Re: [PATCH v2 1/4] init: create cmdline param to disable readonly

Kees Cook
2015-11-30T21:52:13Z

On Wed, Nov 25, 2015 at 11:51 PM, Ingo Molnar <mingo@kernel.org> wrote:
> * Kees Cook <keescook@chromium.org> wrote:
>> +#ifdef CONFIG_DEBUG_RODATA
>
> Btw., could you please remove the Kconfig option altogether in an additional patch
> and make read-only sections an always-on feature? It has been default-y for years
> and all distros have it enabled.

Yeah, this is something I've wanted to do for a while, but I would
point out that only a few architectures have actually implemented it,
and for arm and arm64 it was very recent:

$ git grep 'config DEBUG_RODATA'
arch/arm/mm/Kconfig:config DEBUG_RODATA
arch/arm64/Kconfig.debug:config DEBUG_RODATA
arch/parisc/Kconfig.debug:config DEBUG_RODATA
arch/x86/Kconfig.debug:config DEBUG_RODATA

I think s390 already has strict kernel memory permissions, but they
set it up ahead of time. And now, I see in reading the parisc tree,
they do too, and mark_rodata_ro() is effectively a no-op. How does
powerpc handle permissions for kernel rodata?

For parisc (and maybe powerpc and s390) we'll need additional changes
to support __ro_after_init, since they may be making the ro section ro
_before_ init runs. But, that's okay since this series only uses
__ro_after_init on x86 for the moment. ;)

> The 'debug rodata' naming is purely historic: this started out as a simple
> debugging feature, but meanwhile it has spread and has become an essential kernel
> robustness feature.

I agree completely. I suspect I would turn this into
ARCH_HAS_STRICT_KERNMEM or something.

-Kees

--
Kees Cook
Chrome OS & Brillo Security