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

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <CAMj1kXEJPPWe4JE7SBYuQMcdTwQHvPqBO9Djj=tcW3DsNknkpw@mail.gmail.com>
Date: Tue, 27 Jan 2026 11:47:05 +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 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?

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.