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

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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




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