Re: [PATCHv2 1/2] arch: Move CONFIG_DEBUG_RODATA and CONFIG_SET_MODULE_RONX to be common

[Russell King - ARM Linux <linux@armlinux.org.uk>]

On Fri, Feb 03, 2017 at 11:45:56AM -0800, Kees Cook wrote:
> On Fri, Feb 3, 2017 at 9:52 AM, Laura Abbott <labbott@redhat.com> wrote:
> > diff --git a/arch/Kconfig b/arch/Kconfig
> > index 99839c2..22ee01e 100644
> > --- a/arch/Kconfig
> > +++ b/arch/Kconfig
> > @@ -781,4 +781,32 @@ config VMAP_STACK
> >           the stack to map directly to the KASAN shadow map using a formula
> >           that is incorrect if the stack is in vmalloc space.
> >
> > +config ARCH_NO_STRICT_RWX_DEFAULTS
> > +       def_bool n
> > +
> > +config ARCH_HAS_STRICT_KERNEL_RWX
> > +       def_bool n
> > +
> > +config DEBUG_RODATA
> > +       def_bool y if !ARCH_NO_STRICT_RWX_DEFAULTS
> > +       prompt "Make kernel text and rodata read-only" if ARCH_NO_STRICT_RWX_DEFAULTS
> 
> Ah! Yes, perfect. I totally forgot about using conditional "prompt"
> lines. Nice!

It's no different from the more usual:

	bool "Make kernel text and rodata read-only" if ARCH_NO_STRICT_RWX_DEFAULTS
	default y if !ARCH_NO_STRICT_RWX_DEFAULTS
	depends on ARCH_HAS_STRICT_KERNEL_RWX

But... I really don't like this - way too many negations and negatives
which make it difficult to figure out what's going on here.

The situation we have today is:

-config DEBUG_RODATA
-       bool "Make kernel text and rodata read-only"
-       depends on MMU && !XIP_KERNEL
-       default y if CPU_V7

which is "allow the user to select DEBUG_RODATA if building a MMU non-XIP
kernel", suggesting that the user turns it on for ARMv7 CPUs.

That changes with this and the above:

+       select ARCH_HAS_STRICT_KERNEL_RWX if MMU && !XIP_KERNEL
+       select ARCH_HAS_STRICT_MODULE_RWX if MMU
+       select ARCH_NO_STRICT_RWX_DEFAULTS if !CPU_V7

This means that ARCH_HAS_STRICT_KERNEL_RWX is set for a MMU non-XIP
kernel, which carries the same pre-condition for DEBUG_RODATA - no
problem there.

However, ARCH_NO_STRICT_RWX_DEFAULTS is set for non-ARMv7 CPUs, which
means the "Make kernel text and rodata read-only" prompt _is_ provided
for those.  However, for all ARMv7 systems, we go from "suggesting that
the user enables the option" to "you don't have a choice, you get this
whether you want it or not."

I'd prefer to keep it off for my development systems, where I don't
care about kernel security.  If we don't wish to do that as a general
rule, can we make it dependent on EMBEDDED?

Given that on ARM it can add up to 4MB to the kernel image - there
_will_ be about 1MB before the .text section, the padding on between
__modver and __ex_table which for me is around 626k, the padding
between .notes and the init sections start with .vectors (the space
between __ex_table and end of .notes is only 4124, which gets padded
up to 1MB) and lastly the padding between the .init section and the
data section (for me around 593k).  This all adds up to an increase
in kernel image size of 3.2MB on 14.2MB - an increase of 22%.

So no, I'm really not happy with that.

-- 
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