Re: [kernel-hardening] [PATCH] module: extend 'rodata=off' boot cmdline parameter to module mappings

[Kees Cook]

On Wed, Oct 19, 2016 at 11:24 PM, AKASHI Takahiro
<takahiro.akashi@xxxxxxxxxx> wrote:
> The current "rodata=off" parameter disables read-only kernel mappings
> under CONFIG_DEBUG_RODATA:
>     commit d2aa1acad22f ("mm/init: Add 'rodata=off' boot cmdline parameter
>     to disable read-only kernel mappings")
>
> This patch is a logical extension to module mappings ie. read-only mappings
> at module loading can be disabled even if CONFIG_DEBUG_SET_MODULE_RONX
> (mainly for debug use). Please note, however, that it only affects RO/RW
> permissions, keeping NX set.
>
> This is the first step to make CONFIG_DEBUG_SET_MODULE_RONX mandatory
> (always-on) in the future as CONFIG_DEBUG_RODATA on x86 and arm64.
>
> Suggested-by: Mark Rutland <mark.rutland@xxxxxxx>
> Signed-off-by: AKASHI Takahiro <takahiro.akashi@xxxxxxxxxx>
> Cc: Rusty Russell <rusty@xxxxxxxxxxxxxxx>
> ---
> v1:
>   * remove RFC's "module_ronx=" and merge it with "rodata="
>   * always keep NX set if CONFIG_SET_MODULE_RONX
>
>  include/linux/init.h |  3 ++-
>  init/main.c          |  2 +-
>  kernel/module.c      | 21 ++++++++++++++++++---
>  3 files changed, 21 insertions(+), 5 deletions(-)
>
> diff --git a/include/linux/init.h b/include/linux/init.h
> index e30104c..20aa2eb 100644
> --- a/include/linux/init.h
> +++ b/include/linux/init.h
> @@ -126,7 +126,8 @@ void prepare_namespace(void);
>  void __init load_default_modules(void);
>  int __init init_rootfs(void);
>
> -#ifdef CONFIG_DEBUG_RODATA
> +#if defined(CONFIG_DEBUG_RODATA) || defined(CONFIG_DEBUG_SET_MODULE_RONX)
> +extern bool rodata_enabled;
>  void mark_rodata_ro(void);
>  #endif
>
> diff --git a/init/main.c b/init/main.c
> index 2858be7..92db2f3 100644
> --- a/init/main.c
> +++ b/init/main.c
> @@ -915,7 +915,7 @@ static int try_to_run_init_process(const char *init_filename)
>  static noinline void __init kernel_init_freeable(void);
>
>  #ifdef CONFIG_DEBUG_RODATA
> -static bool rodata_enabled = true;
> +bool rodata_enabled = true;

Is there a mismatch here between the extern ifdef and the bool ifdef?
I.e. shouldn't the ifdef here be || DEBUG_SET_MODULE_RONX too?

Also, can you mark this as __ro_after_init, since nothing changes it
after the kernel command line is parsed?

Otherwise, this looks fine to me.

-Kees


-- 
Kees Cook
Nexus Security