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

[Andrey Konovalov]

On Mon, Nov 29, 2021 at 7:37 AM 'Dmitry Vyukov' via kasan-dev
<kasan-dev@xxxxxxxxxxxxxxxx> wrote:
>
> On Sun, 28 Nov 2021 at 01:43, Miguel Ojeda
> <miguel.ojeda.sandonis@xxxxxxxxx> 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.gjvqsxacw3hcxfvr@xxxxxxxxxxxxxx/
> >
> > 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!