[PATCH 2/6] mm/hugetlb: Fix uffd-wp bit lost when unsharing happens

Peter Xu
2023-04-13T23:12:20Z

When we try to unshare a pinned page for a private hugetlb, uffd-wp bit can
get lost during unsharing. Fix it by carrying it over.

This should be very rare, only if an unsharing happened on a private
hugetlb page with uffd-wp protected (e.g. in a child which shares the same
page with parent with UFFD_FEATURE_EVENT_FORK enabled).

Cc: linux-stable <stable@vger.kernel.org>
Fixes: 166f3ecc0daf ("mm/hugetlb: hook page faults for uffd write protection")
Reported-by: Mike Kravetz <mike.kravetz@oracle.com>
Signed-off-by: Peter Xu <peterx@redhat.com>
---
mm/hugetlb.c | 7 +++++--
1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index 7320e64aacc6..083aae35bff8 100644
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -5637,13 +5637,16 @@ static vm_fault_t hugetlb_wp(struct mm_struct *mm, struct vm_area_struct *vma,
spin_lock(ptl);
ptep = hugetlb_walk(vma, haddr, huge_page_size(h));
if (likely(ptep && pte_same(huge_ptep_get(ptep), pte))) {
+ pte_t newpte = make_huge_pte(vma, &new_folio->page, !unshare);
+
/* Break COW or unshare */
huge_ptep_clear_flush(vma, haddr, ptep);
mmu_notifier_invalidate_range(mm, range.start, range.end);
page_remove_rmap(old_page, vma, true);
hugepage_add_new_anon_rmap(new_folio, vma, haddr);
- set_huge_pte_at(mm, haddr, ptep,
- make_huge_pte(vma, &new_folio->page, !unshare));
+ if (huge_pte_uffd_wp(pte))
+ newpte = huge_pte_mkuffd_wp(newpte);
+ set_huge_pte_at(mm, haddr, ptep, newpte);
folio_set_hugetlb_migratable(new_folio);
/* Make the old page be freed below */
new_folio = page_folio(old_page);
--
2.39.1