Re: [PATCH v2 09/10] arm64: mm: Map the kernel data/bss read-only in the linear map

[Ryan Roberts <ryan.roberts@arm.com>]

On 26/01/2026 09:26, Ard Biesheuvel wrote:
> From: Ard Biesheuvel <ardb@kernel.org>
> 
> 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 <ardb@kernel.org>
> ---
>  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?

>  
>  	/* 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?

Thanks,
Ryan

>  	arm64_kfence_map_pool(early_kfence_pool);
>  }
>