CVE-2025-21984 — mm: fix kernel BUG when userfaultfd_move encounters swapcache

Affected: Linux

In the Linux kernel, the following vulnerability has been resolved:

mm: fix kernel BUG when userfaultfd_move encounters swapcache

userfaultfd_move() checks whether the PTE entry is present or a swap entry.

- If the PTE entry is present, move_present_pte() handles folio migration by setting:

src_folio->index = linear_page_index(dst_vma, dst_addr);

- If the PTE entry is a swap entry, move_swap_pte() simply copies the PTE to the new dst_addr.

This approach is incorrect because, even if the PTE is a swap entry, it can still reference a folio that remains in the swap cache.

This creates a race window between steps 2 and 4. 1. add_to_swap: The folio is added to the swapcache. 2. try_to_unmap: PTEs are converted to swap entries. 3. pageout: The folio is written back. 4. Swapcache is cleared. If userfaultfd_move() occurs in the window between steps 2 and 4, after the swap PTE has been moved to the destination, accessing the destination triggers do_swap_page(), which may locate the folio in the swapcache. However, since the folio's index has not been updated to match the destination VMA, do_swap_page() will detect a mismatch.

This can result in two critical issues depending on the system configuration.

If KSM is disabled, both small and large folios can trigger a BUG during the add_rmap operation due to:

page_pgoff(folio, page) != linear_page_index(vma, address)

[ 13.336953] page: refcount:6 mapcount:1 mapping:00000000f43db19c index:0xffffaf150 pfn:0x4667c […

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CVE-2025-21984 — mm: fix kernel BUG when userfaultfd_move encounters swapcache