Re: [syzbot] [bpf?] [net?] KMSAN: uninit-value in dev_map_lookup_elem

Martin KaFai Lau
2024-03-27T18:46:55Z

On 3/26/24 9:47 PM, Yonghong Song wrote:
>
> On 3/26/24 6:07 PM, Alexei Starovoitov wrote:
>> On Tue, Mar 26, 2024 at 5:54 PM Martin KaFai Lau <martin.lau@linux.dev> wrote:
>>> On 3/25/24 2:36 AM, syzbot wrote:
>>>> Hello,
>>>>
>>>> syzbot found the following issue on:
>>>>
>>>> HEAD commit:    5e74df2f8f15 Merge tag 'x86-urgent-2024-03-24' of git://gi..
>>>> git tree:       upstream
>>>> console+strace: https://syzkaller.appspot.com/x/log.txt?x=148872a5180000
>>>> kernel config:  https://syzkaller.appspot.com/x/.config?x=e6bd769cb793b98a
>>>> dashboard link: https://syzkaller.appspot.com/bug?extid=1a3cf6f08d68868f9db3
>>>> compiler:       Debian clang version 15.0.6, GNU ld (GNU Binutils for
>>>> Debian) 2.40
>>>> syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=15921a6e180000
>>>> C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=12e081f1180000
>>>>
>>>> Downloadable assets:
>>>> disk image:
>>>> https://storage.googleapis.com/syzbot-assets/1a82880723a7/disk-5e74df2f.raw.xz
>>>> vmlinux:
>>>> https://storage.googleapis.com/syzbot-assets/fd3046ac43b9/vmlinux-5e74df2f.xz
>>>> kernel image:
>>>> https://storage.googleapis.com/syzbot-assets/2097be59cbc1/bzImage-5e74df2f.xz
>>>>
>>>> IMPORTANT: if you fix the issue, please add the following tag to the commit:
>>>> Reported-by: syzbot+1a3cf6f08d68868f9db3@syzkaller.appspotmail.com
>>>>
>>>> =====================================================
>>>> BUG: KMSAN: uninit-value in __dev_map_lookup_elem kernel/bpf/devmap.c:441
>>>> [inline]
>>>> BUG: KMSAN: uninit-value in dev_map_lookup_elem+0xf3/0x170
>>>> kernel/bpf/devmap.c:796
>>>>    __dev_map_lookup_elem kernel/bpf/devmap.c:441 [inline]
>>>>    dev_map_lookup_elem+0xf3/0x170 kernel/bpf/devmap.c:796
>>>>    ____bpf_map_lookup_elem kernel/bpf/helpers.c:42 [inline]
>>>>    bpf_map_lookup_elem+0x5c/0x80 kernel/bpf/helpers.c:38
>>>>    ___bpf_prog_run+0x13fe/0xe0f0 kernel/bpf/core.c:1997
>>>>    __bpf_prog_run256+0xb5/0xe0 kernel/bpf/core.c:2237
>>> It should be in the interpreter mode.
>>>
>>> The C reproducer is trying to run the following bpf prog:
>>>
>>>      0: (18) r0 = 0x0
>>>      2: (18) r1 = map[id:49]
>>>      4: (b7) r8 = 16777216
>>>      5: (7b) *(u64 *)(r10 -8) = r8
>>>      6: (bf) r2 = r10
>>>      7: (07) r2 += -229
>>>              ^^^^^^^^^^
>>>
>>>      8: (b7) r3 = 8
>>>      9: (b7) r4 = 0
>>>     10: (85) call dev_map_lookup_elem#1543472
>>>     11: (95) exit
>>>
>>> I think this KMSAN report (and a few others related to lookup/delete_elem)
>>> should only happen in the interpreter mode.
>>>
>>> Does it worth to suppress it by always initializing the stack in the interpreter
>>> mode considering the interpreter is not very speed sensitive ?
>> Maybe we can mark it as initialized from kmsan pov ?
>> There are kasan_poison/unpoison helpers that may fit ?
>
> Maybe use kmsan_unpoison_memory()?

Good idea. It should do. My qemu cannot boot with CONFIG_KMSAN somehow.
I will ask syzbot to test.

#syz test: git://git.kernel.org/pub/scm/linux/kernel/git/martin.lau/bpf-next.git interpreter.kmsan