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

["Kirill A. Shutemov" <kirill@shutemov.name>]

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@shutemov.name> 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@shutemov.name> 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

-- 
 Kirill A. Shutemov

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