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

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Date: Mon, 8 Apr 2024 22:30:21 -0700
From: Martin KaFai Lau <martin.lau@...ux.dev>
To: syzbot <syzbot+9b8be5e35747291236c8@...kaller.appspotmail.com>
Cc: andrii@...nel.org, ast@...nel.org, bpf@...r.kernel.org,
 daniel@...earbox.net, haoluo@...gle.com, john.fastabend@...il.com,
 jolsa@...nel.org, kpsingh@...nel.org, linux-kernel@...r.kernel.org,
 sdf@...gle.com, song@...nel.org, syzkaller-bugs@...glegroups.com,
 yonghong.song@...ux.dev
Subject: Re: [syzbot] [bpf?] KMSAN: uninit-value in strnchr

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@...kaller.appspotmail.com
> 
> =====================================================
> 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