Re: Making KASAN compatible with VMAP_STACK

[Dmitry Vyukov <dvyukov@google.com>]

On Mon, Jul 23, 2018 at 2:51 PM Dmitry Vyukov <dvyukov@google.com> wrote:
>
> On Mon, Jul 23, 2018 at 2:42 PM, Mark Rutland <mark.rutland@arm.com> 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.

A stack overflow just fired in:
https://groups.google.com/forum/#!topic/syzkaller-bugs/HoRZMT92WKk
on incoming network packet parsing (!). Was detected as some "innocent
WARNING" in rcu subsystem.

I filed https://bugzilla.kernel.org/show_bug.cgi?id=202009 to track
KASAN+VMAP_STACK.