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Date: Fri, 3 Nov 2017 18:41:48 -0700
From: Kees Cook <>
To: Al Viro <>
Cc: Laura Abbott <>,, 
	LKML <>, Mark Rutland <>, 
	X86 ML <>
Subject: Re: [RFC PATCH 2/2] x86: Allow paranoid __{get,put}_user

On Fri, Nov 3, 2017 at 6:39 PM, Kees Cook <> wrote:
> On Fri, Nov 3, 2017 at 5:24 PM, Al Viro <> wrote:
>> On Fri, Nov 03, 2017 at 05:14:05PM -0700, Kees Cook wrote:
>>> > x86 turns out to be easier since the safe and unsafe paths are mostly
>>> > disjoint so we don't have to worry about gcc optimizing out access_ok.
>>> > I tweaked the Kconfig to someting a bit more generic.
>>> >
>>> > The size increase was ~8K in text with a config I tested.
>>> Specifically, this feature would have caught the waitid() bug in 4.13
>>> immediately.
>> You mean, as soon as waitid() was given a kernel address.  At which point
>> you'd get a shiny way to generate a BUG(), and if something like that
>> happened under a mutex - it's even more fun...
> Nope, any usage at all would BUG. This would have been immediately noticed. :)

Sorry, ignore that; yes, on any kernel address. But as always
reduction of impact is important: from exploitable flaw to DoS. Much


Kees Cook
Pixel Security

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