linux-kernel - Re: [PATCH] x86, kernel: make dump

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Message-ID: <CAGXu5jJw==6ypJVa3YQu27xBidfbgg4tO6AkJia34CwHF=OOfg@mail.gmail.com>
Date:	Mon, 1 Jul 2013 08:55:03 -0700
From:	Kees Cook <keescook@...omium.org>
To:	Arjan van de Ven <arjan@...ux.intel.com>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>, "x86@...nel.org" <x86@...nel.org>
Subject: Re: [PATCH] x86, kernel: make dump_pagetables a tristate

On Mon, Jul 1, 2013 at 6:58 AM, Arjan van de Ven <arjan@...ux.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).

-Kees

--
Kees Cook
Chrome OS Security
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