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

From: Kirill A. Shutemov
Date: Mon Jul 10 2017 - 08:34:00 EST

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

Any ideas?

If you want to play with this by yourself, qemu supports la57 -- use
-cpu "qemu64,+la57".

Kirill A. Shutemov