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

[Kees Cook]

On Sun, Mar 23, 2014 at 4:37 PM, Nicolas Pitre <nicolas.pitre@xxxxxxxxxx> wrote:
> On Sun, 23 Mar 2014, Kees Cook wrote:
>
>> On Sun, Mar 23, 2014 at 12:47 PM, Laura Abbott <lauraa@xxxxxxxxxxxxxx> wrote:
>> > On 2/17/2014 4:34 AM, Dave Martin 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.
>> >>
>> >>>
>> >>>> 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.
>> >>
>> >>>
>> >>>> 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.
>> >>
>> >
>> > FWIW, we've hit issues not just with kexec but kprobes as well. The same
>> > problems exist with this series:
>>
>> For this stage, how about I make this "depends on KEXEC=n &&
>> KPROBES=n"? Then as these areas get fixed, we can drop those
>> requirements.
>
> Do they really need "fixing"?
>
> The goal here is to increase security by preventing kernel code to be
> modified.  And now it would require hole punching in order to support
> kprobes.
>
> If security is important enough for this option to be attractive to you,
> then wouldn't you want to keep kprobes firmly turned off as well?

I couldn't agree more. :) I don't build with these options, but if
someone wants both of these, we'll have to deal with that. Until then,
we should keep it disabled with the negative "depends on".

-Kees

-- 
Kees Cook
Chrome OS Security
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