Re: [RFC v4 PATCH 00/13] HARDENED_ATOMIC

[Kees Cook]

On Thu, Nov 10, 2016 at 12:37 PM, Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
> On Thu, Nov 10, 2016 at 10:24:35PM +0200, Elena Reshetova wrote:
>> This series brings the PaX/Grsecurity PAX_REFCOUNT
>> feature support to the upstream kernel. All credit for the
>> feature goes to the feature authors.
>>
>> The name of the upstream feature is HARDENED_ATOMIC
>> and it is configured using CONFIG_HARDENED_ATOMIC and
>> HAVE_ARCH_HARDENED_ATOMIC.
>>
>> This series only adds x86 support; other architectures are expected
>> to add similar support gradually.
>>
>> More information about the feature can be found in the following
>> commit messages.
>
> No, this should be here. As it stands this is completely without
> content.
>
> In any case, NAK on this approach. Its the wrong way around.
>
> _IF_ you want to do a non-wrapping variant, it must not be the default.

Unfortunately, we have to do it this way because there are so many
misuses of atomic_t, and they just keep appearing. We can't do opt-in
protections for the kernel -- we need to protect atomic_t and opt OUT
of the protection where it's not needed.

We must change the kernel culture to making things secure-by-default.
Without this, we're wasting our time and continuing to leave people
vulnerable every time some new driver lands that refcounts with
atomic_t. Since education is proven to not work, we have to harden the
_infrastructure_ of the kernel, of which atomic_t is a part.

> Since you need to audit every single atomic_t user in the kernel anyway,
> it doesn't matter. But changing atomic_t to non-wrap by default is not
> robust, if you forgot one, you can then trivially dos the kernel.

Correct: everything must be audited in either case. However, making a
mistake using opt-out means a DoS. Making a mistake using opt-in means
an exploitable kernel escalation. We must have the courage to
recognize this distinction. Right now, every refcount mistake is an
exploitable kernel flaw. Reducing this to a DoS is a giant
improvement.

> That said, I still don't much like this.
>
> I would much rather you make kref useful and use that. It still means
> you get to audit all refcounts in the kernel, but hey, you had to do
> that anyway.

This has already been suggested in the past, and suffers from the same
opt-in problem. I'll let Greg comment on it, though, as he's agreed
with going opt-out in the past when reviewing this work.

-Kees

-- 
Kees Cook
Nexus Security