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

[AKASHI Takahiro <takahiro.akashi@linaro.org>]

On Thu, Oct 20, 2016 at 01:48:15PM -0700, Kees Cook wrote:
> On Wed, Oct 19, 2016 at 11:24 PM, AKASHI Takahiro
> <takahiro.akashi@linaro.org> 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@arm.com>
> > Signed-off-by: AKASHI Takahiro <takahiro.akashi@linaro.org>
> > Cc: Rusty Russell <rusty@rustcorp.com.au>
> > ---
> > 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?

Yes.

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

Yes, yes.

Thanks,
-Takahiro AKASHI

> Otherwise, this looks fine to me.
> 
> -Kees
> 
> 
> -- 
> Kees Cook
> Nexus Security