[RFC PATCH] arm64: mm: Map fixmap PTE tables r/o in the linear map

[Kevin Brodsky]

On 27/08/2026 11:04, Ard Biesheuvel wrote:
> On Thu, 27 Aug 2026, at 10:55, Kevin Brodsky wrote:
>> On 26/08/2026 11:07, Ard Biesheuvel wrote:
>>>>> [...]
>>>>>
>>>>> +static int __init fixmap_remap_ro(void)
>>>>> +{
>>>>> +	return set_direct_map_ro((unsigned long)lm_alias(&bm_pte), NR_BM_PTE_TABLES);
>>>> Should we not also remap bm_pmd and bm_pud?
>>>>
>>>> For that matter, do we need RW access via the linear map for any page
>>>> annotated with __bss_pgtbl? I suppose that might be the case for
>>>> kasan_early_shadow_* but I don't know enough about KASAN to tell for sure.
>>>>
>>> bm_pte[] is special because it is only ever written via the kernel mapping,
>>> and never via the linear map. This is why it is being singled out in this
>>> patch.
>> Right I see __set_fixmap(). This is a very special case of calling
>> __set_pte() on a pointer derived from a global (i.e. pointing to the
>> kernel image and not the LM) so it makes sense to treat it differently.
>>
>> I do wonder whether there is that much value in protecting bm_pte while
>> all page tables in init_pg_dir (also at a fixed offset in the LM) remain
>> writeable though.
>>
> Yes, so those need to be taken care of as well.

But init_pg_dir needs to remain writeable in the LM, right?

>> Either way all this is interesting for my series protecting page tables
>> with pkeys [1] - it seems that to protect the fixmap, the easiest option
>> would be to change the pkey of .pgtlb in both the kernel image and LM
>> (the former for bm_pte, and the latter for everything else).
>>
>> [1] https://lore.kernel.org/all/20260818-kpkeys-v9-0-743ad31b2c8f@arm.com/
>>
> Thanks, I've been meaning to go through those but they are a bit overwhelming :-)

Yes there's a fair bit to go through, sorry for that... You'll probably
be most interested in patch 12, 13 and 19-21 - that's where page tables
are being mapped with a special pkey. The cover letter should give
enough context for them to make sense on their own.

>>> Whether or not bm_pmd[] can be treated as a special case depends on the page
>>> size: with 4k pages, the whole array covers a virtual region of 1G, which is
>>> currently guaranteed to be shared only with the PCI I/O space (but we could
>>> move that out). With 16k pages, it covers 64G, and so it is shared with the
>>> vmemmap and other virtual mappings in the vmalloc region, and so the current
>>> kernel mapping code expects to be able to write those entries.
>>>
>>> What we might do is generalize the logic that uses the fixmap to modify
>>> pgd level entries in swapper_pg_dir, and use it for all modifications
>>> at PMD level or higher if those tables are in .rodata
>>>
>>> But this is a bit more complicated than this change, so I decided to
>>> present this as a separate change.
>> Definitely, this is orthogonal to this patch. I wonder what the
>> performance impact would be if we used the fixmap for setting all kernel
>> entries at PMD and above. It would be nice to keep that logic easily
>> togglable, because with the kpkeys approach I mentioned above we have a
>> (most likely) much cheaper way of protecting these page tables.
>>
> I'm currently experimenting with using __put_kernel_nofault() to update
> the descriptors at all levels (including PTE) and handling the fault
> using the fixmap approach. That way, all statically allocated page
> tables could move to .rodata (except bm_pte[]) and there should be no
> substantial performance impact.

Doesn't that still mean modifying a fixmap PTE every time we update
kernel page tables? That doesn't exactly sound cheap.

>>>>> +}
>>>>> +late_initcall(fixmap_remap_ro);
>>>> Is mark_rodata_ro() definitely too early to remap these pages RO?
>>>>
>>> No, we might just call this from there, afaict.
>> Sounds good, could we then use update_mapping_prot()? I suppose not as
>> that would require mapping bm_pte separately in map_mem()?
>>
> Yeah, but perhaps we should do the latter anyway, so we can remap the
> region even if !can_set_direct_map().

Yep makes sense. Not having to add yet another set_direct_map_* function
is definitely welcome ;)

- Kevin