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SubjectRe: KASAN vs. boot-time switching between 4- and 5-level paging
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On 07/11/2017 06:06 PM, Andy Lutomirski wrote:
> On Tue, Jul 11, 2017 at 3:35 AM, Kirill A. Shutemov
> <kirill@shutemov.name> wrote:
>> On Mon, Jul 10, 2017 at 05:30:38PM -0700, Andy Lutomirski wrote:
>>> On Mon, Jul 10, 2017 at 2:24 PM, Kirill A. Shutemov
>>> <kirill@shutemov.name> wrote:
>>>> On Mon, Jul 10, 2017 at 01:07:13PM -0700, Andy Lutomirski wrote:
>>>>> Can you give the disassembly of the backtrace lines? Blaming the
>>>>> .endr doesn't make much sense to me.
>>>>
>>>> I don't have backtrace. It's before printk() is functional. I only see
>>>> triple fault and reboot.
>>>>
>>>> I had to rely on qemu tracing and gdb.
>>>
>>> Can you ask GDB or objtool to disassemble around those addresses? Can
>>> you also attach the big dump that QEMU throws out that shows register
>>> state? In particular, CR2, CR3, and CR4 could be useful.
>>
>> The last three execptions:
>>
>> check_exception old: 0xffffffff new 0xe, cr2: 0xffffffff7ffffff8, rip: 0xffffffff84bb3036
>> RAX=00000000ffffffff RBX=ffffffff800000d8 RCX=ffffffff84be4021 RDX=dffffc0000000000
>> RSI=0000000000000006 RDI=ffffffff84c57000 RBP=ffffffff800000c8 RSP=ffffffff80000000
>
> So RSP was 0xffffffff80000000, a push happened, and we tried to write
> to 0xffffffff7ffffff8, which failed.
>
>> check_exception old: 0xe new 0xe, cr2: 0xffffffff7ffffff8, rip: 0xffffffff84bb3141
>> RAX=00000000ffffffff RBX=ffffffff800000d8 RCX=ffffffff84be4021 RDX=dffffc0000000000
>> RSI=0000000000000006 RDI=ffffffff84c57000 RBP=ffffffff800000c8 RSP=ffffffff80000000
>
> And #PF doesn't use IST, so it double-faulted.
>
> Either the stack isn't mapped in the page tables, RSP is corrupt, or
> there's a genuine stack overflow here.
>

I reproduced this, and this is kasan bug:

│0xffffffff84864897 <x86_early_init_platform_quirks+5> mov $0xffffffff83f1d0b8,%rdi
│0xffffffff8486489e <x86_early_init_platform_quirks+12> movabs $0xdffffc0000000000,%rax
│0xffffffff848648a8 <x86_early_init_platform_quirks+22> push %rbp
│0xffffffff848648a9 <x86_early_init_platform_quirks+23> mov %rdi,%rdx
│0xffffffff848648ac <x86_early_init_platform_quirks+26> shr $0x3,%rdx
│0xffffffff848648b0 <x86_early_init_platform_quirks+30> mov %rsp,%rbp
>│0xffffffff848648b3 <x86_early_init_platform_quirks+33> mov (%rdx,%rax,1),%al

we crash on the last move which is a read from shadow

(gdb) p/x $rdx
$1 = 0x1ffffffff07e3a17
(gdb) p/x $rax
$2 = 0xdffffc0000000000

(gdb) p/x 0xdffffc0000000000 + 0x1ffffffff07e3a17
$4 = 0xfffffbfff07e3a17
(gdb) p/x *0xfffffbfff07e3a17
Cannot access memory at address 0xfffffbfff07e3a17

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