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

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Date:	Mon, 30 Nov 2015 13:52:10 -0800
From:	Kees Cook <keescook@...omium.org>
To:	Ingo Molnar <mingo@...nel.org>,
	Heiko Carstens <heiko.carstens@...ibm.com>,
	Michael Ellerman <mpe@...erman.id.au>,
	"James E.J. Bottomley" <jejb@...isc-linux.org>,
	Catalin Marinas <catalin.marinas@....com>,
	Russell King - ARM Linux <linux@....linux.org.uk>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Andy Lutomirski <luto@...capital.net>,
	"H. Peter Anvin" <hpa@...or.com>,
	Mathias Krause <minipli@...glemail.com>,
	Ingo Molnar <mingo@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	"x86@...nel.org" <x86@...nel.org>, Arnd Bergmann <arnd@...db.de>,
	PaX Team <pageexec@...email.hu>,
	Emese Revfy <re.emese@...il.com>,
	"kernel-hardening@...ts.openwall.com" 
	<kernel-hardening@...ts.openwall.com>,
	linux-arch <linux-arch@...r.kernel.org>
Subject: Re: [PATCH v2 1/4] init: create cmdline param to disable readonly

On Wed, Nov 25, 2015 at 11:51 PM, Ingo Molnar <mingo@...nel.org> wrote:
> * Kees Cook <keescook@...omium.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
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