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Date: Tue, 25 Apr 2017 20:59:10 -0700
From: Kees Cook <keescook@...omium.org>
To: PaX Team <pageexec@...email.hu>
Cc: LKML <linux-kernel@...r.kernel.org>, Peter Zijlstra <peterz@...radead.org>, 
	Jann Horn <jannh@...gle.com>, Eric Biggers <ebiggers3@...il.com>, 
	Christoph Hellwig <hch@...radead.org>, "axboe@...nel.dk" <axboe@...nel.dk>, 
	James Bottomley <James.Bottomley@...senpartnership.com>, 
	Elena Reshetova <elena.reshetova@...el.com>, Hans Liljestrand <ishkamiel@...il.com>, 
	David Windsor <dwindsor@...il.com>, "x86@...nel.org" <x86@...nel.org>, Ingo Molnar <mingo@...nel.org>, 
	Arnd Bergmann <arnd@...db.de>, Greg Kroah-Hartman <gregkh@...uxfoundation.org>, 
	"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 v2 0/2] x86, refcount: Implement fast refcount overflow

On Tue, Apr 25, 2017 at 7:01 PM, PaX Team <pageexec@...email.hu> wrote:
> On 25 Apr 2017 at 15:56, Kees Cook wrote:
>
>> This protection is a modified version of the x86 PAX_REFCOUNT
>> implementation from PaX/grsecurity. This speeds up the refcount_t API by
>> duplicating the existing atomic_t implementation with a single instruction
>> added to detect if the refcount has wrapped past INT_MAX (or below 0)
>> resulting in a signed value.
>
> 'signed value' sounds somewhat ambiguous given that in C a signed type (such
> as the one beneath refcount_t) can have both negative and positive values yet
> you didn't mean the latter here i guess.

Yeah, the language for the CPU "sign flag" confuses this. I will
attempt to clarify for future versions.

>> Various differences from PaX:
>> - uses "js" instead of "jo" to trap all signed results instead of just
>>   under/overflow transitions
>
> there're differences in my 4.11 port but this isn't one of them.

Any changes you'd suggest for upstreaming?

-Kees

-- 
Kees Cook
Pixel Security

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