[PATCH] ARM: mm: mark section-aligned portion of rodata NX

[Ard Biesheuvel]

On 7 December 2015 at 21:54, Kees Cook <keescook at chromium.org> wrote:
> When rodata is large enough that it crosses a section boundary after the
> kernel text, mark the rest NX. This is as close to full NX of rodata as
> we can get without splitting page tables or doing section alignment via
> CONFIG_DEBUG_ALIGN_RODATA.
>
> Signed-off-by: Kees Cook <keescook at chromium.org>
> ---
> I am baffled why I can't put the ALIGN in the ".start = " initializer.
> GCC seems to think it's non-static, but only because of the "&" operator.
> Does anyone see a way to do this that doesn't require the runtime ALIGN?

Yes, but you need to move the ALIGN() expression to the linker script
as the value of a new symbol

> ---
>  arch/arm/mm/init.c | 9 ++++++---
>  1 file changed, 6 insertions(+), 3 deletions(-)
>
> diff --git a/arch/arm/mm/init.c b/arch/arm/mm/init.c
> index 321d3683dc7c..b40a0536ef60 100644
> --- a/arch/arm/mm/init.c
> +++ b/arch/arm/mm/init.c
> @@ -579,6 +579,8 @@ struct section_perm {
>         pmdval_t clear;
>  };
>
> +#define RODATA_IDX     0
> +
>  static struct section_perm nx_perms[] = {
>         /* Make pages tables, etc before _stext RW (set NX). */
>         {
> @@ -596,16 +598,14 @@ static struct section_perm nx_perms[] = {
>                 .mask   = ~PMD_SECT_XN,
>                 .prot   = PMD_SECT_XN,
>         },
> -#ifdef CONFIG_DEBUG_ALIGN_RODATA
>         /* Make rodata NX (set RO in ro_perms below). */
> -       {
> +       [RODATA_IDX] = {
>                 .name   = "rodata NX",
>                 .start  = (unsigned long)__start_rodata,
>                 .end    = (unsigned long)__init_begin,
>                 .mask   = ~PMD_SECT_XN,
>                 .prot   = PMD_SECT_XN,
>         },
> -#endif
>  };
>
>  static struct section_perm ro_perms[] = {
> @@ -709,6 +709,9 @@ int __fix_kernmem_perms(void *unused)
>
>  void fix_kernmem_perms(void)
>  {
> +       /* rodata may not be section aligned, so cover as much as we can. */
> +       nx_perms[RODATA_IDX].start = ALIGN(nx_perms[RODATA_IDX].start,
> +                                          SECTION_SIZE);
>         stop_machine(__fix_kernmem_perms, NULL, NULL);
>  }
>
> --
> 1.9.1
>
>
> --
> Kees Cook
> Chrome OS & Brillo Security