KASAN Arm: global-out-of-bounds in load_module

[Miguel Ojeda]

Hi KASAN / Arm folks,

I noticed in our CI that inserting and removing a module, and then
inserting it again, e.g.:

insmod bcm2835_thermal.ko
rmmod bcm2835_thermal.ko
insmod bcm2835_thermal.ko

deterministically triggers the report below in v5.16-rc2. I also tried
it on v5.12 to see if it was a recent thing, but same story.

I could find this other report from May, which may be related:
https://lore.kernel.org/lkml/20210510202653....@pengutronix.de/

Cheers,
Miguel

BUG: KASAN: global-out-of-bounds in load_module+0x1b98/0x33b0
Write of size 16384 at addr bf000000 by task busybox/17

CPU: 0 PID: 17 Comm: busybox Not tainted 5.15.0 #7
Hardware name: Generic DT based system
[<c010f968>] (unwind_backtrace) from [<c010c6f8>] (show_stack+0x10/0x14)
[<c010c6f8>] (show_stack) from [<c0210734>]
(print_address_description+0x58/0x384)
[<c0210734>] (print_address_description) from [<c0210cc8>]
(kasan_report+0x168/0x1fc)
[<c0210cc8>] (kasan_report) from [<c0211230>] (kasan_check_range+0x260/0x2a8)
[<c0211230>] (kasan_check_range) from [<c0211c68>] (memset+0x20/0x44)
[<c0211c68>] (memset) from [<c019d21c>] (load_module+0x1b98/0x33b0)
[<c019d21c>] (load_module) from [<c0199f88>] (sys_init_module+0x198/0x1ac)
[<c0199f88>] (sys_init_module) from [<c0100060>] (ret_fast_syscall+0x0/0x48)
Exception stack(0xc113ffa8 to 0xc113fff0)
ffa0: 00000000 00002a98 00098038 00002a98 00081483 00093f88
ffc0: 00000000 00002a98 00000000 00000080 00000001 b66ffef0 00081483 000815c7
ffe0: b66ffbd8 b66ffbc8 000207f5 00011cc2


Memory state around the buggy address:
bf001200: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
bf001280: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
>bf001300: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 f9 f9
^
bf001380: 00 00 07 f9 f9 f9 f9 f9 00 00 00 00 00 00 00 00
bf001400: 00 00 f9 f9 f9 f9 f9 f9 00 00 04 f9 f9 f9 f9 f9

[Dmitry Vyukov]

On Sun, 28 Nov 2021 at 01:43, Miguel Ojeda
<miguel.oje...@gmail.com> wrote:
>
> Hi KASAN / Arm folks,
>
> I noticed in our CI that inserting and removing a module, and then
> inserting it again, e.g.:
>
> insmod bcm2835_thermal.ko
> rmmod bcm2835_thermal.ko
> insmod bcm2835_thermal.ko
>
> deterministically triggers the report below in v5.16-rc2. I also tried
> it on v5.12 to see if it was a recent thing, but same story.
>
> I could find this other report from May, which may be related:
> https://lore.kernel.org/lkml/20210510202653....@pengutronix.de/
>
> Cheers,
> Miguel

HI Miguel,

0xf9 is redzone for global variables:
#define KASAN_GLOBAL_REDZONE 0xF9 /* redzone for global variable */

I would assume this is caused by not clearing shadow of unloaded
modules, so that the next module loaded hits these leftover redzones.

+arm mailing list and Linus W

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[Andrey Konovalov]

On Mon, Nov 29, 2021 at 7:37 AM 'Dmitry Vyukov' via kasan-dev
<kasa...@googlegroups.com> wrote:
>
> On Sun, 28 Nov 2021 at 01:43, Miguel Ojeda
> <miguel.oje...@gmail.com> wrote:
> >
> > Hi KASAN / Arm folks,
> >
> > I noticed in our CI that inserting and removing a module, and then
> > inserting it again, e.g.:
> >
> > insmod bcm2835_thermal.ko
> > rmmod bcm2835_thermal.ko
> > insmod bcm2835_thermal.ko
> >
> > deterministically triggers the report below in v5.16-rc2. I also tried
> > it on v5.12 to see if it was a recent thing, but same story.
> >
> > I could find this other report from May, which may be related:
> > https://lore.kernel.org/lkml/20210510202653....@pengutronix.de/
> >
> > Cheers,
> > Miguel
>
> HI Miguel,
>
> 0xf9 is redzone for global variables:
> #define KASAN_GLOBAL_REDZONE 0xF9 /* redzone for global variable */
>
> I would assume this is caused by not clearing shadow of unloaded
> modules, so that the next module loaded hits these leftover redzones.

Hi Miguel,

Adding to what Dmitry mentioned:

The code that's responsible for allocating&clearing/freeing shadow for
modules is at the very end of mm/kasan/shadow.c. It's only required
when CONFIG_KASAN_VMALLOC is not supported/enabled.

As 32-bit arm doesn't select HAVE_ARCH_KASAN_VMALLOC, perhaps it needs
something along the lines of what kasan_module_alloc() does with
regards to clearing shadow? I assume arm doesn't call that function
directly due to a different shadow allocation scheme.

Just a guess.

Thanks!

[Ard Biesheuvel]

Side note: vmap'ed stacks support is being added to ARM, so it would
be worth it to investigate whether we can support
HAVE_ARCH_KASAN_VMALLOC on ARM as well, otherwise we cannot enable
vmap'ed stacks and KASAN at the same time.