Re: [PATCH v2 08/10] arm64: mm: Don't abuse memblock NOMAP to check for overlaps

[Ryan Roberts <ryan.roberts@arm.com>]

On 27/01/2026 10:27, Ard Biesheuvel wrote:
> On Tue, 27 Jan 2026 at 11:21, Ryan Roberts <ryan.roberts@arm.com> wrote:
>>
>> On 26/01/2026 09:26, Ard Biesheuvel wrote:
>>> From: Ard Biesheuvel <ardb@kernel.org>
>>>
>>> Now that the DRAM mapping routines respect existing table mappings and
>>> contiguous block and page mappings, it is no longer needed to fiddle
>>> with the memblock tables to set and clear the NOMAP attribute. Instead,
>>> map the kernel text and rodata alias first, avoiding contiguous
>>> mappings, so that they will not be added later when mapping the
>>> memblocks.
>>
>> Should we do something similar for kfence? Currently we have
>> arm64_kfence_alloc_pool() which marks some memory NOMAP then
>> arm64_kfence_map_pool() which PTE-maps it and clears NOMAP. Presumably we could
>> rationalize into a single function that does it all, prior to mapping the bulk
>> of the linear map?
>>
> 
> Yeah, good point - I did not spot that but I will address it in the
> next revision.
> 
>>>
>>> Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
>>> ---
>>>  arch/arm64/mm/mmu.c | 27 ++++++++------------
>>>  1 file changed, 10 insertions(+), 17 deletions(-)
>>>
>>> diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c
>>> index 80587cd47ce7..18415d4743bf 100644
>>> --- a/arch/arm64/mm/mmu.c
>>> +++ b/arch/arm64/mm/mmu.c
>>> @@ -1149,12 +1149,17 @@ static void __init map_mem(void)
>>>               flags |= NO_BLOCK_MAPPINGS | NO_CONT_MAPPINGS;
>>>
>>>       /*
>>> -      * Take care not to create a writable alias for the
>>> -      * read-only text and rodata sections of the kernel image.
>>> -      * So temporarily mark them as NOMAP to skip mappings in
>>> -      * the following for-loop
>>> +      * Map the linear alias of the [_text, __init_begin) interval
>>> +      * as non-executable now, and remove the write permission in
>>> +      * mark_linear_text_alias_ro() above (which will be called after
>>> +      * alternative patching has completed). This makes the contents
>>> +      * of the region accessible to subsystems such as hibernate,
>>> +      * but protects it from inadvertent modification or execution.
>>> +      * Note that contiguous mappings cannot be remapped in this way,
>>> +      * so we should avoid them here.
>>>        */
>>> -     memblock_mark_nomap(kernel_start, kernel_end - kernel_start);
>>> +     __map_memblock(kernel_start, kernel_end, PAGE_KERNEL,
>>> +                    flags | NO_CONT_MAPPINGS);
>>
>> So the reason to disallow cont mappings is because we need to modify the
>> permissions later? It _is_ safe to change permissions on a live contiguous
>> mapping in this way. That was clarified in the architecture a couple of years
>> back and we rely on it for contpte_wrprotect_ptes(); see comment there.
>>
> 
> OK, good to know - I was hoping to get your take on this ...
> 
>> I think we could relax this?
>>
> 
> OK, I suppose that means that we can drop the NO_CONT_MAPPINGS here,
> but we still need to map the kernel text/rodata alias initially to
> ensure that no block mappings are created that would need to broken
> down, right?

Yes, but...

I think your intent is that the multiple __map_memblock() calls are just
controlling the allowed leaf mapping sizes. It becomes problematic if the 2
calls use different permissions... which they do.

PAGE_KERNEL vs pgprot_tagged(PAGE_KERNEL).

Is it possible that the text/rodata section ends up tagged, which is not intended?

Thanks,
Ryan