Date: Tue, 25 Feb 2014 09:07:51 -0500 (EST)
From: Vince Weaver <vincent.weaver@maine.edu>
To: "H. Peter Anvin" <hpa@zytor.com>
cc: Peter Zijlstra <peterz@infradead.org>,
        Steven Rostedt <rostedt@goodmis.org>,
        Vince Weaver <vincent.weaver@maine.edu>,
        Linux Kernel <linux-kernel@vger.kernel.org>,
        Ingo Molnar <mingo@redhat.com>
Subject: Re: perf_fuzzer compiled for x32 causes reboot
In-Reply-To: <530C12CA.6070308@zytor.com>
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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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