[RFC PATCH untested] arm64: kernel: implement fast refcount checking

[Ard Biesheuvel]

On 25 July 2017 at 18:13, Kees Cook <keescook at chromium.org> wrote:
> On Tue, Jul 25, 2017 at 4:49 AM, Ard Biesheuvel
> <ard.biesheuvel at linaro.org> 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.