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Date: Wed, 31 May 2017 06:20:25 -0700
From: Kees Cook <keescook@...omium.org>
To: Davidlohr Bueso <dave@...olabs.net>
Cc: LKML <linux-kernel@...r.kernel.org>, Christoph Hellwig <hch@...radead.org>, 
	Peter Zijlstra <peterz@...radead.org>, "Eric W. Biederman" <ebiederm@...ssion.com>, 
	Andrew Morton <akpm@...ux-foundation.org>, Josh Poimboeuf <jpoimboe@...hat.com>, 
	PaX Team <pageexec@...email.hu>, Jann Horn <jannh@...gle.com>, 
	Eric Biggers <ebiggers3@...il.com>, Elena Reshetova <elena.reshetova@...el.com>, 
	Hans Liljestrand <ishkamiel@...il.com>, David Windsor <dwindsor@...il.com>, 
	Greg KH <gregkh@...uxfoundation.org>, Ingo Molnar <mingo@...hat.com>, 
	Alexey Dobriyan <adobriyan@...il.com>, "Serge E. Hallyn" <serge@...lyn.com>, arozansk@...hat.com, 
	Manfred Spraul <manfred@...orfullife.com>, "axboe@...nel.dk" <axboe@...nel.dk>, 
	James Bottomley <James.Bottomley@...senpartnership.com>, "x86@...nel.org" <x86@...nel.org>, 
	Ingo Molnar <mingo@...nel.org>, Arnd Bergmann <arnd@...db.de>, 
	"David S. Miller" <davem@...emloft.net>, Rik van Riel <riel@...hat.com>, 
	linux-arch <linux-arch@...r.kernel.org>, 
	"kernel-hardening@...ts.openwall.com" <kernel-hardening@...ts.openwall.com>
Subject: Re: [PATCH v5 0/3] Implement fast refcount overflow protection

On Wed, May 31, 2017 at 5:27 AM, Davidlohr Bueso <dave@...olabs.net> wrote:
> On Tue, 30 May 2017, Kees Cook wrote:
>
>> A new patch has been added at the start of this series to make the default
>> refcount_t implementation just use an unchecked atomic_t implementation,
>> since many kernel subsystems want to be able to opt out of the full
>> validation, since it includes a small performance overhead. When enabling
>> CONFIG_REFCOUNT_FULL, the full validation is used.
>>
>> The other two patches provide overflow protection on x86 without incurring
>> a performance penalty. The changelog for patch 3 is reproduced here for
>> details:
>
>
> To be sure I'm getting this right, after this all archs with the exception
> of x86 will use the regular atomic_t ("unsecure") flavor, right?

If a build does not select CONFIG_REFCOUNT_FULL and lacks
CONFIG_ARCH_HAS_REFCOUNT, refcount_t will be the same at atomic_t
(i.e. no change from the historical behavior where all the ref
counters in the kernel used atomic_t).

-Kees

-- 
Kees Cook
Pixel Security

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