[PATCH v2 4/5] arm64: mmu: map .text as read-only from the outset

[mark.rutland@arm.com (Mark Rutland)]

On Sat, Feb 11, 2017 at 08:23:05PM +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.
> 
> Reviewed-by: Laura Abbott <labbott@redhat.com>
> Reviewed-by: Kees Cook <keescook@chromium.org>
> Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>

This generally looks good.

One effect of this is that even with rodata=off, external debuggers
can't install SW breakpoints via the executable mapping.

We might want to allow that to be overridden. e.g. make rodata= an
early param, and switch the permissions based on that in map_kernel(),
e.g. have:

	pgprot_t text_prot = rodata_enabled ? PAGE_KERNEL_ROX
					    : PAGE_KERNEL_EXEC);

... and use that for .text and .init.text by default.

Thanks,
Mark.

> ---
>  arch/arm64/mm/mmu.c | 5 +----
>  1 file changed, 1 insertion(+), 4 deletions(-)
> 
> diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c
> index 7ed981c7f4c0..e97f1ce967ec 100644
> --- a/arch/arm64/mm/mmu.c
> +++ b/arch/arm64/mm/mmu.c
> @@ -442,9 +442,6 @@ void mark_rodata_ro(void)
>  {
>  	unsigned long section_size;
>  
> -	section_size = (unsigned long)_etext - (unsigned long)_text;
> -	create_mapping_late(__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.
> @@ -484,7 +481,7 @@ 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);
> +	map_kernel_segment(pgd, _text, _etext, PAGE_KERNEL_ROX, &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
>