Re: [kernel-hardening] Re: [PATCH v9 1/4] syscalls: Verify address limit before returning to user-mode

Kees Cook
2017-05-08T15:26:23Z

On Mon, May 8, 2017 at 8:22 AM, Daniel Micay <danielmicay@gmail.com> wrote:
> On Mon, 2017-05-08 at 09:52 +0200, Ingo Molnar wrote:
>>
>> ... it's just not usable in that form for a regular maintenance flow.
>>
>> So what would be more useful is to add a specific Sparse check that
>> only checks
>> KERNEL_DS, to add it as a regular (.config driven) build option and
>> make sure the
>> kernel build has zero warnings.
>>
>> From that point on we can declare that this kind of bug won't occur
>> anymore, if
>> the Sparse implementation of the check is correct.
>>
>> But there's a (big) problem with that development model: Sparse is not
>> part of the
>> kernel tree and adding a feature to it while making the kernel depend
>> on that
>> brand new feature is a logistical nightmare. The overhead is quite
>> similar to
>> adding new features to a compiler - it happens at a glacial pace and
>> is only done
>> for major features really, at considerable expense. I don't think this
>> is an
>> adequate model for 'extended syntax checking' of the kernel,
>> especially when it
>> comes to correctness that has such obvious security impact.
>>
>> Thanks,
>>
>> Ingo
>
> There's the option of using GCC plugins now that the infrastructure was
> upstreamed from grsecurity. It can be used as part of the regular build
> process and as long as the analysis is pretty simple it shouldn't hurt
> compile time much.

Well, and that the situation may arise due to memory corruption, not
from poorly-matched set_fs() calls, which static analysis won't help
solve. We need to catch this bad kernel state because it is a very bad
state to run in.

-Kees

--
Kees Cook
Pixel Security