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Date: Thu, 10 Nov 2016 16:23:08 -0500
From: David Windsor <>
Cc: Will Deacon <>, Greg KH <>, 
	Peter Zijlstra <>, Elena Reshetova <>, 
	Arnd Bergmann <>, Thomas Gleixner <>, Ingo Molnar <>, 
	"H. Peter Anvin" <>, LKML <>
Subject: Re: Re: [RFC v4 PATCH 00/13] HARDENED_ATOMIC

On Thu, Nov 10, 2016 at 4:01 PM, Kees Cook <> wrote:
> On Thu, Nov 10, 2016 at 12:48 PM, Will Deacon <> 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
>>> >
>>> > 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

> 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

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