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

[Dmitry Vyukov]

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?