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

[Andrii Nakryiko <andrii.nakryiko@gmail.com>]

On Mon, May 18, 2020 at 6:00 PM Yonghong Song <yhs@fb.com> wrote:
>
>
>
> On 5/18/20 5:25 PM, Andrii Nakryiko wrote:
> > On Mon, May 18, 2020 at 5:09 PM Qian Cai <cai@lca.pw> wrote:
> >>
> >> On Mon, May 18, 2020 at 7:55 PM Andrii Nakryiko
> >> <andrii.nakryiko@gmail.com> wrote:
> >>>
> >>> On Sun, May 17, 2020 at 7:45 PM Qian Cai <cai@lca.pw> 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.
>
> Maybe something like below?
>
>    struct bpf_array {
>          struct bpf_map map;
>          u32 elem_size;
>          u32 index_mask;
>          struct bpf_array_aux *aux;
>          union {
>                  char value;
>                  void *ptrs;
>                  void __percpu *pptrs;
>          } u[] __aligned(8);

That will require wider code changes, and would look quite unnatural:

array->u[whatever].pptrs

instead of current

array->pptrs[whatever]

>    };