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mm/userfaultfd: allow hugetlb change protection upon poison entry
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After UFFDIO_POISON, there can be two kinds of hugetlb pte markers, either
the POISON one or UFFD_WP one.

Allow change protection to run on a poisoned marker just like !hugetlb
cases, ignoring the marker irrelevant of the permission.

Here the two bits are mutual exclusive.  For example, when install a
poisoned entry it must not be UFFD_WP already (by checking pte_none()
before such install).  And it also means if UFFD_WP is set there must have
no POISON bit set.  It makes sense because UFFD_WP is a bit to reflect
permission, and permissions do not apply if the pte is poisoned and
destined to sigbus.

So here we simply check uffd_wp bit set first, do nothing otherwise.

Attach the Fixes to UFFDIO_POISON work, as before that it should not be
possible to have poison entry for hugetlb (e.g., hugetlb doesn't do swap,
so no chance of swapin errors).

Link: https://lkml.kernel.org/r/[email protected]
Link: https://lore.kernel.org/r/[email protected]
Fixes: fc71884 ("mm: userfaultfd: add new UFFDIO_POISON ioctl")
Signed-off-by: Peter Xu <[email protected]>
Reported-by: [email protected]
Reviewed-by: David Hildenbrand <[email protected]>
Reviewed-by: Axel Rasmussen <[email protected]>
Cc: <[email protected]>	[6.6+]
Signed-off-by: Andrew Morton <[email protected]>
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xzpeter authored and akpm00 committed Apr 16, 2024
1 parent 7401745 commit c5977c9
Showing 1 changed file with 7 additions and 3 deletions.
10 changes: 7 additions & 3 deletions mm/hugetlb.c
Original file line number Diff line number Diff line change
Expand Up @@ -7044,9 +7044,13 @@ long hugetlb_change_protection(struct vm_area_struct *vma,
if (!pte_same(pte, newpte))
set_huge_pte_at(mm, address, ptep, newpte, psize);
} else if (unlikely(is_pte_marker(pte))) {
/* No other markers apply for now. */
WARN_ON_ONCE(!pte_marker_uffd_wp(pte));
if (uffd_wp_resolve)
/*
* Do nothing on a poison marker; page is
* corrupted, permissons do not apply. Here
* pte_marker_uffd_wp()==true implies !poison
* because they're mutual exclusive.
*/
if (pte_marker_uffd_wp(pte) && uffd_wp_resolve)
/* Safe to modify directly (non-present->none). */
huge_pte_clear(mm, address, ptep, psize);
} else if (!huge_pte_none(pte)) {
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