Re: [RFC PATCH v2 0/4] arm64: mm: Map fixmap page tables read-only

[Ard Biesheuvel]

On Tue, 1 Sep 2026, at 11:21, Kevin Brodsky wrote:
> On 27/08/2026 18:44, Ard Biesheuvel wrote:
>> From: Ard Biesheuvel <ardb@xxxxxxxxxx>
>>
>> This v2 now covers intermediate level page tables as well as the PTE
>> level page table for the fixmap. The latter is a special case, as it
>>
>> a) is only accessed via the kernel image's mapping, and never via the
>>    linear map (except for ptdump etc)
>>
>> b) must be accessible via a read-write mapping, as all manipulation of
>>    read-only page table descriptors relies on the fixmap itself
>>
>> and so it is treated separately. The intermediate page tables may be
>> shared with other mappings in the upper kernel/vmalloc region, so they
>> must be updatable using the ordinary APIs.
>>
>> Build tested and boot tested on a Lenovo Yoga C630 using 16k pages.
>>
>> v1: https://lore.kernel.org/all/20260805104042.1107678-2-ardb+git@xxxxxxxxxx/
>>
>> Cc: Ryan Roberts <ryan.roberts@xxxxxxx>
>> Cc: Anshuman Khandual <anshuman.khandual@xxxxxxx>
>> Cc: Kevin Brodsky <kevin.brodsky@xxxxxxx>
>> Cc: Liz Prucka <lizprucka@xxxxxxxxxx>
>> Cc: Seth Jenkins <sethjenkins@xxxxxxxxxx>
>> Cc: Kees Cook <kees@xxxxxxxxxx>
>> Cc: Jann Horn <jannh@xxxxxxxxxx>
>> Cc: linux-hardening@xxxxxxxxxxxxxxx
>
> Looks like the Cc's didn't propagate to the actual patches, fortunately
> my lei filters did catch this series ;)
>

Ugh I must have forgotten to put --cc-cover

> Either way I quite like this series, it's an elegant approach and it
> should increase security without overhead, what's not to like!
>
> I also considered it from the perspective of kpkeys protection [1] and I
> think they should work together fine. The kpkeys series still allows
> page table setters to write to all page tables, so if we get a fault
> there it must be because the target is read only, and not because of a
> pkey fault.
>

Excellent, thanks for confirming.