userfaultfd: fix a crash in UFFDIO_MOVE when PMD is a migration entry

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.

[surenb@google.com: remove extra folio check, per David]
Link: https://lkml.kernel.org/r/20250807200418.1963585-1-surenb@google.com
Link: https://lkml.kernel.org/r/20250806220022.926763-1-surenb@google.com
Fixes: adef440691ba ("userfaultfd: UFFDIO_MOVE uABI")
Signed-off-by: Suren Baghdasaryan <surenb@google.com>
Reported-by: syzbot+b446dbe27035ef6bd6c2@syzkaller.appspotmail.com
Closes: https://lore.kernel.org/all/68794b5c.a70a0220.693ce.0050.GAE@google.com/
Reviewed-by: Peter Xu <peterx@redhat.com>
Acked-by: David Hildenbrand <david@redhat.com>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: Lokesh Gidra <lokeshgidra@google.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

diff --git a/mm/userfaultfd.c b/mm/userfaultfd.c
index cbed91b0964013568b24ce12c4e04c5058c4f82f..45e6290e2e8b3349c268c558171380e853662ed4 100644 (file)
--- a/mm/userfaultfd.c
+++ b/mm/userfaultfd.c
@@ -1821,13 +1821,16 @@ 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;
+                                       }
                                }
 
                                spin_unlock(ptl);