Re: [PATCH v6 11/15] arm64: mm: Map the kernel data/bss read-only in the linear map

[Kevin Brodsky]

On 26/05/2026 19:58, Ard Biesheuvel wrote:
> From: Ard Biesheuvel <ardb@xxxxxxxxxx>
>
> 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@xxxxxxxxxx>

Reviewed-by: Kevin Brodsky <kevin.brodsky@xxxxxxx>

> ---
>  arch/arm64/mm/mmu.c | 15 +++++++++++++--
>  1 file changed, 13 insertions(+), 2 deletions(-)
>
> diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c
> index b20c76b8381d..e7ca53d20b87 100644
> --- a/arch/arm64/mm/mmu.c
> +++ b/arch/arm64/mm/mmu.c
> @@ -1138,7 +1138,9 @@ 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(__bss_stop);
>  	phys_addr_t start, end;
>  	int flags = NO_EXEC_MAPPINGS;
>  	u64 i;
> @@ -1171,7 +1173,11 @@ static void __init map_mem(void)
>  	 * of the region accessible to subsystems such as hibernate,
>  	 * but protects it from inadvertent modification or execution.
>  	 */
> -	__map_memblock(kernel_start, kernel_end, PAGE_KERNEL, flags);
> +	__map_memblock(kernel_start, init_begin, PAGE_KERNEL, flags);
> +
> +	/* Map the kernel data/bss so it can be remapped later */
> +	__map_memblock(init_end, kernel_end, pgprot_tagged(PAGE_KERNEL),
> +		       flags);
>  
>  	/* map all the memory banks */
>  	for_each_mem_range(i, &start, &end) {
> @@ -1183,6 +1189,11 @@ static void __init map_mem(void)
>  		__map_memblock(start, end, pgprot_tagged(PAGE_KERNEL),
>  			       flags);
>  	}
> +
> +	/* Map the kernel data/bss read-only in the linear map */
> +	__map_memblock(init_end, kernel_end, PAGE_KERNEL_RO, flags);
> +	flush_tlb_kernel_range((unsigned long)lm_alias(__init_end),
> +			       (unsigned long)lm_alias(__bss_stop));
>  }
>  
>  void mark_rodata_ro(void)