linux-kernel - Re: 2.6.25-rc6: BUG: unable to handle kernel NULL pointer dereference

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Date:	Fri, 28 Mar 2008 00:50:39 +0100
From:	Björn Steinbrink <B.Steinbrink@....de>
To:	Thomas Gleixner <tglx@...utronix.de>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Christian Kujau <lists@...dbynature.de>,
	LKML <linux-kernel@...r.kernel.org>,
	Markus Rehbach <markus.rehbach@....de>,
	"Rafael J. Wysocki" <rjw@...k.pl>, Ingo Molnar <mingo@...e.hu>
Subject: Re: 2.6.25-rc6: BUG: unable to handle kernel NULL pointer
	dereference

On 2008.03.27 16:20:53 +0100, Thomas Gleixner wrote:
> On Tue, 25 Mar 2008, Andrew Morton wrote:
> > > Code: 53 c0 e8 20 08 fc ff c1 e3 02 8b 14 33 89 54 24 20 8b 44 24 20 85 c0 75 10 eb 51 8b 12 89 54 24 20 8b 44 24 20 85 c0 74 43 8b 02 <0f> 18 00 90 8d 5a d8 39 6b 34 75 e4 8b 7c 24 0c 39 7b 30 75 db 
> 
> It faults in a prefetch.
> 
> > Markus reported what looks to be the same thing here:
> > http://lkml.org/lkml/2008/3/21/202 and it's already in the regresison list.
> 
> Same here. And both are AMD X2 early stepping machines.
>  
> > I guess you've confirmed that this wasn't a mystery
> > once-off-on-that-machine.
> > 
> > I can't think what we did to cause this.
> 
> I had a lengthy bug decoding session with Ingo and we found the root
> cause:
> 
> A dropped workaround for the prefetch bug in early X2s and
> Opterons. Patch below.
> 
> Thanks,
> 
> 	tglx
> 
> --------------->
> Subject: x86: fix prefetch workaround
> From: Ingo Molnar <mingo@...e.hu>
> Date: Thu Mar 27 15:58:28 CET 2008
> 
> some early Athlon XP's and Opterons generate bogus faults on prefetch
                    ^^

Umh, XP? Didn't you say X2 above? And looking at the patch, X2 seems
more plausible as well, I don't think that the XP supported the NX bit,
did it?

Björn

> instructions. The workaround for this regressed over .24 - reinstate it.
> 
> Signed-off-by: Ingo Molnar <mingo@...e.hu>
> Signed-off-by: Thomas Gleixner <tglx@...utronix.de>
> 
> ---
>  arch/x86/mm/fault.c |    3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> Index: linux-x86.q/arch/x86/mm/fault.c
> ===================================================================
> --- linux-x86.q.orig/arch/x86/mm/fault.c
> +++ linux-x86.q/arch/x86/mm/fault.c
> @@ -104,7 +104,8 @@ static int is_prefetch(struct pt_regs *r
>  	unsigned char *max_instr;
>  
>  #ifdef CONFIG_X86_32
> -	if (!(__supported_pte_mask & _PAGE_NX))
> +	/* Catch an obscure case of prefetch inside an NX page: */
> +	if ((__supported_pte_mask & _PAGE_NX) && (error_code & 16))
>  		return 0;
>  #endif
>  
> 
> 
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