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

From: Yonghong Song <yhs@fb.com>
To: Andrii Nakryiko <andrii.nakryiko@gmail.com>
Cc: Qian Cai <cai@lca.pw>, Alexei Starovoitov <ast@kernel.org>,
	Daniel Borkmann <daniel@iogearbox.net>,
	Martin KaFai Lau <kafai@fb.com>, Song Liu <songliubraving@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>
Subject: Re: UBSAN: array-index-out-of-bounds in kernel/bpf/arraymap.c:177
Date: Mon, 18 May 2020 18:36:26 -0700	[thread overview]
Message-ID: <f2b1ee99-6250-13b7-83ea-d6f870ecf95d@fb.com> (raw)
In-Reply-To: <CAEf4Bza4++AxxU4ikMEfjeYLMiudWqc=Tk=5iTeBBNRjZjZZkQ@mail.gmail.com>

On 5/18/20 6:30 PM, Andrii Nakryiko wrote:
> 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]

Right. There will be a tradeoff between to make it work vs.
some code ugliness :-). BTW, I don't have a strong preference
on how to solve this particular issue.

> 
>>     };