Re: [kernel-hardening] Re: [PATCH] lkdtm: Add tests for LIST_POISON and ZERO_SIZE_PTR

[Michael Ellerman <mpe@ellerman.id.au>]

Kees Cook <keescook@chromium.org> writes:

> On Thu, Nov 17, 2016 at 2:53 AM, Michael Ellerman <mpe@ellerman.id.au> wrote:
>> kbuild test robot <lkp@intel.com> writes:
>>> [auto build test ERROR on char-misc/char-misc-testing]
>>> [also build test ERROR on v4.9-rc5]
>>> [cannot apply to next-20161115]
>>> [if your patch is applied to the wrong git tree, please drop us a note to help improve the system]
>>>
>>> url:    https://github.com/0day-ci/linux/commits/Michael-Ellerman/lkdtm-Add-tests-for-LIST_POISON-and-ZERO_SIZE_PTR/20161115-235441
>>> config: i386-randconfig-i0-201646 (attached as .config)
>>> compiler: gcc-4.8 (Debian 4.8.4-1) 4.8.4
>>> reproduce:
>>>         # save the attached .config to linux build tree
>>>         make ARCH=i386
>>>
>>> All errors (new ones prefixed by >>):
>>>
>>>>> ERROR: "mmap_min_addr" [drivers/misc/lkdtm.ko] undefined!
>>
>> I couldn't see what was causing this, but of course it's just that
>> mmap_min_addr is not exported, and in that config LKDTM is a module.
>>
>> I always build it in which is why I didn't notice.
>>
>> I'm not sure we want to EXPORT_SYMBOL mmap_min_addr just for this test
>> do we? I could change the test to use mmap_min_addr if LKDTM is
>> built-in, and otherwise just map at zero. Dunno.
>
> Because of lkdtm's moduleness, I've had to put a lot of weird stuff
> into EXPORT_SYMBOL_GPL(). mmap_min_addr is more sensitive than most,
> though. How about just hard-coding it as 64k instead of using
> mmap_min_addr?

We can just use CONFIG_DEFAULT_MMAP_MIN_ADDR, which should work unless
someone's actually changed the value at runtime. Will test.

I'd rather that than 64K hard-coded, as I'm thinking of increasing the
minimum on ppc64.

cheers