Re: [PATCH v3 01/15] arm64/mm: Make set_ptes() robust when OAs cross 48-bit boundary

David Hildenbrand
2024-02-09T22:36:23Z

On 08.02.24 07:10, Mike Rapoport wrote:
> On Mon, Jan 29, 2024 at 01:46:35PM +0100, David Hildenbrand wrote:
>> From: Ryan Roberts <ryan.roberts@arm.com>
>>
>> Since the high bits [51:48] of an OA are not stored contiguously in the
>> PTE, there is a theoretical bug in set_ptes(), which just adds PAGE_SIZE
>> to the pte to get the pte with the next pfn. This works until the pfn
>> crosses the 48-bit boundary, at which point we overflow into the upper
>> attributes.
>>
>> Of course one could argue (and Matthew Wilcox has :) that we will never
>> see a folio cross this boundary because we only allow naturally aligned
>> power-of-2 allocation, so this would require a half-petabyte folio. So
>> its only a theoretical bug. But its better that the code is robust
>> regardless.
>>
>> I've implemented pte_next_pfn() as part of the fix, which is an opt-in
>> core-mm interface. So that is now available to the core-mm, which will
>> be needed shortly to support forthcoming fork()-batching optimizations.
>>
>> Link: https://lkml.kernel.org/r/20240125173534.1659317-1-ryan.roberts@arm.com
>> Fixes: 4a169d61c2ed ("arm64: implement the new page table range API")
>> Closes: https://lore.kernel.org/linux-mm/fdaeb9a5-d890-499a-92c8-d171df43ad01@arm.com/
>> Signed-off-by: Ryan Roberts <ryan.roberts@arm.com>
>> Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>
>> Reviewed-by: David Hildenbrand <david@redhat.com>
>> Signed-off-by: David Hildenbrand <david@redhat.com>
>
> Reviewed-by: Mike Rapoport (IBM) <rppt@kernel.org>

Thanks for the review Mike, appreciated!

--
Cheers,

David / dhildenb