linux-kernel - Re: [PATCH v3 05/19] x86/kasan/64: Teach KASAN about the cpu_entry

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Message-ID: <CALCETrW7=WDxt_ZiXKBez9mTGz92qeVxWJp3BJauEtAs3r4cTg@mail.gmail.com>
Date:   Fri, 24 Nov 2017 08:27:09 -0800
From:   Andy Lutomirski <luto@...nel.org>
To:     Andrey Ryabinin <aryabinin@...tuozzo.com>
Cc:     Andy Lutomirski <luto@...nel.org>, X86 ML <x86@...nel.org>,
        Borislav Petkov <bpetkov@...e.de>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Brian Gerst <brgerst@...il.com>,
        Dave Hansen <dave.hansen@...el.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Alexander Potapenko <glider@...gle.com>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        kasan-dev <kasan-dev@...glegroups.com>
Subject: Re: [PATCH v3 05/19] x86/kasan/64: Teach KASAN about the cpu_entry_area

On Fri, Nov 24, 2017 at 5:16 AM, Andrey Ryabinin
<aryabinin@...tuozzo.com> wrote:
>
>
> 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@...tuozzo.com>
>> Cc: Alexander Potapenko <glider@...gle.com>
>> Cc: Dmitry Vyukov <dvyukov@...gle.com>
>> Cc: kasan-dev@...glegroups.com
>> Signed-off-by: Andy Lutomirski <luto@...nel.org>
>> ---
>>  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.

Any chance you could send a fixup patch or a replacement patch?  You
obviously understand this code *way* better than I do.

Or you could do my table-based approach and fix it permanently... :)

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