Re: [PATCH v2] ARM: mm: flip priority of CONFIG_DEBUG_RODATA

[Laura Abbott]

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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