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

[Andrey Ryabinin]

On 07/11/2017 06:06 PM, Andy Lutomirski wrote:
> On Tue, Jul 11, 2017 at 3:35 AM, Kirill A. Shutemov
> <kirill@xxxxxxxxxxxxx> 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@xxxxxxxxxxxxx> 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:

   â0xffffffff84864897 <x86_early_init_platform_quirks+5>   mov    $0xffffffff83f1d0b8,%rdi 
   â0xffffffff8486489e <x86_early_init_platform_quirks+12>  movabs $0xdffffc0000000000,%rax 
   â0xffffffff848648a8 <x86_early_init_platform_quirks+22>  push   %rbp
   â0xffffffff848648a9 <x86_early_init_platform_quirks+23>  mov    %rdi,%rdx  
   â0xffffffff848648ac <x86_early_init_platform_quirks+26>  shr    $0x3,%rdx
   â0xffffffff848648b0 <x86_early_init_platform_quirks+30>  mov    %rsp,%rbp
  >â0xffffffff848648b3 <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