linux-kernel - Re: [PATCH 2/2] ARM: mm: keep rodata non-executable

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Date:	Fri, 14 Feb 2014 11:11:07 -0800
From:	Kees Cook <keescook@...omium.org>
To:	Dave Martin <Dave.Martin@....com>
Cc:	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	Catalin Marinas <catalin.marinas@....com>,
	Will Deacon <will.deacon@....com>,
	Larry Bassel <lbassel@...eaurora.org>,
	Stephen Rothwell <sfr@...b.auug.org.au>,
	Russell King <linux@....linux.org.uk>,
	Nicolas Pitre <nico@...aro.org>,
	Ben Dooks <ben.dooks@...ethink.co.uk>,
	Uwe Kleine-König 
	<u.kleine-koenig@...gutronix.de>,
	Grant Likely <grant.likely@...aro.org>,
	Jiang Liu <liuj97@...il.com>,
	Christoffer Dall <cdall@...columbia.edu>,
	Laura Abbott <lauraa@...eaurora.org>,
	Marc Zyngier <marc.zyngier@....com>,
	Rob Herring <rob.herring@...xeda.com>,
	Vitaly Andrianov <vitalya@...com>,
	Jonathan Austin <jonathan.austin@....com>,
	Simon Baatz <gmbnomis@...il.com>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Santosh Shilimkar <santosh.shilimkar@...com>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH 2/2] ARM: mm: keep rodata non-executable

On Fri, Feb 14, 2014 at 8:22 AM, Dave Martin <Dave.Martin@....com> wrote:
> On Thu, Feb 13, 2014 at 05:04:10PM -0800, Kees Cook wrote:
>> Introduce "CONFIG_DEBUG_RODATA" to mostly match the x86 config, though
>> the behavior is different: it depends on STRICT_KERNMEM_PERMS, which
>> sets rodata read-only (but executable), where as this option additionally
>> splits rodata from the kernel text (resulting in potentially more memory
>> lost to padding) and sets it non-executable as well. The end result is
>> that on builds with CONFIG_DEBUG_RODATA=y (like x86) the rodata with be
>> marked purely read-only.
>
> This triggers an Oops in kexec, because we have a block of code in .text
> which is a template for generating baremetal code to relocate the new
> kernel, and some literal words are written into it before copying.

You're writing into the text area? I would imagine that
CONFIG_ARM_KERNMEM_PERMS would break that. However, that's not the
right place to be building code -- shouldn't the module area be used
for that?

> Possibly this should be in .rodata, not .text.

Well, rodata should be neither writable nor executable.

> There may be a few other instances of this kind of thing.

This config will certainly find them! :) But, that's why it's behind a config.

> Are you aware of similar situations on other arches?

I think there were some problems a long time ago on x86 for rodata too.

-Kees

-- 
Kees Cook
Chrome OS Security
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