Re: [RFC v4 PATCH 00/13] HARDENED_ATOMIC

[Kees Cook]

On Thu, Nov 10, 2016 at 12:48 PM, Will Deacon <will.deacon@xxxxxxx> wrote:
> On Thu, Nov 10, 2016 at 09:37:49PM +0100, Peter Zijlstra 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.
>>
>> 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.
>
> Completely agreed.
>
> Whilst I understand that you're addressing an important and commonly
> exploited vulnerability, this really needs to be opt-in rather than
> opt-out given the prevalence of atomic_t users in the kernel. Having a
> "hardened" kernel that does the wrong thing is useless.

I (obviously) disagree. It's not useless. Such a kernel is totally
safe against refcount errors and would be exposed to DoS issues only
where mistakes were made. This is the fundamental shift here:

- we already have exploitable privilege escalation refcount flaws on a
regular basis
- this changes things to have zero exploitable refcount flaws now and
into the future
- the risk is bugs leading to DoS instead of the risk of exploitable flaws

That's the real trade.

>> 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.
>
> What needs to happen to kref to make it useful? Like many others, I've
> been guilty of using atomic_t for refcounts in the past.

That's the point: expecting everyone to get this right and not miss
mistake from now into the future is not a solution. This solves the
privilege escalation issue for refcounts now and forever.

-Kees

-- 
Kees Cook
Nexus Security