Re: [PATCH] x86, kernel: make dump_pagetables a tristate

[Arjan van de Ven <arjan@linux.intel.com>]

On 7/1/2013 8:55 AM, Kees Cook wrote:
> On Mon, Jul 1, 2013 at 6:58 AM, Arjan van de Ven <arjan@linux.intel.com> wrote:
>> On 6/29/2013 9:05 PM, Kees Cook wrote:
>>>
>>> Being able to examine page tables is handy, so make this a module that
>>> can be loaded as needed.
>>
>> I personally don't think this is a good idea due to the various
>> security/etc implications of this feature... should really just
>> be off for non-debug kernels, not "off unless you load the module"
>
> I struggled with this too, but I couldn't come up with any reason that
> made sense. If a system is running without modules_disabled, this code
> is still loadable:
> https://www.outflux.net/blog/archives/2011/04/27/non-executable-kernel-memory-progress/
>
> The root user just needs to look at /proc/kallsyms before passing an
> argument. So having it NOT a tristate doesn't actually change anything
> except make it awkward to get it done.
>
> If a system is running with verified modules, then just not
> signing/including ptdump makes it unavailable. And running with
> modules_disabled, obviously, blocks it.
>
>>> +#ifdef CONFIG_X86_64
>>> +EXPORT_SYMBOL_GPL(init_level4_pgt);
>>> +#else
>>> +EXPORT_SYMBOL_GPL(swapper_pg_dir);
>>> +#endif
>>
>> like these really have no business in any module
>
> Well, that's why I took me 2 years to send this patch. Those symbols
> shouldn't be used outside of page table debugging, so it didn't really
> seem upstreamable. However, now that I need to do regular examination
> of the page tables, I wanted to do it without the hacky thing above. I
> want to do at will on our test images (we use the same kernel for
> production and test, but production images leave out the test modules,
> etc).

the code is small...
how about making it a command line option to enable?

rather than making something like this a module