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Date: Fri, 3 Feb 2017 10:09:18 -0800
From: Kees Cook <keescook@...omium.org>
To: Peter Zijlstra <peterz@...radead.org>
Cc: "Reshetova, Elena" <elena.reshetova@...el.com>, Greg KH <gregkh@...uxfoundation.org>, 
	Arnd Bergmann <arnd@...db.de>, Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...nel.org>, 
	"H. Peter Anvin" <h.peter.anvin@...el.com>, Will Deacon <will.deacon@....com>, 
	David Windsor <dwindsor@...il.com>, David Howells <dhowells@...hat.com>, 
	LKML <linux-kernel@...r.kernel.org>, 
	"kernel-hardening@...ts.openwall.com" <kernel-hardening@...ts.openwall.com>
Subject: Re: [PATCH 1/5] refcount_t: A special purpose refcount type

On Fri, Feb 3, 2017 at 5:25 AM, Peter Zijlstra <peterz@...radead.org> wrote:
> Provide refcount_t, an atomic_t like primitive built just for
> refcounting.
>
> It provides saturation semantics such that overflow becomes impossible
> and thereby 'spurious' use-after-free is avoided.

Wheee :) Thanks for working on this

>
> Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
> ---
> [...]
> --- a/lib/Kconfig.debug
> +++ b/lib/Kconfig.debug
> @@ -716,6 +716,19 @@ source "lib/Kconfig.kmemcheck"
>
>  source "lib/Kconfig.kasan"
>
> +config DEBUG_REFCOUNT
> +       bool "Verbose refcount checks"
> +       --help--

Quick feedback as I start playing with this: this isn't valid Kconfig
syntax (build breaks). It should either be "---help---" or just
"help", latter preferred.

> +         Say Y here if you want reference counters (refcount_t and kref) to
> +         generate WARNs on dubious usage. Without this refcount_t will still
> +         be a saturating counter and avoid Use-After-Free by turning it into
> +         a resource leak Denial-Of-Service.
> +
> +         Use of this option will increase kernel text size but will alert the
> +         admin of potential abuse.
> +
> +         If in doubt, say "N".
> +
>  endmenu # "Memory Debugging"
>
>  config ARCH_HAS_KCOV
>
>

-Kees

-- 
Kees Cook
Pixel Security

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