Re: [RFC PATCH] UML: add support for KASAN under x86_64

[Dmitry Vyukov]

On Thu, Jan 16, 2020 at 10:39 PM Patricia Alfonso
<trishalfonso@xxxxxxxxxx> wrote:
>
> On Thu, Jan 16, 2020 at 1:23 AM Dmitry Vyukov <dvyukov@xxxxxxxxxx> wrote:
> >
> > On Thu, Jan 16, 2020 at 10:20 AM Johannes Berg
> > <johannes@xxxxxxxxxxxxxxxx> wrote:
> > >
> > > On Thu, 2020-01-16 at 10:18 +0100, Dmitry Vyukov wrote:
> > > >
> > > > Looking at this problem and at the number of KASAN_SANITIZE := n in
> > > > Makefiles (some of which are pretty sad, e.g. ignoring string.c,
> > > > kstrtox.c, vsprintf.c -- that's where the bugs are!), I think we
> > > > initialize KASAN too late. I think we need to do roughly what we do in
> > > > user-space asan (because it is user-space asan!). Constructors run
> > > > before main and it's really good, we need to initialize KASAN from
> > > > these constructors. Or if that's not enough in all cases, also add own
> > > > constructor/.preinit array entry to initialize as early as possible.
> > >
>
> I am not too happy with the number of KASAN_SANITIZE := n's either.
> This sounds like a good idea. Let me look into it; I am not familiar
> with constructors or .preint array.
>
> > > We even control the linker in this case, so we can put something into
> > > the .preinit array *first*.
> >
> > Even better! If we can reliably put something before constructors, we
> > don't even need lazy init in constructors.
> >
> > > > All we need to do is to call mmap syscall, there is really no
> > > > dependencies on anything kernel-related.
> > >
> > > OK. I wasn't really familiar with those details.
> > >
> > > > This should resolve the problem with constructors (after they
> > > > initialize KASAN, they can proceed to do anything they need) and it
> > > > should get rid of most KASAN_SANITIZE (in particular, all of
> > > > lib/Makefile and kernel/Makefile) and should fix stack instrumentation
> > > > (in case it does not work now). The only tiny bit we should not
> > > > instrument is the path from constructor up to mmap call.
>
> This sounds like a great solution. I am getting this KASAN report:
> "BUG: KASAN: stack-out-of-bounds in syscall_stub_data+0x2a5/0x2c7",
> which is probably because of this stack instrumentation problem you
> point out.

[reposting to the list]

If that part of the code I mentioned is instrumented, manifestation
would be different -- stack instrumentation will try to access shadow,
shadow is not mapped yet, so it would crash on the shadow access.

What you are seeing looks like, well, a kernel bug where it does a bad
stack access. Maybe it's KASAN actually _working_? :)