linux-kernel - Re: sched, timers: use after free in __lock_task

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Date:	Mon, 14 Jul 2014 14:25:48 +0400
From:	Andrey Ryabinin <a.ryabinin@...sung.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Sasha Levin <sasha.levin@...cle.com>,
	Ingo Molnar <mingo@...nel.org>,
	John Stultz <john.stultz@...aro.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Frederic Weisbecker <fweisbec@...il.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Dave Jones <davej@...hat.com>, Oleg Nesterov <oleg@...hat.com>
Subject: Re: sched, timers: use after free in __lock_task_sighand when exiting
 a process

On 07/14/14 13:58, Peter Zijlstra wrote:
> On Mon, Jul 14, 2014 at 01:34:40PM +0400, Andrey Ryabinin wrote:
>> On 07/14/14 13:04, Peter Zijlstra wrote:
>>> On Sun, Jul 13, 2014 at 07:45:56PM -0400, Sasha Levin wrote:
>>>> On 07/13/2014 05:51 PM, Sasha Levin wrote:
>>>>> Hi all,
>>>>>
>>>>> While fuzzing with trinity inside a KVM tools guest running the latest -next
>>>>> kernel with the KASAN patchset, I've stumbled on the following spew:
>>>
>>> WTH is a KASAN?
>>>
>>
>> It's dynamic memory checker, detects use after free, out of bounds accesses - https://lkml.org/lkml/2014/7/9/990
> 
> How is it better/different from the existing use after free stuff? That
> email fails to tell me.
> 

DEBUG_PAGEALLOC works on page-granularity level, kasan can do checks at sub-page granularity (for slab objects).

SLUB_DEBUG(poisoning, etc.) doesn't guarantee that you will catch use-after-free.
For example if you are only reading and checking a single bit in some already free object. Kasan will catch this.

KASAN is very similar to kmemcheck, but definitely faster.
The only thing that currently kasan can't catch, while kmemcheck can - is reads of uninitialized memory.
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