Re: KASAN vs. boot-time switching between 4- and 5-level paging

[Dmitry Vyukov]

On Mon, May 29, 2017 at 1:18 PM, Andrey Ryabinin
<aryabinin@xxxxxxxxxxxxx> wrote:
>
>
> On 05/29/2017 01:02 PM, Dmitry Vyukov wrote:
>> On Sat, May 27, 2017 at 12:10 AM, Kirill A. Shutemov
>> <kirill@xxxxxxxxxxxxx> wrote:
>>> On Thu, May 25, 2017 at 11:33:33PM +0300, Kirill A. Shutemov wrote:
>>>> diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
>>>> index 0bf81e837cbf..c795207d8a3c 100644
>>>> --- a/arch/x86/Kconfig
>>>> +++ b/arch/x86/Kconfig
>>>> @@ -100,7 +100,7 @@ config X86
>>>>       select HAVE_ARCH_AUDITSYSCALL
>>>>       select HAVE_ARCH_HUGE_VMAP              if X86_64 || X86_PAE
>>>>       select HAVE_ARCH_JUMP_LABEL
>>>> -     select HAVE_ARCH_KASAN                  if X86_64 && SPARSEMEM_VMEMMAP
>>>> +     select HAVE_ARCH_KASAN                  if X86_64 && SPARSEMEM_VMEMMAP && !X86_5LEVEL
>>>>       select HAVE_ARCH_KGDB
>>>>       select HAVE_ARCH_KMEMCHECK
>>>>       select HAVE_ARCH_MMAP_RND_BITS          if MMU
>>>
>>> Looks like KASAN will be a problem for boot-time paging mode switching.
>>> It wants to know CONFIG_KASAN_SHADOW_OFFSET at compile-time to pass to
>>> gcc -fasan-shadow-offset=. But this value varies between paging modes...
>>>
>>> I don't see how to solve it. Folks, any ideas?
>>
>> +kasan-dev
>>
>> I wonder if we can use the same offset for both modes. If we use
>> 0xFFDFFC0000000000 as start of shadow for 5 levels, then the same
>> offset that we use for 4 levels (0xdffffc0000000000) will also work
>> for 5 levels. Namely, ending of 5 level shadow will overlap with 4
>> level mapping (both end at 0xfffffbffffffffff), but 5 level mapping
>> extends towards lower addresses. The current 5 level start of shadow
>> is actually close -- 0xffd8000000000000 and it seems that the required
>> space after it is unused at the moment (at least looking at mm.txt).
>> So just try to move it to 0xFFDFFC0000000000?
>>
>
> Yeah, this should work, but note that 0xFFDFFC0000000000 is not PGDIR aligned address. Our init code
> assumes that kasan shadow stars and ends on the PGDIR aligned address.
> Fortunately this is fixable, we'd need two more pages for page tables to map unaligned start/end
> of the shadow.

I think we can extend the shadow backwards (to the current address),
provided that it does not affect shadow offset that we pass to
compiler.