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