linux-kernel - Re: perf_fuzzer compiled for x32 causes reboot

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Date:	Thu, 27 Feb 2014 20:00:04 -0500 (EST)
From:	Vince Weaver <vincent.weaver@...ne.edu>
To:	"H. Peter Anvin" <hpa@...or.com>
cc:	Steven Rostedt <rostedt@...dmis.org>,
	Vince Weaver <vincent.weaver@...ne.edu>,
	Peter Zijlstra <peterz@...radead.org>,
	Linux Kernel <linux-kernel@...r.kernel.org>,
	Ingo Molnar <mingo@...hat.com>
Subject: Re: perf_fuzzer compiled for x32 causes reboot

On Thu, 27 Feb 2014, H. Peter Anvin wrote:

> On 02/27/2014 03:30 PM, Steven Rostedt wrote:
> > On Thu, 27 Feb 2014 14:52:54 -0800
> > "H. Peter Anvin" <hpa@...or.com> wrote:
> > 
> >> On 02/27/2014 02:31 PM, Steven Rostedt wrote:
> >>>
> >>> Yeah, something is getting mesed up.
> >>>
> >>
> >> What it *looks* like to me is that we try to nest the cr2 save/restore,
> >> which doesn't nest because it is a percpu variable.
> >>
> >> ... except in the x86-64 case, we *ALSO* save/restore cr2 inside
> >> entry_64.S, which makes the stuff in do_nmi completely redundant and
> >> there for no good reason.
> > 
> > Peter, look at the code. That percpu cr2 is in a #ifdef CONFIG_X86_32
> > section. That is, it isn't even executed. That's i386 code. The only
> > place the cr2 is saved for x86_64 is in entry_64.S.
> > 
> 
> Right, egg on my face.  However, I still think it would make more sense
> for it to nest the way entry_64.S does if at all possible.
> 
> That makes this even more confusing, though.  I would still like to see
> what happens with the patch I sent Vince.

I'll try your patch momentarily, first I had some other changes I started 
running before I left work (for some reason it recompiled the whole 
kernel).

8: function:             perf_output_begin
8: bprint:               perf_output_begin: VMW: event type 2 config 2a st: 2c3e
8: bputs:                perf_output_begin: VMW: before rcu_dereference
9: function:             __do_page_fault
9: function:                down_read_trylock
9: function:                _cond_resched
9: function:                find_vma

so it looks like the fault happens 

rcu_read_lock();

116         /*
117          * For inherited events we send all the output towards the parent.
118          */
119         if (event->parent)
120                 event = event->parent;
121 

somewhere between here

122         rb = rcu_dereference(event->rb);
123         if (unlikely(!rb))
124                 goto out;

and here

125 
126         if (unlikely(!rb->nr_pages))
127                 goto out;

although if rcu locks do anything to turn off tracing then this could be 
suspect.

Vince
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