Message-ID: <0202fc62-0918-493a-98e0-3fd543c383fb@arm.com>
Date: Tue, 27 Jan 2026 11:09:36 +0000
From: Ryan Roberts <ryan.roberts@....com>
To: Ard Biesheuvel <ardb@...nel.org>
Cc: linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
 will@...nel.org, catalin.marinas@....com, mark.rutland@....com,
 Anshuman Khandual <anshuman.khandual@....com>,
 Liz Prucka <lizprucka@...gle.com>, Seth Jenkins <sethjenkins@...gle.com>,
 Kees Cook <kees@...nel.org>, linux-hardening@...r.kernel.org
Subject: Re: [PATCH v2 08/10] arm64: mm: Don't abuse memblock NOMAP to check
 for overlaps

On 27/01/2026 11:03, Ard Biesheuvel wrote:
> On Tue, 27 Jan 2026 at 12:00, Ryan Roberts <ryan.roberts@....com> wrote:
>>
>> On 27/01/2026 10:47, Ard Biesheuvel wrote:
>>> On Tue, 27 Jan 2026 at 11:39, Ryan Roberts <ryan.roberts@....com> wrote:
>>>>
>>>> On 27/01/2026 10:27, Ard Biesheuvel wrote:
>>>>> On Tue, 27 Jan 2026 at 11:21, Ryan Roberts <ryan.roberts@....com> wrote:
>>>>>>
>>>>>> On 26/01/2026 09:26, Ard Biesheuvel wrote:
>>>>>>> From: Ard Biesheuvel <ardb@...nel.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@...nel.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.
>>>
>>> Indeed.
>>>
>>>> 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?
>>>>
>>>
>>>
>>> OK so toggling the R/O attribute on a live contiguous mapping is
>>> permitted (provided that ultimately, the entire contiguous region is
>>> updated accordingly) but the same doesn't apply to MT_NORMAL vs
>>> MT_NORMAL_TAGGED, right?
>>
>> This is the rule:
>>
>> RJQQTC
>> For a TLB lookup in a contiguous region mapped by translation table entries that
>> have consistent values for the Contiguous bit, but have the OA, attributes, or
>> permissions misprogrammed, that TLB lookup is permitted to produce an OA, access
>> permissions, and memory attributes that are consistent with any one of the
>> programmed translation table values.
>>
>> So, yes, it is fine to modify the attributes (and I assume the memory type
>> counts as attributes) on the live mapping...
>>
> 
> OK.
> 
>>>
>>> So let's use just the same prot and flags for the initial mapping, and
>>> use MT_NORMAL_TAGGED when we remap the text alias R/O.
>>
>> ...but that wasn't my point. I belive the intent is that all of the linear map
>> uses MT_NORMAL_TAGGED except for the kernel text and rodata, which uses
>> MT_NORMAL. In some configs (e.g. force_pte_mapping() == true), your change will
>> result in all the linear map, including text and rodata being MT_NORMAL_TAGGED.
>>
> 
> I didn't think that would matter tbh. But if it does, are we fine as
> long as we change it back to MT_NORMAL by the time we remap the region
> read-only?

I have no idea. I worry MT_NORMAL_TAGGED on the text might imply a performance
cost? But not sure about functional... Will ask around.

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