linux-kernel - Re: [RFC][PATCH 0/3] Speed up SLUB poisoning + disable checks

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Date:	Wed, 3 Feb 2016 21:23:11 -0600 (CST)
From:	Christoph Lameter <cl@...ux.com>
To:	Laura Abbott <labbott@...hat.com>
cc:	Kees Cook <keescook@...omium.org>,
	Joonsoo Kim <iamjoonsoo.kim@....com>,
	Laura Abbott <labbott@...oraproject.org>,
	Pekka Enberg <penberg@...nel.org>,
	David Rientjes <rientjes@...gle.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linux-MM <linux-mm@...ck.org>,
	LKML <linux-kernel@...r.kernel.org>,
	"kernel-hardening@...ts.openwall.com" 
	<kernel-hardening@...ts.openwall.com>
Subject: Re: [RFC][PATCH 0/3] Speed up SLUB poisoning + disable checks

On Wed, 3 Feb 2016, Laura Abbott wrote:

> I also notice that __CMPXCHG_DOUBLE is turned off when the debug
> options are turned on. I don't see any details about why. What's
> the reason for turning it off when the debug options are enabled?

Because operations on the object need to be locked out while the debug
code is running. Otherwise concurrent operations from other processors
could lead to weird object states. The object needs to be stable for
debug checks. Poisoning and the related checks need that otherwise you
will get sporadic false positives.