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

Alexei Starovoitov
2024-03-27T01:07:48Z

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 ?