linux-kernel - Re: [PATCH v2] ARM: mm: flip priority of CONFIG_DEBUG

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Date:	Wed, 23 Dec 2015 13:26:14 -0800
From:	Laura Abbott <labbott@...hat.com>
To:	Russell King - ARM Linux <linux@....linux.org.uk>,
	Kees Cook <keescook@...omium.org>
Cc:	Laura Abbott <labbott@...oraproject.org>,
	Catalin Marinas <catalin.marinas@....com>,
	linux-arm-kernel@...ts.infradead.org,
	LKML <linux-kernel@...r.kernel.org>,
	Linux-MM <linux-mm@...ck.org>,
	Ard Biesheuvel <ard.biesheuvel@...aro.org>,
	Will Deacon <will.deacon@....com>,
	Nicolas Pitre <nico@...aro.org>, Arnd Bergmann <arnd@...db.de>,
	kernel-hardening@...ts.openwall.com
Subject: Re: [PATCH v2] ARM: mm: flip priority of CONFIG_DEBUG_RODATA

On 12/23/2015 12:15 PM, Russell King - ARM Linux wrote:
> On Wed, Dec 02, 2015 at 12:27:25PM -0800, Kees Cook wrote:
>> The use of CONFIG_DEBUG_RODATA is generally seen as an essential part of
>> kernel self-protection:
>> http://www.openwall.com/lists/kernel-hardening/2015/11/30/13
>> Additionally, its name has grown to mean things beyond just rodata. To
>> get ARM closer to this, we ought to rearrange the names of the configs
>> that control how the kernel protects its memory. What was called
>> CONFIG_ARM_KERNMEM_PERMS is really doing the work that other architectures
>> call CONFIG_DEBUG_RODATA.
>
> Kees,
>
> There is a subtle problem with the kernel memory permissions and the
> DMA debugging.
>
> DMA debugging checks whether we're trying to do DMA from the kernel
> mappings (text, rodata, data etc).  It checks _text.._etext.  However,
> when RODATA is enabled, we have about one section between _text and
> _stext which are freed into the kernel's page pool, and then become
> available for allocation and use for DMA.
>
> This then causes the DMA debugging sanity check to fire.
>
> So, I think I'll revert this change for the time being as it seems to
> be causing many people problems, and having this enabled is creating
> extra warnings when kernel debug options are enabled along with it.
>
> Sorry.
>

in include/asm-generic/sections.h:

/*
  * Usage guidelines:
  * _text, _data: architecture specific, don't use them in arch-independent code
  * [_stext, _etext]: contains .text.* sections, may also contain .rodata.*
  *                   and/or .init.* sections


So based on that comment it seems like the dma-debug should be checking for
_stext not _text since only _stext is guaranteed across all architectures.
I'll submit a patch to dma-debug.c if this seems appropriate or if you
haven't done so already.

Thanks,
Laura
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