[PATCH v3 1/1] mm/rmap: fix potential out-of-bounds page table access during batched unmap

[Lance Yang]

On 2025/7/1 22:03, David Hildenbrand wrote:
> On 30.06.25 03:13, Lance Yang wrote:
>> From: Lance Yang <lance.yang at linux.dev>
>>
>> As pointed out by David[1], the batched unmap logic in try_to_unmap_one()
>> may read past the end of a PTE table when a large folio's PTE mappings
>> are not fully contained within a single page table.
>>
>> While this scenario might be rare, an issue triggerable from userspace 
>> must
>> be fixed regardless of its likelihood. This patch fixes the out-of-bounds
>> access by refactoring the logic into a new helper, 
>> folio_unmap_pte_batch().
>>
>> The new helper correctly calculates the safe batch size by capping the 
>> scan
>> at both the VMA and PMD boundaries. To simplify the code, it also 
>> supports
>> partial batching (i.e., any number of pages from 1 up to the calculated
>> safe maximum), as there is no strong reason to special-case for fully
>> mapped folios.
>>
>> [1] https://lore.kernel.org/linux-mm/ 
>> a694398c-9f03-4737-81b9-7e49c857fcbe at redhat.com
>>
>> Fixes: 354dffd29575 ("mm: support batched unmap for lazyfree large 
>> folios during reclamation")
>> Cc: <stable at vger.kernel.org>
>> Acked-by: Barry Song <baohua at kernel.org>
>> Suggested-by: David Hildenbrand <david at redhat.com>
> 
> Realized this now: This should probably be a "Reported-by:" with the 
> "Closes:" and and a link to my mail.

Got it. Both tags (Reported-by/Closes) will be in the next commit ;)