On 07.08.25 22:04, Suren Baghdasaryan wrote:

When UFFDIO_MOVE encounters a migration PMD entry, it proceeds with obtaining a folio and accessing it even though the entry is swp_entry_t. Add the missing check and let split_huge_pmd() handle migration entries. While at it also remove unnecessary folio check.

Fixes: adef440691ba ("userfaultfd: UFFDIO_MOVE uABI") Reported-by: syzbot+b446dbe27035ef6bd6c2@syzkaller.appspotmail.com Closes: https://lore.kernel.org/all/68794b5c.a70a0220.693ce.0050.GAE@google.com/ Signed-off-by: Suren Baghdasaryan surenb@google.com Reviewed-by: Peter Xu peterx@redhat.com Cc: stable@vger.kernel.org

---

Applies to mm-unstable after reverting older v4 [1] version.

Changes since v4 [1]

• Removed extra folio check, per David Hildenbrand

[1] https://lore.kernel.org/all/20250806220022.926763-1-surenb@google.com/

mm/userfaultfd.c | 17 ++++++++++------- 1 file changed, 10 insertions(+), 7 deletions(-)

diff --git a/mm/userfaultfd.c b/mm/userfaultfd.c index 5431c9dd7fd7..aefdf3a812a1 100644 --- a/mm/userfaultfd.c +++ b/mm/userfaultfd.c @@ -1826,13 +1826,16 @@ ssize_t move_pages(struct userfaultfd_ctx *ctx, unsigned long dst_start, /* Check if we can move the pmd without splitting it. */ if (move_splits_huge_pmd(dst_addr, src_addr, src_start + len) || !pmd_none(dst_pmdval)) {

• 		struct folio *folio = pmd_folio(*src_pmd);
  

• 		if (!folio || (!is_huge_zero_folio(folio) &&
  

• 			       !PageAnonExclusive(&folio->page))) {
  

• 			spin_unlock(ptl);
  

• 			err = -EBUSY;
  

• 			break;
  

• 		/* Can be a migration entry */
  

• 		if (pmd_present(*src_pmd)) {
  

• 			struct folio *folio = pmd_folio(*src_pmd);
  

• 			if (!is_huge_zero_folio(folio) &&
  

• 			    !PageAnonExclusive(&folio->page)) {
  

• 				spin_unlock(ptl);
  

• 				err = -EBUSY;
  

• 				break;
  

• 			}
  

Acked-by: David Hildenbrand david@redhat.com