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

[Andy Lutomirski]

On Mon, Jul 10, 2017 at 11:47 AM, Kirill A. Shutemov
<kirill@xxxxxxxxxxxxx> wrote:
> On Mon, Jul 10, 2017 at 08:56:37AM -0700, Andy Lutomirski wrote:
>>
>>
>> > On Jul 10, 2017, at 7:17 AM, Kirill A. Shutemov <kirill@xxxxxxxxxxxxx> wrote:
>> >
>> >> On Mon, Jul 10, 2017 at 02:43:17PM +0200, Dmitry Vyukov wrote:
>> >> On Mon, Jul 10, 2017 at 2:33 PM, Kirill A. Shutemov
>> >> <kirill@xxxxxxxxxxxxx> wrote:
>> >>> On Thu, Jun 01, 2017 at 05:56:30PM +0300, Andrey Ryabinin wrote:
>> >>>>> On 05/29/2017 03:46 PM, Andrey Ryabinin wrote:
>> >>>>> On 05/29/2017 02:45 PM, Andrey Ryabinin wrote:
>> >>>>>>>>>> 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.
>> >>>>>>
>> >>>>>> I thought about this. We can round down shadow start to 0xffdf000000000000, but we can't
>> >>>>>> round up shadow end, because in that case shadow would end at 0xffffffffffffffff.
>> >>>>>> So we still need at least one more page to cover unaligned end.
>> >>>>>
>> >>>>> Actually, I'm wrong here. I assumed that we would need an additional page to store p4d entries,
>> >>>>> but in fact we don't need it, as such page should already exist. It's the same last pgd where kernel image
>> >>>>> is mapped.
>> >>>>>
>> >>>>
>> >>>>
>> >>>> Something like bellow might work. It's just a proposal to demonstrate the idea, so some code might look ugly.
>> >>>> And it's only build-tested.
>> >>>
>> >>> [Sorry for loong delay.]
>> >>>
>> >>> The patch works for me for legacy boot. But it breaks EFI boot with
>> >>> 5-level paging. And I struggle to understand why.
>> >>>
>> >>> What I see is many page faults at mm/kasan/kasan.c:758 --
>> >>> "DEFINE_ASAN_LOAD_STORE(4)". Handling one of them I get double-fault at
>> >>> arch/x86/kernel/head_64.S:298 -- "pushq %r14", which ends up with triple
>> >>> fault.
>> >>>
>> >>> Any ideas?
>> >>
>> >>
>> >> Just playing the role of the rubber duck:
>> >> - what is the fault address?
>> >> - is it within the shadow range?
>> >> - was the shadow mapped already?
>> >
>> > I misread trace. The initial fault is at arch/x86/kernel/head_64.S:270,
>> > which is ".endr" in definition of early_idt_handler_array.
>> >
>> > The fault address for all three faults is 0xffffffff7ffffff8, which is
>> > outside shadow range. It's just before kernel text mapping.
>> >
>> > Codewise, it happens in load_ucode_bsp() -- after kasan_early_init(), but
>> > before kasan_init().
>>
>> My theory is that, in 5 level mode, the early IDT code isn't all mapped
>> in the page tables.  This could sometimes be papered over by lazy page
>> table setup, but lazy setup can't handle faults in the page fault code
>> or data structures.
>>
>> EFI sometimes uses separate page tables, which could contribute.
>
> As far as I can see all involved code is within the same page:
>
> (gdb) p/x &x86_64_start_kernel
> $1 = 0xffffffff84bad2ae
> (gdb) p/x &early_idt_handler_array
> $2 = 0xffffffff84bad000
> (gdb) p/x &early_idt_handler_common
> $3 = 0xffffffff84bad120
> (gdb) p/x &early_make_pgtable
> $4 = 0xffffffff84bad3b4
>

Can you give the disassembly of the backtrace lines?  Blaming the
.endr doesn't make much sense to me.

Or maybe Andrey will figure it out quickly.