Re: [PATCH 1/5] gcc-plugins/stackleak: Exclude alloca() from the instrumentation logic

Alexander Popov
2020-06-10T15:24:33Z

On 09.06.2020 21:39, Kees Cook wrote:
> On Thu, Jun 04, 2020 at 06:23:38PM +0300, Alexander Popov wrote:
>> On 04.06.2020 17:01, Jann Horn wrote:
>>> On Thu, Jun 4, 2020 at 3:51 PM Alexander Popov <alex.popov@linux.com> wrote:
>>>> Some time ago Variable Length Arrays (VLA) were removed from the kernel.
>>>> The kernel is built with '-Wvla'. Let's exclude alloca() from the
>>>> instrumentation logic and make it simpler. The build-time assertion
>>>> against alloca() is added instead.
>>> [...]
>>>> + /* Variable Length Arrays are forbidden in the kernel */
>>>> + gcc_assert(!is_alloca(stmt));
>>>
>>> There is a patch series from Elena and Kees on the kernel-hardening
>>> list that deliberately uses __builtin_alloca() in the syscall entry
>>> path to randomize the stack pointer per-syscall - see
>>> <https://lore.kernel.org/kernel-hardening/20200406231606.37619-4-keescook@chromium.org/>.
>>
>> Thanks, Jann.
>>
>> At first glance, leaving alloca() handling in stackleak instrumentation logic
>> would allow to integrate stackleak and this version of random_kstack_offset.
>
> Right, it seems there would be a need for this coverage to remain,
> otherwise the depth of stack erasure might be incorrect.
>
> It doesn't seem like the other patches strictly depend on alloca()
> support being removed, though?

Ok, I will leave alloca() support, reorganize the patch series and send v2.

>> Kees, Elena, did you try random_kstack_offset with upstream stackleak?
>
> I didn't try that combination yet, no. It seemed there would likely
> still be further discussion about the offset series first (though the
> thread has been silent -- I'll rebase and resend it after rc2).

Ok, please add me to CC list.

Best regards,
Alexander

>> It looks to me that without stackleak erasing random_kstack_offset can be
>> weaker. I mean, if next syscall has a bigger stack randomization gap, the data
>> on thread stack from the previous syscall is not overwritten and can be used. Am
>> I right?
>
> That's correct. I think the combination is needed, but I don't think
> they need to be strictly tied together.
>
>> Another aspect: CONFIG_STACKLEAK_METRICS can be used for guessing kernel stack
>> offset, which is bad. It should be disabled if random_kstack_offset is on.
>
> Agreed.