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

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Date:   Mon, 18 May 2020 18:00:01 -0700
From:   Yonghong Song <yhs@...com>
To:     Andrii Nakryiko <andrii.nakryiko@...il.com>, Qian Cai <cai@....pw>
CC:     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 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);
   };