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(). -- Kirill A. Shutemov
