Re: [PATCH v3 05/19] x86/kasan/64: Teach KASAN about the cpu_entry_area

[Andrey Ryabinin]

On 11/24/2017 07:32 AM, Andy Lutomirski wrote:
> The cpu_entry_area will contain stacks.  Make sure that KASAN has
> appropriate shadow mappings for them.
> 
> Cc: Andrey Ryabinin <aryabinin@xxxxxxxxxxxxx>
> Cc: Alexander Potapenko <glider@xxxxxxxxxx>
> Cc: Dmitry Vyukov <dvyukov@xxxxxxxxxx>
> Cc: kasan-dev@xxxxxxxxxxxxxxxx
> Signed-off-by: Andy Lutomirski <luto@xxxxxxxxxx>
> ---
>  arch/x86/mm/kasan_init_64.c | 13 ++++++++++++-
>  1 file changed, 12 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/x86/mm/kasan_init_64.c b/arch/x86/mm/kasan_init_64.c
> index 99dfed6dfef8..54561dce742e 100644
> --- a/arch/x86/mm/kasan_init_64.c
> +++ b/arch/x86/mm/kasan_init_64.c
> @@ -277,6 +277,7 @@ void __init kasan_early_init(void)
>  void __init kasan_init(void)
>  {
>  	int i;
> +	void *cpu_entry_area_begin, *cpu_entry_area_end;
>  
>  #ifdef CONFIG_KASAN_INLINE
>  	register_die_notifier(&kasan_die_notifier);
> @@ -329,8 +330,18 @@ void __init kasan_init(void)
>  			      (unsigned long)kasan_mem_to_shadow(_end),
>  			      early_pfn_to_nid(__pa(_stext)));
>  
> +	cpu_entry_area_begin = (void *)(__fix_to_virt(FIX_CPU_ENTRY_AREA_BOTTOM));
> +	cpu_entry_area_end = (void *)(__fix_to_virt(FIX_CPU_ENTRY_AREA_TOP) + PAGE_SIZE);
> +
>  	kasan_populate_zero_shadow(kasan_mem_to_shadow((void *)MODULES_END),
> -			(void *)KASAN_SHADOW_END);
> +				   kasan_mem_to_shadow(cpu_entry_area_begin));
> +
> +	kasan_populate_shadow((unsigned long)kasan_mem_to_shadow(cpu_entry_area_begin),
> +			      (unsigned long)kasan_mem_to_shadow(cpu_entry_area_end),
> +		0);
> +
> +	kasan_populate_zero_shadow(kasan_mem_to_shadow(cpu_entry_area_end),

Seems we need to round_up kasan_mem_to_shadow(cpu_entry_area_end) to the next page
(or alternatively - round_up(cpu_entry_area_end, KASAN_SHADOW_SCALE_SIZE*PAGE_SIZE)).
Otherwise, kasan_populate_zero_shadow() will overpopulate the last shadow page of cpu_entry area with kasan_zero_page.

We don't necessarily need to round_down(kasan_mem_to_shadow(cpu_entry_area_begin), PAGE_SIZE) because
kasan_populate_zero_shadow() will not populate the last 'incomplete' page and kasan_populate_shadow()
does round_down() internally, which is exactly what we want here. But it might be better to round_down()
explicitly anyway, to avoid relying on such subtle implementation details.

> +				   (void *)KASAN_SHADOW_END);
>  
>  	load_cr3(init_top_pgt);
>  	__flush_tlb_all();
>