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

[Martin KaFai Lau]

On 3/7/24 12:30 AM, syzbot wrote:
> Hello,
>
> syzbot found the following issue on:
>
> HEAD commit:    04b8076df253 Merge tag 'firewire-fixes-6.8-rc7' of git://g..
> git tree:       upstream
> console+strace: https://syzkaller.appspot.com/x/log.txt?x=10bb9306180000
> kernel config:  https://syzkaller.appspot.com/x/.config?x=80c7a82a572c0de3
> dashboard link: https://syzkaller.appspot.com/bug?extid=9b8be5e35747291236c8
> 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=11093316180000
> C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=15a53082180000
>
> Downloadable assets:
> disk image: https://storage.googleapis.com/syzbot-assets/a4610b1ff2a7/disk-04b8076d.raw.xz
> vmlinux: https://storage.googleapis.com/syzbot-assets/991e9d902d39/vmlinux-04b8076d.xz
> kernel image: https://storage.googleapis.com/syzbot-assets/a5b8e8e98121/bzImage-04b8076d.xz
>
> IMPORTANT: if you fix the issue, please add the following tag to the commit:
> Reported-by: syzbot+9b8be5e35747291236c8@xxxxxxxxxxxxxxxxxxxxxxxxx
>
> =====================================================
> BUG: KMSAN: uninit-value in strnchr+0x90/0xd0 lib/string.c:388
>   strnchr+0x90/0xd0 lib/string.c:388
>   bpf_bprintf_prepare+0x1c2/0x23b0 kernel/bpf/helpers.c:829
>   ____bpf_trace_printk kernel/trace/bpf_trace.c:385 [inline]
>   bpf_trace_printk+0xec/0x3e0 kernel/trace/bpf_trace.c:375
>   ___bpf_prog_run+0x2180/0xdb80 kernel/bpf/core.c:1986
>   __bpf_prog_run32+0xb2/0xe0 kernel/bpf/core.c:2225

This is the same as the dev_map_lookup_elem report when running in interpreter 
mode. It loads a different program to call bpf_trace_printk instead.

   0: (6a) *(u16 *)(r10 -8) = 628106613
   1: (bf) r1 = r10
   2: (07) r1 += -8
   3: (b7) r2 = 5
   4: (bf) r3 = r1
   5: (85) call bpf_trace_printk#-315920
   6: (b7) r0 = 0
   7: (95) exit


#syz dup: [syzbot] [bpf?] [net?] KMSAN: uninit-value in dev_map_lookup_elem