Re: [PATCH 0/5] v2: block subsystem refcounter conversions

Kees Cook
2017-04-21T19:23:01Z

On Fri, Apr 21, 2017 at 3:55 AM, Reshetova, Elena
<elena.reshetova@intel.com> wrote:
> On Thu, Apr 20, 2017 at 11:33 AM, Eric Biggers <ebiggers3@gmail.com> wrote:
>> Of course, having extra checks behind a debug option is fine. But they should
>> not be part of the base feature; the base feature should just be mitigation of
>> reference count *overflows*. It would be nice to do more, of course; but when
>> the extra stuff prevents people from using refcount_t for performance reasons,
>> it defeats the point of the feature in the first place.
>
> Sure, but as I said above, I think the smaller tricks and fixes won't be convincing enough,
> so their value is questionable.
>
>> I strongly encourage anyone who has been involved in refcount_t to experiment
>> with removing a reference count decrement somewhere in their kernel, then trying
>> to exploit it to gain code execution. If you don't know what you're trying to
>> protect against, you will not know which defences work and which don't.
>
> Well, we had successful CVEs and even exploits on this in the past.
> @Kees, do you store a list of them in the project?

Here are two from last year:
http://perception-point.io/2016/01/14/analysis-and-exploitation-of-a-linux-kernel-vulnerability-cve-2016-0728/
http://cyseclabs.com/page?n=02012016

I don't disagree with Eric on the threat model: the primary concern
for reference count protection is the overflow condition. Detecting a
prior use-after-free is certainly nice to have, but the more important
case is the initial overflow.

How about we introduce something like CONFIG_HAVE_ARCH_FAST_REFCOUNT_T
for per-arch implementations and CONFIG_FAST_REFCOUNT_T that trades
coverage for speed, and checks only the overflow condition. This gets
us the critical coverage without the changes in performance. This is
basically what PaX/grsecurity already did: there is a tiny change to
the atomic inc functions to detect the wrap.

-Kees

--
Kees Cook
Pixel Security