linux-kernel - Re: [RFC/PATCH v2 02/10] x86

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Date:	Wed, 01 Oct 2014 20:28:18 +0400
From:	Andrey Ryabinin <a.ryabinin@...sung.com>
To:	"H. Peter Anvin" <hpa@...or.com>, linux-kernel@...r.kernel.org
Cc:	Dmitry Vyukov <dvyukov@...gle.com>,
	Konstantin Serebryany <kcc@...gle.com>,
	Dmitry Chernenkov <dmitryc@...gle.com>,
	Andrey Konovalov <adech.fo@...il.com>,
	Yuri Gribov <tetra2005@...il.com>,
	Konstantin Khlebnikov <koct9i@...il.com>,
	Sasha Levin <sasha.levin@...cle.com>,
	Christoph Lameter <cl@...ux.com>,
	Joonsoo Kim <iamjoonsoo.kim@....com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Dave Hansen <dave.hansen@...el.com>,
	Andi Kleen <andi@...stfloor.org>,
	Vegard Nossum <vegard.nossum@...il.com>, x86@...nel.org,
	linux-mm@...ck.org, Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>
Subject: Re: [RFC/PATCH v2 02/10] x86_64: add KASan support

On 10/01/2014 07:31 PM, H. Peter Anvin wrote:
> On 09/10/2014 10:31 PM, Andrey Ryabinin wrote:
>> On 09/11/2014 08:01 AM, H. Peter Anvin wrote:
>>> On 09/10/2014 07:31 AM, Andrey Ryabinin wrote:
>>>> This patch add arch specific code for kernel address sanitizer.
>>>>
>>>> 16TB of virtual addressed used for shadow memory.
>>>> It's located in range [0xffff800000000000 - 0xffff900000000000]
>>>> Therefore PAGE_OFFSET has to be changed from 0xffff880000000000
>>>> to 0xffff900000000000.
>>>
>>> NAK on this.
>>>
>>> 0xffff880000000000 is the lowest usable address because we have agreed
>>> to leave 0xffff800000000000-0xffff880000000000 for the hypervisor or
>>> other non-OS uses.
>>>
>>> Bumping PAGE_OFFSET seems needlessly messy, why not just designate a
>>> zone higher up in memory?
>>>
>>
>> I already answered to Dave why I choose to place shadow bellow PAGE_OFFSET (answer copied bellow).
>> In short - yes, shadow could be higher. But for some sort of kernel bugs we could have confusing oopses in kasan kernel.
>>
> 
> Confusing how?  I presume you are talking about something trying to
> touch a non-canonical address, which is usually a very blatant type of bug.
> 
> 	-hpa
> 

For those kinds of bugs we normally get general protection fault.

With inline instrumented kasan we could get either general protection fault,
or unhandled page fault on "kasan_mem_to_shadow(non_canonical_address)" address.
I assume that the last case could be a bit confusing.

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