Message-ID: <CAMj1kXHA_ndSDVtJ+obzh1ijPd_vuKQTiA+S1xZ=aqOcnbitmA@mail.gmail.com>
Date: Tue, 27 Jan 2026 12:03:07 +0100
From: Ard Biesheuvel <ardb@...nel.org>
To: Ryan Roberts <ryan.roberts@....com>
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 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?

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