Re: [PATCH 0/1] Zero initialise kernel stack variables

[Kees Cook <keescook@chromium.org>]

On Tue, Feb 27, 2018 at 3:26 PM, Laura Abbott <labbott@redhat.com> wrote:
> On 02/27/2018 11:28 AM, Kees Cook wrote:
>>
>> On Tue, Feb 27, 2018 at 3:15 AM, P J P <ppandit@redhat.com> wrote:
>>>
>>> Hello,
>>
>>
>> Hi!
>>
>>>
>>> Please see:
>>>    -> https://gcc.gnu.org/ml/gcc-patches/2014-06/msg00615.html
>>>
>>> This experimental patch by Florian Weimer(CC'd) adds an option
>>> '-finit-local-vars' to gcc(1) compiler. When a program(or kernel)
>>> is built using this option, its automatic(local) variables are
>>> initialised with zero(0). This could significantly reduce the kernel
>>> information leakage issues.
>>>
>>> A dnf(8) repository of the latest gcc-7.3.1 package built with the above
>>> patch and kernel-4.15.5 package built using '-finit-local-vars' option
>>> on Fedora-27 is available below
>>>
>>>    -> https://pjp.fedorapeople.org/init-vars/
>>>
>>> This same kernel is running on my F27 test machine as I write this.
>>> There is no slowness or notice-able performance impact as such.
>>
>>
>> Unfortunately "noticeable" isn't going to be a viable metric. You'll
>> need to do some real-world benchmarks (i.e. kernel builds, hackbench,
>> etc), and compare the results. Even just initializing
>> passed-by-reference variables (GCC_PLUGIN_STRUCTLEAK_BYREF_ALL) had
>> measurable performance impact.
>>
>
>
> For comparison (-300 is official Fedora, -301 is from the repo):
>
>    text    data     bss     dec     hex filename
> 16948437        6771094 1777872 25497403        1850f3b
> /lib/debug/lib/modules/4.15.5-300.fc27.x86_64/vmlinux
> 16970359        6776078 1777872 25524309        1857855
> /lib/debug/lib/modules/4.15.5-301.fc27.x86_64/vmlinux

That's a surprisingly small text change! I'd love to see benchmarks
too. Are you able to verify this is initializing the
passed-by-reference variables too? Hmm, I suspect it's time for
another LKDTM test. ;)

-Kees

-- 
Kees Cook
Pixel Security