mm/userfaultfd: fix kmap_local LIFO ordering for CONFIG_HIGHPTE

With CONFIG_HIGHPTE on 32-bit ARM, move_pages_pte() maps PTE pages using
kmap_local_page(), which requires unmapping in Last-In-First-Out order.

The current code maps dst_pte first, then src_pte, but unmaps them in the
same order (dst_pte, src_pte), violating the LIFO requirement.  This
causes the warning in kunmap_local_indexed():

  WARNING: CPU: 0 PID: 604 at mm/highmem.c:622 kunmap_local_indexed+0x178/0x17c
  addr \!= __fix_to_virt(FIX_KMAP_BEGIN + idx)

Fix this by reversing the unmap order to respect LIFO ordering.

This issue follows the same pattern as similar fixes:
- commit eca6828403b8 ("crypto: skcipher - fix mismatch between mapping and unmapping order")
- commit 8cf57c6df818 ("nilfs2: eliminate staggered calls to kunmap in nilfs_rename")

Both of which addressed the same fundamental requirement that kmap_local
operations must follow LIFO ordering.

Link: https://lkml.kernel.org/r/20250731144431.773923-1-sashal@kernel.org
Fixes: adef440691ba ("userfaultfd: UFFDIO_MOVE uABI")
Signed-off-by: Sasha Levin <sashal@kernel.org>
Acked-by: David Hildenbrand <david@redhat.com>
Reviewed-by: Suren Baghdasaryan <surenb@google.com>
Cc: Andrea Arcangeli <aarcange@redhat.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 45e6290e2e8b3349c268c558171380e853662ed4..aefdf3a812a185a8d066a3968df2b5e1b742bb64 100644 (file)
--- a/mm/userfaultfd.c
+++ b/mm/userfaultfd.c
@@ -1453,10 +1453,15 @@ out:
                folio_unlock(src_folio);
                folio_put(src_folio);
        }
-       if (dst_pte)
-               pte_unmap(dst_pte);
+       /*
+        * Unmap in reverse order (LIFO) to maintain proper kmap_local
+        * index ordering when CONFIG_HIGHPTE is enabled. We mapped dst_pte
+        * first, then src_pte, so we must unmap src_pte first, then dst_pte.
+        */
        if (src_pte)
                pte_unmap(src_pte);
+       if (dst_pte)
+               pte_unmap(dst_pte);
        mmu_notifier_invalidate_range_end(&range);
        if (si)
                put_swap_device(si);