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From:   Qian Cai <cai@lca.pw>
Date:   Tue, 19 May 2020 11:00:31 -0400
Message-ID: <CAG=TAF5rYmMXBcxno0pPxVZdcyz=ik-enh03E-V8wupjDS0K5g@mail.gmail.com>
Subject: Re: UBSAN: array-index-out-of-bounds in kernel/bpf/arraymap.c:177
To:     Andrii Nakryiko <andrii.nakryiko@gmail.com>
Cc:     Alexei Starovoitov <ast@kernel.org>,
        Daniel Borkmann <daniel@iogearbox.net>,
        Martin KaFai Lau <kafai@fb.com>,
        Song Liu <songliubraving@fb.com>, Yonghong Song <yhs@fb.com>,
        Andrii Nakryiko <andriin@fb.com>,
        John Fastabend <john.fastabend@gmail.com>,
        KP Singh <kpsingh@chromium.org>,
        Linux Netdev List <netdev@vger.kernel.org>,
        bpf <bpf@vger.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        clang-built-linux <clang-built-linux@googlegroups.com>,
        Kees Cook <keescook@chromium.org>
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On Mon, May 18, 2020 at 8:25 PM Andrii Nakryiko
<andrii.nakryiko@gmail.com> 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.

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--;
+
+       return elem;
 }