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

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Date:   Mon, 18 May 2020 17:25:27 -0700
From:   Andrii Nakryiko <andrii.nakryiko@...il.com>
To:     Qian Cai <cai@....pw>
Cc:     Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Martin KaFai Lau <kafai@...com>,
        Song Liu <songliubraving@...com>, Yonghong Song <yhs@...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 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.