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. 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? Thanks, Ryan
