Re: Making KASAN compatible with VMAP_STACK

[Dmitry Vyukov]

On Mon, Jul 23, 2018 at 2:42 PM, Mark Rutland <mark.rutland@xxxxxxx> wrote:
>> >> > Hi all-
>> >> >
>> >> > It would be really nice to make KASAN compatible with VMAP_STACK.
>> >> > Both are valuable memory debugging features, and the fact that you
>> >> > can't use both is disappointing.
>> >> >
>> >> > As far as I know, there are only two problems:
>> >> >
>> >> > 1. The KASAN shadow population code is a mess, and adding *anything*
>> >> > to the KASAN shadow requires magical, fragile incantations.  It should
>> >> > be cleaned up so that ranges can be easily populated without needing
>> >> > to very carefully align things, call helpers in the right order, etc.
>> >> > The core KASAN code should figure it out by itself.
>> >> >
>> >> > 2. The vmalloc area is potentially extremely large.  It might be
>> >> > necessary to have a way to *depopulate* shadow space when stacks get
>> >> > freed or, more generally, when vmap areas are freed.  Ideally KASAN
>> >> > would integrate with the core vmalloc/vmap code and it would Just Work
>> >> > (tm).  And, as a bonus, we'd get proper KASAN protection of vmalloced
>> >> > memory.
>> >> >
>> >> > Any volunteers to fix this?
>> >>
>> >> Hi Andy,
>> >>
>> >> I understand that having most configs as orthogonal settings that can
>> >> be enabled independently is generally good in intself, but I would
>> >> like to understand what does VMAP_STACK add on top of KASAN in terms
>> >> of debugging capabilities?
>> >
>> > VMAP_STACK makes it possible to detect stack overflows reliably at the
>> > point of overflow.
>> >
>> > KASAN can't handle this reliably, even if it detects that an access is
>> > out of the stack bounds, since handling this requires stack space.
>> > Depending on a number of factors, this may be reported, might result in
>> > recursive exceptions, etc.
>>
>> Interesting. Does VMAP_STACK detect task_struct smashing today? As far
>> as I remember, the first version didn't.
>
> I assume you mean thread_info? Both arm64 and x86 moved the thread_info
> out of the stack region by moving it into task_struct, which has always
> been allocated separately. So thread_info smashing by stack overflow is
> not possible.
>
> Regardless of VMAP_STACK, the stack region is purely stack on arm64 and
> x86.
>
>> As an orthogonal measure we could add KASAN redzone between stack and
>> task_struct, and make KASAN instrumentation detect when the new frame
>> hits this redzone. We bump stack order under KASAN significantly, so
>> adding, say 128 byte redzone should not be a problem. Does it make any
>> sense?
>
> I don't think this is necessary since the two are allocated separately.


I see. Thanks.

So current KASAN failure mode would be silently smashing whatever page
happens to be after the stack. If so, I guess combining it with
VMAP_STACK would be useful, in particular, to prevent random assorted
crashes coming out of syzbot.

But I think I am not well qualified to actually do this.