Re: Dealing with the NMI mess

[Peter Zijlstra <peterz@infradead.org>]

On Fri, Jul 24, 2015 at 07:58:41AM -0400, Steven Rostedt wrote:
> On Fri, 24 Jul 2015 10:13:26 +0200
> Peter Zijlstra <peterz@infradead.org> wrote:
> 
> > On Thu, Jul 23, 2015 at 02:59:56PM -0700, Linus Torvalds wrote:
> > > Hmmm. I thought watchpoints were "before the instruction" too, but
> > > that's just because I haven't used them in ages, and I didn't remember
> > > the details. I just looked it up.
> > > 
> > > You're right - the memory watchpoints trigger after the instruction
> > > has executed, so RF isn't an issue. So yes, the only issue is
> > > instruction breakpoints, and those are the only ones we need to clear.
> > > 
> > > And that makes it really easy.
> > > 
> > > So yes, I agree. We only need to clear all kernel breakpoints.
> > 
> > But but but, we can access userspace with !IF, imagine someone doing:
> > 
> >   local_irq_disable();
> >   copy_from_user_inatomic();
> > 
> > and as luck would have it, there's a breakpoint on the user memory we
> > just touched. And we go and disable a user breakpoint.
> 
> Where does the kernel do that to user text? I would think that user
> data would only have watchpoints, and Andy and Linus said that those
> would not be disabled (I'm guessing because they don't have the RF flag
> set, and forward progress can proceed). If the kernel does the above to
> user code and there's a breakpoint there, would it even trigger?
> 
> I'm not too familiar with how to use hw breakpoints, but I'm guessing
> (correct me if I'm wrong) that breakpoints on code that trigger when
> executed, but watchpoints on data trigger when accessed. Then
> copy_from_user_inatomic() would only trigger on watchpoints (it's not
> executing that code, at least I hope it isn't!), and those wont bother
> us.

These things can be: RW, W, X.

Sure, hitting a user X watchpoint is going to be 'interesting', but its
fairly easy to hit a RW one.

Just watch an on-stack variable and get perf to copy a huge chunk of
stack (like it does for the dwarf stuff).