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

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Date:   Mon, 18 May 2020 18:30:20 -0700
From:   Andrii Nakryiko <andrii.nakryiko@...il.com>
To:     Yonghong Song <yhs@...com>
Cc:     Qian Cai <cai@....pw>, Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Martin KaFai Lau <kafai@...com>,
        Song Liu <songliubraving@...com>,
        Andrii Nakryiko <andriin@...com>,
        John Fastabend <john.fastabend@...il.com>,
        KP Singh <kpsingh@...omium.org>,
        Linux Netdev List <netdev@...r.kernel.org>,
        bpf <bpf@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        clang-built-linux <clang-built-linux@...glegroups.com>,
        Kees Cook <keescook@...omium.org>
Subject: Re: UBSAN: array-index-out-of-bounds in kernel/bpf/arraymap.c:177

On Mon, May 18, 2020 at 6:00 PM Yonghong Song <yhs@...com> wrote:
>
>
>
> On 5/18/20 5:25 PM, Andrii Nakryiko wrote:
> > On Mon, May 18, 2020 at 5:09 PM Qian Cai <cai@....pw> wrote:
> >>
> >> On Mon, May 18, 2020 at 7:55 PM Andrii Nakryiko
> >> <andrii.nakryiko@...il.com> wrote:
> >>>
> >>> On Sun, May 17, 2020 at 7:45 PM Qian Cai <cai@....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]

>    };