linux-kernel - Re: [kernel-hardening] Re: [RFC v4 PATCH 00/13] HARDENED

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Date:   Fri, 11 Nov 2016 12:41:27 +0000
From:   Mark Rutland <mark.rutland@....com>
To:     kernel-hardening@...ts.openwall.com
Cc:     Kees Cook <keescook@...omium.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Greg KH <gregkh@...uxfoundation.org>,
        Will Deacon <will.deacon@....com>,
        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 Fri, Nov 11, 2016 at 01:29:21AM +0100, Colin Vidal wrote:
> On Fri, 2016-11-11 at 00:57 +0100, Peter Zijlstra wrote:
> > On Thu, Nov 10, 2016 at 03:15:44PM -0800, Kees Cook wrote:
> > > On Thu, Nov 10, 2016 at 2:27 PM, Greg KH <gregkh@...uxfoundation.org> wrote:
> > > > On Thu, Nov 10, 2016 at 10:13:10PM +0100, Peter Zijlstra wrote:

> I wonder if we didn't make a confusion between naming and
> specifications. I have thought about Kees idea and what you're saying:
> 
> - The name "atomic_t" name didn't tell anything about if the variable
>   can wrap or not. It just tells there is no race condition on
>   concurrent access, nothing else, and users are well with that. OK
>   then, we don't modify atomic_t, it makes sense.
> 
> - Hence, let's say a new type "refcount_t". It names exactly what we
>   try to protect in this patch set. A much more simpler interface than
>   atomic_t would be needed, and it protects on race condition and
>   overflows (precisely what is expected of a counter reference). Not
>   an opt-in solution, but it is much less invasive since we "just"
>   have to modify the kref implementation and some vfs reference
>   counters.
> 
> That didn't tell us how actually implements refcount_t: reuse some
> atomic_t code or not (it would be simpler anyways, since we don't have
> to implement the whole atomic_t interface). Still, this is another
> problem.
> 
> Sounds better?

Regardless of atomic_t semantics, a refcount_t would be far more obvious
to developers than atomic_t and/or kref, and better documents the intent
of code using it.

We'd still see abuse of atomic_t (and so this won't solve the problems
Kees mentioned), but even as something orthogonal I think that would
make sense to have.

Thanks,
Mark.