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

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Date:	Tue, 25 Feb 2014 06:34:55 -0800
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Vince Weaver <vincent.weaver@...ne.edu>
CC:	Peter Zijlstra <peterz@...radead.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Linux Kernel <linux-kernel@...r.kernel.org>,
	Ingo Molnar <mingo@...hat.com>
Subject: Re: perf_fuzzer compiled for x32 causes reboot

#2 is what I really don't understand.

I worry something else is going on there

On February 25, 2014 6:07:51 AM PST, Vince Weaver <vincent.weaver@...ne.edu> wrote:
>On Mon, 24 Feb 2014, H. Peter Anvin wrote:
>
>> On 02/24/2014 11:30 AM, Peter Zijlstra wrote:
>> > On Mon, Feb 24, 2014 at 02:13:29PM -0500, Steven Rostedt wrote:
>> >> Ah, and x86_64 saves off the cr2 register when entering NMI and
>restores
>> >> it before returning. But it seems to be missing from the i386
>code.
>> > 
>> > arch/x86/kernel/nmi.c:
>> > 
>> > #define nmi_nesting_preprocess(regs)					\
>> > 	do {								\
>> > 		if (this_cpu_read(nmi_state) != NMI_NOT_RUNNING) {	\
>> > 			this_cpu_write(nmi_state, NMI_LATCHED);		\
>> > 			return;						\
>> > 		}							\
>> > 		this_cpu_write(nmi_state, NMI_EXECUTING);		\
>> > 		this_cpu_write(nmi_cr2, read_cr2());			\
>> > 	} while (0);							\
>> > 	nmi_restart:
>> > 
>> > #define nmi_nesting_postprocess()					\
>> > 	do {								\
>> > 		if (unlikely(this_cpu_read(nmi_cr2) != read_cr2()))	\
>> > 			write_cr2(this_cpu_read(nmi_cr2));		\
>> > 		if (this_cpu_dec_return(nmi_state))			\
>> > 			goto nmi_restart;				\
>> > 	} while (0)
>> > 
>> > That very much looks like saving/restoring CR2 to me.
>> > 
>> > FWIW; I hate how the x86_64 and i386 versions of this NMI nesting
>magic
>> > are so completely different.
>> 
>> Is there any way that nmi_cr2 can end up getting overwritten by
>multiple
>> nestings of some kind?  I would have thought it would have made more
>> sense to spill cr2 onto the stack after the stack has been properly
>set up.
>
>So how can I help with debugging this?
>
>While the missing cr2 issue made debugging frustrating, I find the
>other 
>aspects of the bug more serious:
>
>  1.  Programs that are doing valid memory accesses can segfault
>and worse
>2.  This bug can cause an instant-reboot of the system (I assume
>somehow
>      with the right combination of memory accesses  it causes a 
>      triple-fault?)
>
>#2 is why I spent all of this time tracking this down, I couldn't leave
>a 
>machine fuzzing overnight without the machine rebooting.
>
>Vince

-- 
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