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

[David Windsor]

On Thu, Nov 10, 2016 at 4:27 PM, Kees Cook <keescook@xxxxxxxxxxxx> wrote:
> 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.
>

I completely agree with you: I'm just bringing to light now the
arguments that were voiced about this (particularly by someone from
the PaX/grsecurity team; I forget which thread).

To address Peter's concerns about trivially DoS-ing a kernel, would it
be feasible to still use atomic_t as a protected type, but change
hardened_atomic_overflow() to not kill the process?  Just for a time:
we let this bake in distros for a while, fix all of the
yet-to-be-identified wrappable users of atomic_t, then re-enable
process killing?

>>> 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