Re: KASAN vs vmapped stacks

From: Andy Lutomirski
Date: Mon Jul 11 2016 - 10:52:57 EST

On Mon, Jul 11, 2016 at 1:30 AM, Dmitry Vyukov <dvyukov@xxxxxxxxxx> wrote:
> On Sun, Jul 10, 2016 at 2:47 PM, Andy Lutomirski <luto@xxxxxxxxxxxxxx> 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.
>> 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?
> Hi Andy,
> What is "virtually mapped stacks"? How are they enabled?

It's a patchset I'm working on, and I'm hoping to get them in to 4.8.
They're only supported (so far) on x86, and they're enabled by a
config option. The init task has a conventional stack, but all other
stacks are vmalloced.

> KASAN does not have real shadow for vmalloc range. Fixing this will
> probably involve allocating real shadow memory for that range which
> will increase memory consumption.
> You said that you disabled the virtually mapped stacks, which means
> that it is not a critical feature for your environment. When is it
> supposed to be used? What are the benefits? For now I am leaning
> towards automatically disabling virtually mapped stacks when KASAN is
> enabled.

That's what I'm doing right now. Enabling virtually mapped stacks
gives reliably stack overflow detection and avoids needing
higher-order pages. It's certainly not a critical feature, but it's
nice, and supporting both it and KASAN at the same time would be nice.