Re: KASAN vs vmapped stacks

[Andrey Ryabinin]

On 07/11/2016 01:29 PM, Dmitry Vyukov wrote:
> On Mon, Jul 11, 2016 at 11:57 AM, Andrey Ryabinin
> <aryabinin@xxxxxxxxxxxxx> 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?
> 

I have no idea what do you mean by eager allocation.
What we need is to allocate shadow per vmalloc()/vmap() call if such allocation requires non-zero shadow, like stacks or modules.
Sometimes we certainly know that the whole shadow page will contain only zeros. In such cases we can use zero page to not waste memory.