Re: perf: fuzzer KASAN unwind_get_return_address

From: Josh Poimboeuf
Date: Thu Nov 17 2016 - 12:44:29 EST

On Thu, Nov 17, 2016 at 10:04:46AM +0100, Peter Zijlstra wrote:
> On Wed, Nov 16, 2016 at 10:48:28PM -0600, Josh Poimboeuf wrote:
> > Peter or Vince, can you try to recreate with this patch? It dumps the
> > raw stack contents during a stack dump. Hopefully that would give a
> > clue about what's going wrong.
> Here goes... I'll do another run and get you the results of that as
> well.

Thanks, I just waded through this and it turned up some good clues. And
according to 'git blame', you might be able to help :-)

It's not stack corruption. Instead it looks like
__intel_pmu_pebs_event() is creating a bad or stale pt_regs which gets
passed to the unwinder. Specifically, regs->bp points to a seemingly
random address on the NMI stack. Which seems odd, considering the code
itself is running on the same NMI stack.

I don't know much about the PEBS code but it seems like it's passing
some stale data. Either that or there's some NMI nesting going on.