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