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Date: Fri, 11 Nov 2016 10:04:42 -0800
From: Kees Cook <>
To: Peter Zijlstra <>
Cc: Will Deacon <>, Greg KH <>, 
	David Windsor <>, 
	"" <>, 
	Elena Reshetova <>, Arnd Bergmann <>, 
	Thomas Gleixner <>, Ingo Molnar <>, 
	"H. Peter Anvin" <>
Subject: Re: Re: [RFC v4 PATCH 00/13] HARDENED_ATOMIC

On Fri, Nov 11, 2016 at 9:46 AM, Peter Zijlstra <> wrote:
> On Fri, Nov 11, 2016 at 09:43:00AM -0800, Kees Cook wrote:
>> > 1) kref: Used for honest-to-goodness reference counters that want
>> > overflow protection.  Uses a new type: atomic_nowrap_t that has
>> > HARDENED_ATOMIC protection.
>> Based on other feedback, it sounds like we're better off with
>> refcount_t (which kref could be implemented on top of). And refcount_t
>> would have a limited API: inc, dec_and_test (or whatever is determined
>> as sanely minimal).
>> > 2) statistical counters: Atomic in all cases, but wraps.
>> Yup. sequence_t seems to make the most sense on naming, I think. If we
>> want to get crazy, the type could be sequence_wrap_t.
> Why? atomic_t is still perfectly fine here, right?
>> > 3) atomic_t: All other users of atomics (locks, etc.).  Wrapping
>> > behavior depends on a CONFIG setting.
>> Correct: if CONFIG_PARANOID_ATOMIC (or something) is set, atomic_t is
>> implemented as refcount_t, otherwise as sequence_t.
> Can't happen. There is far more atomic_t usage than reference and/or
> statistics counters.

The opt-out direction we need to take means that we can't leave
atomic_t as a possible refcount implementation. As such, we need to
convert everything to be either wrapping or non-wrapping, and define
atomic_t as one or the other via CONFIG (to allow the user to choose
their risk level).

Just replacing known-atomic_t refcounters with refcount_t is opt-in,
and won't cover new drivers that get missed by maintainers. We need a
hardened infrastructure, not just "stuff people can maybe remember to

I'm totally open about how to get there, but things can't just be opt-in.


Kees Cook
Nexus Security

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