linux-kernel - Re: [PATCH 1/5] x86_64: fix kasan shadow region page tables

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Date:	Thu, 2 Jul 2015 09:56:50 +0200
From:	Ingo Molnar <mingo@...nel.org>
To:	Andrey Ryabinin <a.ryabinin@...sung.com>
Cc:	Ingo Molnar <mingo@...hat.com>, "H. Peter Anvin" <hpa@...or.com>,
	Thomas Gleixner <tglx@...utronix.de>, x86@...nel.org,
	Andrey Konovalov <adech.fo@...il.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Borislav Petkov <bp@...en8.de>,
	Alexander Popov <alpopov@...ecurity.com>,
	Dmitry Vyukov <dvyukov@...gle.com>,
	Alexander Potapenko <glider@...gle.com>,
	linux-kernel@...r.kernel.org, stable@...r.kernel.org
Subject: Re: [PATCH 1/5] x86_64: fix kasan shadow region page tables


* Andrey Ryabinin <a.ryabinin@...sung.com> wrote:

> On 07/01/2015 12:07 PM, Ingo Molnar wrote:
> > 
> > * Andrey Ryabinin <a.ryabinin@...sung.com> wrote:
> > 
> >> diff --git a/arch/x86/kernel/head64.c b/arch/x86/kernel/head64.c
> >> index 5a46681..f129a9a 100644
> >> --- a/arch/x86/kernel/head64.c
> >> +++ b/arch/x86/kernel/head64.c
> >> @@ -161,11 +161,12 @@ asmlinkage __visible void __init x86_64_start_kernel(char * real_mode_data)
> >>  	/* Kill off the identity-map trampoline */
> >>  	reset_early_page_tables();
> >>  
> >> -	kasan_map_early_shadow(early_level4_pgt);
> >> -
> >> -	/* clear bss before set_intr_gate with early_idt_handler */
> >>  	clear_bss();
> >>  
> >> +	clear_page(init_level4_pgt);
> >> +
> >> +	kasan_early_init();
> >> +
> >>  	for (i = 0; i < NUM_EXCEPTION_VECTORS; i++)
> >>  		set_intr_gate(i, early_idt_handler_array[i]);
> >>  	load_idt((const struct desc_ptr *)&idt_descr);
> >> @@ -177,12 +178,9 @@ asmlinkage __visible void __init x86_64_start_kernel(char * real_mode_data)
> >>  	 */
> >>  	load_ucode_bsp();
> >>  
> >> -	clear_page(init_level4_pgt);
> >>  	/* set init_level4_pgt kernel high mapping*/
> >>  	init_level4_pgt[511] = early_level4_pgt[511];
> >>  
> >> -	kasan_map_early_shadow(init_level4_pgt);
> >> -
> >>  	x86_64_start_reservations(real_mode_data);
> >>  }
> >>  
> > 
> > So this changes generic code (moves the clear_page(init_level4_pgt) call), but the 
> > changelog claims it's a KASAN-specific change.
> > 
> > Please split this into two patches: the first one does the generic change, the 
> > second one the KASAN specific one.
> > 
> 
> Hm...  We will need to backport that generic change to stable, because second change depends on it.
> So, maybe split this on three changes:
> 
> 	#1 fix kasan page tables (without touching clear_page() or kasan_map_early_shadow(init_level4_pgt))
> 	#2 generic move clear_page()
> 	#3 clean up kasan initialization (move kasan_map_early_shadow(init_level4_pgt)).
> 
> Only #1 will have stable tag. Does that makes sense?

I wouldn't overcomplicate it - just split it up and both patches can get the 
stable tag just fine...

Thanks,

	Ingo
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