[PATCH v2 1/1] mm/rmap: fix potential out-of-bounds page table access during batched unmap

[Andrew Morton]

On Fri, 27 Jun 2025 14:23:19 +0800 Lance Yang <ioworker0 at gmail.com> wrote:

> As pointed out by David[1], the batched unmap logic in try_to_unmap_one()
> can read past the end of a PTE table if a large folio is mapped starting at
> the last entry of that table. It would be quite rare in practice, as
> MADV_FREE typically splits the large folio ;)
> 
> So let's fix the potential out-of-bounds read by refactoring the logic into
> a new helper, folio_unmap_pte_batch().
> 
> The new helper now correctly calculates the safe number of pages to scan by
> limiting the operation to the boundaries of the current VMA and the PTE
> table.
> 
> In addition, the "all-or-nothing" batching restriction is removed to
> support partial batches. The reference counting is also cleaned up to use
> folio_put_refs().

I'll queue this for testing while the updated changelog is being prepared.