[kernel-hardening] Re: [PATCH v4 4/6] arm64: mmu: map .text as read-only from the outset

[Mark Rutland <mark.rutland@arm.com>]

On Sat, Mar 04, 2017 at 02:30:46PM +0000, Ard Biesheuvel wrote:
> Now that alternatives patching code no longer relies on the primary
> mapping of .text being writable, we can remove the code that removes
> the writable permissions post-init time, and map it read-only from
> the outset.
> 
> To preserve the existing behavior under rodata=off, which is relied
> upon by external debuggers to manage software breakpoints (as pointed
> out by Mark), add an early_param() check for rodata=, and use RWX
> permissions if it set to 'off'.
> 
> Reviewed-by: Laura Abbott <labbott@redhat.com>
> Reviewed-by: Kees Cook <keescook@chromium.org>
> Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
> ---
>  arch/arm64/mm/mmu.c | 13 +++++++++----
>  1 file changed, 9 insertions(+), 4 deletions(-)
> 
> diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c
> index df377fbe464e..edd982f88714 100644
> --- a/arch/arm64/mm/mmu.c
> +++ b/arch/arm64/mm/mmu.c
> @@ -416,9 +416,6 @@ void mark_rodata_ro(void)
>  {
>  	unsigned long section_size;
>  
> -	section_size = (unsigned long)_etext - (unsigned long)_text;
> -	update_mapping_prot(__pa_symbol(_text), (unsigned long)_text,
> -			    section_size, PAGE_KERNEL_ROX);
>  	/*
>  	 * mark .rodata as read only. Use __init_begin rather than __end_rodata
>  	 * to cover NOTES and EXCEPTION_TABLE.
> @@ -451,6 +448,12 @@ static void __init map_kernel_segment(pgd_t *pgd, void *va_start, void *va_end,
>  	vm_area_add_early(vma);
>  }
>  
> +static int __init parse_rodata(char *arg)
> +{
> +	return strtobool(arg, &rodata_enabled);
> +}
> +early_param("rodata", parse_rodata);
> +
>  /*
>   * Create fine-grained mappings for the kernel.
>   */
> @@ -458,7 +461,9 @@ static void __init map_kernel(pgd_t *pgd)
>  {
>  	static struct vm_struct vmlinux_text, vmlinux_rodata, vmlinux_init, vmlinux_data;
>  
> -	map_kernel_segment(pgd, _text, _etext, PAGE_KERNEL_EXEC, &vmlinux_text);
> +	pgprot_t text_prot = rodata_enabled ? PAGE_KERNEL_ROX : PAGE_KERNEL_EXEC;
> +

It might be worth having a comment as to why, e.g.

	/*
	 * External debuggers may need to write directly to the text
	 * mapping to install SW breakpoints. Allow this (only) when
	 * explicitly requested with rodata=off.
	 */

... otherwise this looks fine. FWIW, either way:

Reviewed-by: Mark Rutland <mark.rutland@arm.com>

Thanks,
Mark.

> +	map_kernel_segment(pgd, _text, _etext, text_prot, &vmlinux_text);
>  	map_kernel_segment(pgd, __start_rodata, __init_begin, PAGE_KERNEL, &vmlinux_rodata);
>  	map_kernel_segment(pgd, __init_begin, __init_end, PAGE_KERNEL_EXEC,
>  			   &vmlinux_init);
> -- 
> 2.7.4
>