Re: Making KASAN compatible with VMAP_STACK

[Mark Rutland]

On Mon, Jul 23, 2018 at 01:55:49PM +0200, Dmitry Vyukov wrote:
> On Mon, Jul 23, 2018 at 1:18 PM, Mark Rutland <mark.rutland@xxxxxxx> wrote:
> > On Mon, Jul 23, 2018 at 09:40:36AM +0200, Dmitry Vyukov wrote:
> >> On Sun, Jul 22, 2018 at 7:52 PM, Andy Lutomirski <luto@xxxxxxxxxx> 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.

Thanks,
Mark.