[PATCH v2] mm/khugepaged: Don't install PMDs in uffd-minor-registered VMAs

[James Houghton <jthoughton@google.com>]

Userfaultfd minor faults provides userspace with the ability to manually
install PTEs with UFFDIO_CONTINUE. Right now, khugepaged collapse can
map holes in the VMA when a naturally-aligned THP is present without
explicit action from userspace.

This is a problem, as it bypasses userfaultfd minor faults that
userspace is expecting to handle.

If userspace implements post-copy live migration using userfaultfd minor
faults, this situation is currently possible:
1. The VMA for guest memory is userfaultfd-minor-registered and nothing
   is mapped in the page tables.
2. A stale copy of a page is present in a naturally-aligned THP (from
   pre-copy live migration).
3. khugepaged collapses the mapping of the THP, installs a PMD.
4. The VM now has access to the stale contents => VM is broken.
5. After installing the correct contents, userspace attempts to map the
   page with UFFDIO_CONTINUE; it gets EEXIST, indicating that something
   unexpectedly mapped the page.

The naturally-aligned THP case is the only case where this is a problem.
khugepaged otherwise requires all PTEs to be present for
userfaultfd-registered VMAs (i.e., max none PTEs is 0), which is
correct. This check is essentially bypassed for naturally-aligned THPs.

No changes are needed for file_backed_vma_is_retractable(), as zapping
PTEs is safe. Userspace must already handle cases where PTEs are zapped
without explicit action (e.g. due to reclaim).

Fixes: 58ac9a8993a1 ("mm/khugepaged: attempt to map file/shmem-backed pte-mapped THPs by pmds")
Cc: <stable@vger.kernel.org> # 6.1
Suggested-by: Lance Yang <lance.yang@linux.dev>
Tested-by: Lance Yang <lance.yang@linux.dev>
Signed-off-by: James Houghton <jthoughton@google.com>
---
v1->v2:
 - Applied Lance's change to move the userfaultfd_minor() check to the
   right place.
 - Suggested-by: and Tested-by: Lance. Thank you!
 - Adjusted the commit description.

v1: https://lore.kernel.org/linux-mm/20260828005004.2870750-1-jthoughton@google.com/
---
 mm/khugepaged.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/mm/khugepaged.c b/mm/khugepaged.c
index b237f6e7662a..22356229c1e4 100644
--- a/mm/khugepaged.c
+++ b/mm/khugepaged.c
@@ -1899,6 +1899,13 @@ static enum scan_result try_collapse_pte_mapped_thp(struct mm_struct *mm, unsign
 	if (userfaultfd_protected(vma))
 		return SCAN_PTE_UFFD;
 
+	/*
+	 * Userfaultfd-minor-registered VMAs should not be collapsed, as
+	 * userspace is expecting to explicitly install PTEs.
+	 */
+	if (userfaultfd_minor(vma))
+		return SCAN_PTE_UFFD;
+
 	folio = filemap_lock_folio(vma->vm_file->f_mapping,
 			       linear_page_index(vma, haddr));
 	if (IS_ERR(folio))

base-commit: 26260251022fbc2f248a3d747a9b2b961b18d2d8
-- 
2.55.0.897.gb25b4bd76c-goog