Date: Fri, 28 Feb 2014 12:11:11 +0100
From: Peter Zijlstra <peterz@infradead.org>
To: Steven Rostedt <rostedt@goodmis.org>
Cc: Vince Weaver <vincent.weaver@maine.edu>, "H. Peter Anvin" <hpa@zytor.com>,
        Linux Kernel <linux-kernel@vger.kernel.org>,
        Ingo Molnar <mingo@redhat.com>
Subject: Re: perf_fuzzer compiled for x32 causes reboot
Message-ID: <20140228111111.GI27965@twins.programming.kicks-ass.net>
References: <alpine.DEB.2.10.1402250904400.23234@vincent-weaver-1.um.maine.edu>
 <dde99a03-9f97-4cd9-aeef-cb0dcc62a7dc@email.android.com>
 <20140225094352.73e0e28c@gandalf.local.home>
 <alpine.DEB.2.10.1402251032230.23234@vincent-weaver-1.um.maine.edu>
 <alpine.DEB.2.10.1402261003390.18730@vincent-weaver-1.um.maine.edu>
 <alpine.DEB.2.10.1402271621000.2571@vincent-weaver-1.um.maine.edu>
 <20140227173150.4e5ed747@gandalf.local.home>
 <530FC1C6.5040209@zytor.com>
 <alpine.DEB.2.10.1402272032140.6271@vincent-weaver-1.um.maine.edu>
 <20140227215726.7018c861@gandalf.local.home>
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Sender: linux-kernel-owner@vger.kernel.org

On Thu, Feb 27, 2014 at 09:57:26PM -0500, Steven Rostedt wrote:
> @@ -512,8 +508,21 @@ static inline void nmi_nesting_postprocess(void)
>  dotraplinkage notrace __kprobes void
>  do_nmi(struct pt_regs *regs, long error_code)
>  {
> +	unsigned long cr2;
> +	
>  	nmi_nesting_preprocess(regs);
>  
> +	/*
> +	 * Save off the CR2 register. If we take a page fault in the NMI then
> +	 * it could corrupt the CR2 value. If the NMI preempts a page fault
> +	 * handler before it was able to read the CR2 register, and then the
> +	 * NMI itself takes a page fault, the page fault that was preempted
> +	 * will read the information from the NMI page fault and not the
> +	 * origin fault. Save it off and restore it if it changes.

> +	 * Use the r12 callee-saved register.

You might want to make that line go away :-)

> +	 */
> +	cr2 = read_cr2();
> +
>  	nmi_enter();
>  
>  	inc_irq_stat(__nmi_count);
> @@ -523,6 +532,10 @@ do_nmi(struct pt_regs *regs, long error_code)
>  
>  	nmi_exit();
>  
> +	/* Reads are cheaper than writes */
> +	if (unlikely(cr2 != read_cr2()))
> +		write_cr2(cr2);
> +
>  	/* On i386, may loop back to preprocess */
>  	nmi_nesting_postprocess();
>  }
--
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