[PATCH] mm/hugetlb: Fix uffd wr-protection for CoW optimization path

[Peter Xu <peterx@redhat.com>]

This patch fixes an issue that a hugetlb uffd-wr-protected mapping can be
writable even with uffd-wp bit set.  It only happens with all these
conditions met: (1) hugetlb memory (2) private mapping (3) original mapping
was missing, then (4) being wr-protected (IOW, pte marker installed).  Then
write to the page to trigger.

Userfaultfd-wp trap for hugetlb was implemented in hugetlb_fault() before
even reaching hugetlb_wp() to avoid taking more locks that userfault won't
need.  However there's one CoW optimization path for missing hugetlb page
that can trigger hugetlb_wp() inside hugetlb_no_page(), that can bypass the
userfaultfd-wp traps.

A few ways to resolve this:

  (1) Skip the CoW optimization for hugetlb private mapping, considering
  that private mappings for hugetlb should be very rare, so it may not
  really be helpful to major workloads.  The worst case is we only skip the
  optimization if userfaultfd_wp(vma)==true, because uffd-wp needs another
  fault anyway.

  (2) Move the userfaultfd-wp handling for hugetlb from hugetlb_fault()
  into hugetlb_wp().  The major cons is there're a bunch of locks taken
  when calling hugetlb_wp(), and that will make the changeset unnecessarily
  complicated due to the lock operations.

  (3) Carry over uffd-wp bit in hugetlb_wp(), so it'll need to fault again
  for uffd-wp privately mapped pages.

This patch chose option (3) which contains the minimum changeset (simplest
for backport) and also make sure hugetlb_wp() itself will start to be
always safe with uffd-wp ptes even if called elsewhere in the future.

This patch will be needed for v5.19+ hence copy stable.

Reported-by: Muhammad Usama Anjum <usama.anjum@collabora.com>
Cc: linux-stable <stable@vger.kernel.org>
Fixes: 166f3ecc0daf ("mm/hugetlb: hook page faults for uffd write protection")
Signed-off-by: Peter Xu <peterx@redhat.com>
---
 mm/hugetlb.c | 8 +++++---
 1 file changed, 5 insertions(+), 3 deletions(-)

diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index 8bfd07f4c143..22337b191eae 100644
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -5478,7 +5478,7 @@ static vm_fault_t hugetlb_wp(struct mm_struct *mm, struct vm_area_struct *vma,
 		       struct folio *pagecache_folio, spinlock_t *ptl)
 {
 	const bool unshare = flags & FAULT_FLAG_UNSHARE;
-	pte_t pte;
+	pte_t pte, newpte;
 	struct hstate *h = hstate_vma(vma);
 	struct page *old_page;
 	struct folio *new_folio;
@@ -5622,8 +5622,10 @@ static vm_fault_t hugetlb_wp(struct mm_struct *mm, struct vm_area_struct *vma,
 		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));
+		newpte = 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