On Tue, 27 Jan 2026 at 12:00, Ryan Roberts <[email protected]> wrote: > > On 27/01/2026 10:47, Ard Biesheuvel wrote: > > On Tue, 27 Jan 2026 at 11:39, Ryan Roberts <[email protected]> wrote: > >> > >> On 27/01/2026 10:27, Ard Biesheuvel wrote: > >>> On Tue, 27 Jan 2026 at 11:21, Ryan Roberts <[email protected]> wrote: > >>>> > >>>> On 26/01/2026 09:26, Ard Biesheuvel wrote: > >>>>> From: Ard Biesheuvel <[email protected]> > >>>>> > >>>>> 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 <[email protected]> > >>>>> --- > >>>>> 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?
