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

[Kees Cook]

On Mon, Feb 17, 2014 at 4:34 AM, Dave Martin <Dave.Martin@xxxxxxx> wrote:
> On Fri, Feb 14, 2014 at 11:11:07AM -0800, Kees Cook wrote:
>> On Fri, Feb 14, 2014 at 8:22 AM, Dave Martin <Dave.Martin@xxxxxxx> 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.
>
> We're not writing into code exactly.
>
> This code is never executed in-place in vmlinux.  It gets copied, and
> only copies are ever executed.
>
> Some pointers and offsets get poked into the code to configure it.
>
> I think it would be better simply to put the code in .rodata, and
> poke paramaters into the copy, not the original -- but that's a bit
> more awkward to code up, since the values can't be poked simply by
> writing global variables.

Okay, interesting. I'll be curious to see what the patch for this looks like.

>> > 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.
>
> I haven't tested exhaustively, but it think this is sufficient for a
> Tested-by.  The patch does seem to be doing what it is intended to
> do, and doesn't seem to be triggering false positives all over the
> place.

Great, thanks for taking the time to check on it!

Should I send this to the patch tracker, or wait for more feedback?

>> > Are you aware of similar situations on other arches?
>>
>> I think there were some problems a long time ago on x86 for rodata too.
>
> It would be good to get this kexec case fixed -- I'll try to hack up
> a separate patch.

Thanks!

-Kees

-- 
Kees Cook
Chrome OS Security
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