleaf_is_migration(softleaf))) {
bool uffd_wp = pte_swp_uffd_wp(entry);
@@ -4948,7 +4952,7 @@ again:
set_huge_pte_at(src, addr, src_pte, entry, sz);
}
if (!userfaultfd_wp(dst_vma))
- entry = huge_pte_clear_uffd_wp(entry);
+ entry = pte_swp_clear_uffd_wp(entry);
set_huge_pte_at(dst, addr, dst_pte, entry, sz);
} else if (unlikely(pte_is_marker(entry))) {
const pte_marker marker = copy_pte_marker(softleaf, dst_vma);
_
Patches currently in -mm which might be from kas@kernel.org are
fs-proc-task_mmu-fix-pagemap_scan-written-state-for-unpopulated-ptes.patch
fs-proc-task_mmu-fix-pagemap_scan-written-state-for-pmd-holes.patch
mm-hugetlb-fix-swap-entry-corruption-when-clearing-uffd-wp-at-fork.patch
mm-decouple-protnone-helpers-from-config_numa_balancing.patch
mm-rename-uffd-wp-pte-bit-macros-to-uffd.patch
mm-rename-uffd-wp-pte-accessors-to-uffd.patch
userfaultfd-test-uffd-vma-flags-through-the-vma_flags_t-api.patch
mm-add-vm_uffd_rwp-vma-flag.patch
mm-add-mm_cp_uffd_rwp-change_protection-flag.patch
mm-preserve-rwp-marker-across-pte-rewrites.patch
mm-handle-vm_uffd_rwp-in-khugepaged-rmap-and-gup.patch
userfaultfd-add-uffdio_register_mode_rwp-and-uffdio_rwprotect-plumbing.patch
mm-userfaultfd-add-rwp-fault-delivery-and-expose-uffdio_register_mode_rwp.patch
mm-pagemap-add-page_is_accessed-for-rwp-tracking.patch
userfaultfd-add-uffd_feature_rwp_async-for-async-fault-resolution.patch
userfaultfd-add-uffdio_set_mode-for-runtime-sync-async-toggle.patch
selftests-mm-add-userfaultfd-rwp-tests.patch
documentation-userfaultfd-document-rwp-working-set-tracking.patch
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