On 27/01/2026 10:36, Ard Biesheuvel wrote: > On Tue, 27 Jan 2026 at 11:33, Ryan Roberts <[email protected]> wrote: >> >> On 26/01/2026 09:26, Ard Biesheuvel wrote: >>> From: Ard Biesheuvel <[email protected]> >>> >>> On systems where the bootloader adheres to the original arm64 boot >>> protocol, the placement of the kernel in the physical address space is >>> highly predictable, and this makes the placement of its linear alias in >>> the kernel virtual address space equally predictable, given the lack of >>> randomization of the linear map. >>> >>> The linear aliases of the kernel text and rodata regions are already >>> mapped read-only, but the kernel data and bss are mapped read-write in >>> this region. This is not needed, so map them read-only as well. >>> >>> Note that the statically allocated kernel page tables do need to be >>> modifiable via the linear map, so leave these mapped read-write. >>> >>> Signed-off-by: Ard Biesheuvel <[email protected]> >>> --- >>> arch/arm64/include/asm/sections.h | 1 + >>> arch/arm64/mm/mmu.c | 10 ++++++++-- >>> 2 files changed, 9 insertions(+), 2 deletions(-) >>> >>> diff --git a/arch/arm64/include/asm/sections.h >>> b/arch/arm64/include/asm/sections.h >>> index 51b0d594239e..f7fe2bcbfd03 100644 >>> --- a/arch/arm64/include/asm/sections.h >>> +++ b/arch/arm64/include/asm/sections.h >>> @@ -23,6 +23,7 @@ extern char __irqentry_text_start[], >>> __irqentry_text_end[]; >>> extern char __mmuoff_data_start[], __mmuoff_data_end[]; >>> extern char __entry_tramp_text_start[], __entry_tramp_text_end[]; >>> extern char __relocate_new_kernel_start[], __relocate_new_kernel_end[]; >>> +extern char __pgdir_start[]; >>> >>> static inline size_t entry_tramp_text_size(void) >>> { >>> diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c >>> index 18415d4743bf..fdbbb018adc5 100644 >>> --- a/arch/arm64/mm/mmu.c >>> +++ b/arch/arm64/mm/mmu.c >>> @@ -1122,7 +1122,9 @@ static void __init map_mem(void) >>> { >>> static const u64 direct_map_end = _PAGE_END(VA_BITS_MIN); >>> phys_addr_t kernel_start = __pa_symbol(_text); >>> - phys_addr_t kernel_end = __pa_symbol(__init_begin); >>> + phys_addr_t init_begin = __pa_symbol(__init_begin); >>> + phys_addr_t init_end = __pa_symbol(__init_end); >>> + phys_addr_t kernel_end = __pa_symbol(__pgdir_start); >>> phys_addr_t start, end; >>> phys_addr_t early_kfence_pool; >>> int flags = NO_EXEC_MAPPINGS; >>> @@ -1158,7 +1160,9 @@ static void __init map_mem(void) >>> * Note that contiguous mappings cannot be remapped in this way, >>> * so we should avoid them here. >>> */ >>> - __map_memblock(kernel_start, kernel_end, PAGE_KERNEL, >>> + __map_memblock(kernel_start, init_begin, PAGE_KERNEL, >>> + flags | NO_CONT_MAPPINGS); >>> + __map_memblock(init_end, kernel_end, PAGE_KERNEL, >>> flags | NO_CONT_MAPPINGS); >> >> I'm probably being dumb again... why map [init_end, kernel_end) RW here, >> only to >> remap RO below? Why not just map RO here? >> > > Because the loop that iterates over the memblocks will remap it RW > again anyway, so whether we map RW or RO at this point is irrelevant. > This call just needs to occur here to ensure that no block mappings > needs to be broken up later.
Ahh yes, got it. > > >>> >>> /* map all the memory banks */ >>> @@ -1172,6 +1176,8 @@ static void __init map_mem(void) >>> flags); >>> } >>> >>> + __map_memblock(init_end, kernel_end, PAGE_KERNEL_RO, >>> + flags | NO_CONT_MAPPINGS); >> >> This seems iffy since __map_memblock() doesn't flush the tlb. If you want to >> update an existing mapping you want to be calling update_mapping_prot() >> right? >> > > Fair enough - I'll use that here.
