Re: [kernel-hardening] Re: [RFC v4 PATCH 00/13] HARDENED_ATOMIC

[Kees Cook]

On Thu, Nov 10, 2016 at 1:23 PM, David Windsor <dave@xxxxxxxxxxxx> wrote:
> On Thu, Nov 10, 2016 at 4:01 PM, Kees Cook <keescook@xxxxxxxxxxxx> wrote:
>> 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.
>>
>
> Discussions have been occurring since KSPP has begun: do we need a
> specialized type for reference counters?  Oh, wait, we do: kref.
> Wait!  kref is implemented with atomic_t.
>
> So, what?  We obviously need an atomicity for a reference counter
> type.  So, do we simply implement the HARDENED_ATOMIC protected
> version of atomic_t "inside" of kref and leave atomic_t alone?
>
> That would certainly reduce the number of users using atomic_t when
> they don't need a refcounter: kernel users using kref probably meant
> to use it as a reference counter, so wrap protection wouldn't cause a
> DoS.

But it leaves all the newly added drivers that get it wrong (by not
using wrap-protected kref) exposed to privilege escalation. We have to
kill the entire class of vulnerability. It needs to be impossible to
get refcounting wrong from a pragmatic approach: we can't educate
everyone, so the infrastructure must be safe.

>> 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