[PATCH v4 1/1] userfaultfd: fix a crash in UFFDIO_MOVE when PMD is a migration entry

[Suren Baghdasaryan <surenb@google.com>]

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.

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
---
Changes since v3 [1]
- Updated the title and changelog, per Peter Xu
- Added Reviewed-by: per Peter Xu

[1] https://lore.kernel.org/all/20250806154015.769024-1-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..116481606be8 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 (!folio || (!is_huge_zero_folio(folio) &&
+						       !PageAnonExclusive(&folio->page))) {
+						spin_unlock(ptl);
+						err = -EBUSY;
+						break;
+					}
 				}
 
 				spin_unlock(ptl);

base-commit: 8e7e0c6d09502e44aa7a8fce0821e042a6ec03d1
-- 
2.50.1.565.gc32cd1483b-goog