Re: BUG: KASAN: stack-out-of-bounds

[Dmitry Vyukov <dvyukov@google.com>]

On Thu, Feb 28, 2019 at 10:22 AM Andrey Ryabinin
<aryabinin@virtuozzo.com> wrote:
>
>
>
> On 2/27/19 4:11 PM, Christophe Leroy wrote:
> >
> >
> > Le 27/02/2019 à 10:19, Andrey Ryabinin a écrit :
> >>
> >>
> >> On 2/27/19 11:25 AM, Christophe Leroy wrote:
> >>> With version v8 of the series implementing KASAN on 32 bits powerpc (https://patchwork.ozlabs.org/project/linuxppc-dev/list/?series=94309), I'm now able to activate KASAN on a mac99 is QEMU.
> >>>
> >>> Then I get the following reports at startup. Which of the two reports I get seems to depend on the option used to build the kernel, but for a given kernel I always get the same report.
> >>>
> >>> Is that a real bug, in which case how could I spot it ? Or is it something wrong in my implementation of KASAN ?
> >>>
> >>> I checked that after kasan_init(), the entire shadow memory is full of 0 only.
> >>>
> >>> I also made a try with the strong STACK_PROTECTOR compiled in, but no difference and nothing detected by the stack protector.
> >>>
> >>> ==================================================================
> >>> BUG: KASAN: stack-out-of-bounds in memchr+0x24/0x74
> >>> Read of size 1 at addr c0ecdd40 by task swapper/0
> >>>
> >>> CPU: 0 PID: 0 Comm: swapper Not tainted 5.0.0-rc7+ #1133
> >>> Call Trace:
> >>> [c0e9dca0] [c01c42a0] print_address_description+0x64/0x2bc (unreliable)
> >>> [c0e9dcd0] [c01c4684] kasan_report+0xfc/0x180
> >>> [c0e9dd10] [c089579c] memchr+0x24/0x74
> >>> [c0e9dd30] [c00a9e38] msg_print_text+0x124/0x574
> >>> [c0e9dde0] [c00ab710] console_unlock+0x114/0x4f8
> >>> [c0e9de40] [c00adc60] vprintk_emit+0x188/0x1c4
> >>> --- interrupt: c0e9df00 at 0x400f330
> >>>      LR = init_stack+0x1f00/0x2000
> >>> [c0e9de80] [c00ae3c4] printk+0xa8/0xcc (unreliable)
> >>> [c0e9df20] [c0c28e44] early_irq_init+0x38/0x108
> >>> [c0e9df50] [c0c16434] start_kernel+0x310/0x488
> >>> [c0e9dff0] [00003484] 0x3484
> >>>
> >>> The buggy address belongs to the variable:
> >>>   __log_buf+0xec0/0x4020
> >>> The buggy address belongs to the page:
> >>> page:c6eac9a0 count:1 mapcount:0 mapping:00000000 index:0x0
> >>> flags: 0x1000(reserved)
> >>> raw: 00001000 c6eac9a4 c6eac9a4 00000000 00000000 00000000 ffffffff 00000001
> >>> page dumped because: kasan: bad access detected
> >>>
> >>> Memory state around the buggy address:
> >>>   c0ecdc00: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
> >>>   c0ecdc80: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
> >>>> c0ecdd00: 00 00 00 00 00 00 00 00 f1 f1 f1 f1 00 00 00 00
> >>>                                     ^
> >>>   c0ecdd80: f3 f3 f3 f3 00 00 00 00 00 00 00 00 00 00 00 00
> >>>   c0ecde00: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
> >>> ==================================================================
> >>>
> >>
> >> This one doesn't look good. Notice that it says stack-out-of-bounds, but at the same time there is
> >>     "The buggy address belongs to the variable:  __log_buf+0xec0/0x4020"
> >>   which is printed by following code:
> >>     if (kernel_or_module_addr(addr) && !init_task_stack_addr(addr)) {
> >>         pr_err("The buggy address belongs to the variable:\n");
> >>         pr_err(" %pS\n", addr);
> >>     }
> >>
> >> So the stack unrelated address got stack-related poisoning. This could be a stack overflow, did you increase THREAD_SHIFT?
> >> KASAN with stack instrumentation significantly increases stack usage.
> >>
> >
> > I get the above with THREAD_SHIFT set to 13 (default value).
> > If increasing it to 14, I get the following instead. That means that in that case the problem arises a lot earlier in the boot process (but still after the final kasan shadow setup).
> >
>
> We usually use 15 (with 4k pages), but I think 14 should be enough for the clean boot.
>
> > ==================================================================
> > BUG: KASAN: stack-out-of-bounds in pmac_nvram_init+0x1f8/0x5d0
> > Read of size 1 at addr f6f37de0 by task swapper/0
> >
> > CPU: 0 PID: 0 Comm: swapper Not tainted 5.0.0-rc7+ #1143
> > Call Trace:
> > [c0e9fd60] [c01c43c0] print_address_description+0x164/0x2bc (unreliable)
> > [c0e9fd90] [c01c46a4] kasan_report+0xfc/0x180
> > [c0e9fdd0] [c0c226d4] pmac_nvram_init+0x1f8/0x5d0
> > [c0e9fef0] [c0c1f73c] pmac_setup_arch+0x298/0x314
> > [c0e9ff20] [c0c1ac40] setup_arch+0x250/0x268
> > [c0e9ff50] [c0c151dc] start_kernel+0xb8/0x488
> > [c0e9fff0] [00003484] 0x3484
> >
> >
> > Memory state around the buggy address:
> >  f6f37c80: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
> >  f6f37d00: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
> >>f6f37d80: 00 00 00 00 00 00 00 00 00 00 00 00 f1 f1 f1 f1
> >                                                ^
> >  f6f37e00: 00 00 01 f4 f2 f2 f2 f2 00 00 00 00 f2 f2 f2 f2
> >  f6f37e80: 00 00 00 00 f3 f3 f3 f3 00 00 00 00 00 00 00 00
> > ==================================================================
>
> Powerpc's show_stack() prints stack addresses, so we know that stack is something near 0xc0e9f... address.
> f6f37de0 is definitely not stack address and it's to far for the stack overflow.
> So it looks like shadow for stack  - kasan_mem_to_shadow(0xc0e9f...) and shadow for address in report - kasan_mem_to_shadow(0xf6f37de0)
> point to the same physical page.

Shouldn't shadow start at 0xf8 for powerpc32? I did some math
yesterday which I think lead me to 0xf8.
This allows to cover at most 1GB of memory. Do you have more by any chance?