linux-kernel - Re: KASAN vs vmapped stacks

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Date:	Mon, 11 Jul 2016 12:29:19 +0200
From:	Dmitry Vyukov <dvyukov@...gle.com>
To:	Andrey Ryabinin <aryabinin@...tuozzo.com>
Cc:	Andy Lutomirski <luto@...capital.net>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	kasan-dev <kasan-dev@...glegroups.com>, X86 ML <x86@...nel.org>,
	Alexander Potapenko <glider@...gle.com>,
	Kees Cook <keescook@...omium.org>,
	Borislav Petkov <bp@...en8.de>
Subject: Re: KASAN vs vmapped stacks

On Mon, Jul 11, 2016 at 11:57 AM, Andrey Ryabinin
<aryabinin@...tuozzo.com> wrote:
>
>
> On 07/10/2016 03:47 PM, Andy Lutomirski wrote:
>> Hi all-
>>
>> I found two nasty issues with virtually mapped stacks if KASAN is
>> enabled.  The first issue is a crash: the first non-init stack is
>> allocated and accessed before KASAN initializes its zero shadow
>> AFAICT, which means that we switch to that stack and then blow up when
>> we start recursively faulting on failed accesses to the shadow.
>>
>
> KASAN initialized quite early, before any non-init task exists. The crash happens
> because non-init task writes to write-protected zero shadow.
> Currently KASAN doesn't allocate shadow memory for vmalloc addresses, we just map single
> zero page and write protect it.
>
>
>> The second issue is that, even if we survive (we initialize the zero
>> shadow on time), KASAN will fail to protect hte stack.
>>
>> For now, I just disabled use of virtually mapped stacks if KASAN is
>> on.  Do you have any easy ideas to fix it?
>>
>
> Allocate shadow memory which backs vmalloc/vmap allocations is the only way to fix this.
> I can do this, and post the patches soon enough.

Do you want to allocate it eagerly? Won't it consume 1/8 of vmalloc
range worth of physical memory?