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Date: Tue, 25 Jul 2017 10:50:20 -0700
From: Kees Cook <>
To: Ard Biesheuvel <>
Cc: Li Kun <>, 
	"" <>, 
	"" <>, Will Deacon <>, 
	Mark Rutland <>, Laura Abbott <>
Subject: Re: [RFC PATCH untested] arm64: kernel: implement fast refcount checking

On Tue, Jul 25, 2017 at 10:20 AM, Ard Biesheuvel
<> wrote:
> On 25 July 2017 at 18:13, Kees Cook <> wrote:
>> On Tue, Jul 25, 2017 at 4:49 AM, Ard Biesheuvel
>> <> wrote:
>>> Hi all,
>>> I had a stab at porting the fast refcount checks to arm64. It is slightly
>>> less straight-forward than x86 given that we need to support both LSE and
>>> LL/SC, and fallback to the latter if running a kernel built with support
>>> for the former on hardware that does not support it.
>>> It is build tested with and without LSE support, and boots fine on non-LSE
>>> hardware in both cases.
>> Ah! Very cool. Hopefully you and Li can compare notes; I think they've
>> been working on an implementation too.
> I wasn't aware of that.
>>> Suggestions welcome as to how to test and/or benchmark this,
>> I'll post a patch for LKDTM that I've been using. It's more
>> comprehensive than the existing ATOMIC checks (which predated the
>> refcount-only protection).
> OK. One thing I couldn't figure out: is refcount_t signed or not? The
> saturate tests set the initial value to UINT_MAX - 1, but this is
> interpreted as a negative value and so the refcount manipulations that
> are expected to succeed also fail in my case.

refcount_t under REFCOUNT_FULL is unsigned. Under the x86 fast
refcount, it's signed to gain the CPU flag detection for overflow. The
understanding is basically "omg, if you've got INT_MAX-many references
to something you already DoSed your machine".

I'll have the full LKDTM tests up in a moment here, just doing another
pass on them now...


Kees Cook
Pixel Security

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