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? 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.
