Re: UBSAN: array-index-out-of-bounds in kernel/bpf/arraymap.c:177

[Andrii Nakryiko]

On Tue, May 19, 2020 at 8:00 AM Qian Cai <cai@xxxxxx> wrote:
>
> On Mon, May 18, 2020 at 8:25 PM Andrii Nakryiko
> <andrii.nakryiko@xxxxxxxxx> wrote:
> >
> > On Mon, May 18, 2020 at 5:09 PM Qian Cai <cai@xxxxxx> wrote:
> > >
> > > On Mon, May 18, 2020 at 7:55 PM Andrii Nakryiko
> > > <andrii.nakryiko@xxxxxxxxx> wrote:
> > > >
> > > > On Sun, May 17, 2020 at 7:45 PM Qian Cai <cai@xxxxxx> wrote:
> > > > >
> > > > > With Clang 9.0.1,
> > > > >
> > > > > return array->value + array->elem_size * (index & array->index_mask);
> > > > >
> > > > > but array->value is,
> > > > >
> > > > > char value[0] __aligned(8);
> > > >
> > > > This, and ptrs and pptrs, should be flexible arrays. But they are in a
> > > > union, and unions don't support flexible arrays. Putting each of them
> > > > into anonymous struct field also doesn't work:
> > > >
> > > > /data/users/andriin/linux/include/linux/bpf.h:820:18: error: flexible
> > > > array member in a struct with no named members
> > > >    struct { void *ptrs[] __aligned(8); };
> > > >
> > > > So it probably has to stay this way. Is there a way to silence UBSAN
> > > > for this particular case?
> > >
> > > I am not aware of any way to disable a particular function in UBSAN
> > > except for the whole file in kernel/bpf/Makefile,
> > >
> > > UBSAN_SANITIZE_arraymap.o := n
> > >
> > > If there is no better way to do it, I'll send a patch for it.
> >
> >
> > That's probably going to be too drastic, we still would want to
> > validate the rest of arraymap.c code, probably. Not sure, maybe
> > someone else has better ideas.
>
> This works although it might makes sense to create a pair of
> ubsan_disable_current()/ubsan_enable_current() for it.
>
> diff --git a/kernel/bpf/arraymap.c b/kernel/bpf/arraymap.c
> index 11584618e861..6415b089725e 100644
> --- a/kernel/bpf/arraymap.c
> +++ b/kernel/bpf/arraymap.c
> @@ -170,11 +170,16 @@ static void *array_map_lookup_elem(struct
> bpf_map *map, void *key)
>  {
>         struct bpf_array *array = container_of(map, struct bpf_array, map);
>         u32 index = *(u32 *)key;
> +       void *elem;
>
>         if (unlikely(index >= array->map.max_entries))
>                 return NULL;
>
> -       return array->value + array->elem_size * (index & array->index_mask);
> +       current->in_ubsan++;
> +       elem = array->value + array->elem_size * (index & array->index_mask);
> +       current->in_ubsan--;

This is an unnecessary performance hit for silencing what is clearly a
false positive. I'm not sure that's the right solution here. It seems
like something that's lacking on the tooling side instead. C language
doesn't allow to express the intent here using flexible array
approach. That doesn't mean that what we are doing here is wrong or
undefined.

> +
> +       return elem;
>  }