Re: perf: fuzzer KASAN unwind_get_return_address

[Josh Poimboeuf <jpoimboe@redhat.com>]

On Thu, Nov 17, 2016 at 09:25:58AM -0500, Vince Weaver wrote:
> On Thu, 17 Nov 2016, Josh Poimboeuf wrote:
> 
> > On Thu, Nov 17, 2016 at 10:48:27AM +0100, Dmitry Vyukov wrote:
> > > Just in case, there is currently a known KASAN false positive related
> > > to longjmp's on GPFs. When a syscall hits GPF stack is unwound to
> > > kernel entry point, this leaves a bunch of stray poisoned redzones on
> > > the thread stack. They later cause false stack-out-of-bounds reports.
> > > 
> > > But this does not seem to be the case here. Kernel is not tainted. And
> > > shadow at the bottom of the reports looks sane.
> > > 
> > > But if that's the case somehow, we will need to add
> > > kasan_unpoison_remaining_stack() call before a longjmp like we did for
> > > jprobe_return():
> > > https://groups.google.com/d/msg/kasan-dev/Hzox58yZ4MU/TOdFoWMuBQAJ
> > 
> > I'm pretty sure this isn't a KASAN false positive.  The unwinder does
> > actually seem to be accessing a bad area of the stack, in the middle of
> > a function's stack frame.
> 
> I'm having trouble reproducing it on a few other machines I have fuzzing.
> So there might be some kernel option contributing, I need to compare 
> .configs.
> 
> Also the machine that easily triggers the problem I'm compiling with 
> gcc-5.4 where the machines I can't are using gcc-4.9.

I believe KASAN only works with gcc 5 and later, so that would explain
why you aren't seeing it with gcc 4.9.

-- 
Josh