Message-ID: <CAEXv5_i3Ju5JNs6g-jA-WkMR38g43Aj5we0+GXeHcuUAM3nziQ@mail.gmail.com>
Date:   Thu, 10 Nov 2016 16:23:08 -0500
From:   David Windsor <dave@...gbits.org>
To:     kernel-hardening@...ts.openwall.com
Cc:     Will Deacon <will.deacon@....com>,
        Greg KH <gregkh@...uxfoundation.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Elena Reshetova <elena.reshetova@...el.com>,
        Arnd Bergmann <arnd@...db.de>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        "H. Peter Anvin" <h.peter.anvin@...el.com>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [kernel-hardening] Re: [RFC v4 PATCH 00/13] HARDENED_ATOMIC

On Thu, Nov 10, 2016 at 4:01 PM, Kees Cook <keescook@...omium.org> wrote:
> On Thu, Nov 10, 2016 at 12:48 PM, Will Deacon <will.deacon@....com> 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
>>> > HAVE_ARCH_HARDENED_ATOMIC.
>>> >
>>> > 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
DoS.

> 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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