Re: perf_fuzzer compiled for x32 causes reboot

[Peter Zijlstra <peterz@infradead.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();
>  }