Re: [PATCH 1/2] security: Change name of CONFIG_DEBUG_RODATA

[Laura Abbott]

On 01/19/2017 12:33 PM, Heiko Carstens wrote:
> On Thu, Jan 19, 2017 at 10:56:46AM +0000, Mark Rutland wrote:
> > > +config HARDENED_PAGE_MAPPINGS
> > > +	bool "Mark kernel mappings with stricter permissions (RO/W^X)"
> > > +	default y
> > > +	depends on ARCH_HAS_HARDENED_MAPPINGS
> > > +	help
> > > +          If this is set, kernel text and rodata memory will be made read-only,
> > > +	  and non-text memory will be made non-executable. This provides
> > > +	  protection against certain security attacks (e.g. executing the heap
> > > +	  or modifying text).
> > > +
> > > +	  Unless your system has known restrictions or performance issues, it
> > > +	  is recommended to say Y here.
> >
> > It's somewhat unfortunate that this means the feature is no longer
> > mandatory on arm64 (and s390+x86). We have a boot-time switch to turn
> > the protections off, so I was hoping we could make this mandatory on all
> > architectures with support.
> >
> > It would be good to see if we could make this mandatory for arm and
> > parisc, or if it really needs to be optional for either of those.
>
> Looks like the config option is a no-op on parisc just like it is on
> s390. Irrelavant of the config option at least on s390 the page tables for
> kernel text and rodata will be read-only anyway.
>
> This works since ages and I don't see a reason why this should be
> changed. Also trying to disable this with the "rodata=" command line option
> does not work at least on s390, and I guess this is true for parisc as
> well.
>
> The only thing implemented with CONFIG_DEBUG_RODATA on both architectures
> is to emit a message that states memory has been protected
> (mark_rodata_ro).
> This just avoids a wrong "Kernel memory protection disabled." message.
>
> So yes, I'd really like to keep this option mandatory.

(Apologies, my SMTP server was set up incorrectly so this didn't get
sent out when I thought it did)

Okay, that's useful to know. I think I'm going to add a
'select HARDENED_MAPPINGS' (or whatever it gets changed to) to arches
that were previously def_bool. This is a slight Kconfig semantic change
but as has been pointed out we now have the command line option.

Thanks,
Laura