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

[Martin KaFai Lau]

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@xxxxxxxxx> 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@xxxxxxxxxxxxxxxxxxxxxxxxx
> > > >
> > > > =====================================================
> > > > 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