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

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Date: Fri, 29 Mar 2024 11:03:16 -0700
From: Martin KaFai Lau <martin.lau@...ux.dev>
To: syzbot <syzbot+8ac8b7b2292ea867a162@...kaller.appspotmail.com>,
 syzkaller-bugs@...glegroups.com
Cc: alexei.starovoitov@...il.com, andrii@...nel.org, ast@...nel.org,
 bpf@...r.kernel.org, daniel@...earbox.net, eddyz87@...il.com,
 haoluo@...gle.com, john.fastabend@...il.com, jolsa@...nel.org,
 kpsingh@...nel.org, linux-kernel@...r.kernel.org, samsun1006219@...il.com,
 sdf@...gle.com, song@...nel.org, xrivendell7@...il.com,
 yonghong.song@...ux.dev
Subject: Re: [syzbot] [bpf?] KMSAN: uninit-value in __bpf_strtoull

On 3/28/24 7:59 PM, syzbot wrote:
> syzbot has found a reproducer for the following issue on:
> 
> HEAD commit:    8d025e2092e2 Merge tag 'erofs-for-6.9-rc2-fixes' of git://..
> git tree:       upstream
> console+strace: https://syzkaller.appspot.com/x/log.txt?x=10f2ffe6180000
> kernel config:  https://syzkaller.appspot.com/x/.config?x=e2599baf258ef795
> dashboard link: https://syzkaller.appspot.com/bug?extid=8ac8b7b2292ea867a162
> 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=15b3ac29180000
> C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=160153c9180000
> 
> Downloadable assets:
> disk image: https://storage.googleapis.com/syzbot-assets/5ccde1a19e22/disk-8d025e20.raw.xz
> vmlinux: https://storage.googleapis.com/syzbot-assets/45420817e7d9/vmlinux-8d025e20.xz
> kernel image: https://storage.googleapis.com/syzbot-assets/354bdafd8c8f/bzImage-8d025e20.xz
> 
> IMPORTANT: if you fix the issue, please add the following tag to the commit:
> Reported-by: syzbot+8ac8b7b2292ea867a162@...kaller.appspotmail.com
> 
> =====================================================
> BUG: KMSAN: uninit-value in __bpf_strtoull+0x245/0x5b0 kernel/bpf/helpers.c:465
>   __bpf_strtoull+0x245/0x5b0 kernel/bpf/helpers.c:465
>   __bpf_strtoll kernel/bpf/helpers.c:504 [inline]
>   ____bpf_strtol kernel/bpf/helpers.c:525 [inline]
>   bpf_strtol+0x7c/0x270 kernel/bpf/helpers.c:519
>   ___bpf_prog_run+0x13fe/0xe0f0 kernel/bpf/core.c:1997
>   __bpf_prog_run96+0xb5/0xe0 kernel/bpf/core.c:2236

This should be similar to the other KMSAN reports on the interpreter using
the uninit stack in the map_lookup/delete_elem helpers. The bpf prog used
in this C reproducer:

    0: (18) r0 = 0x0
    2: (b7) r8 = 0
    3: (7b) *(u64 *)(r10 -72) = r8
    4: (b7) r8 = 0
    5: (7b) *(u64 *)(r10 -16) = r8
    6: (bf) r1 = r10
    7: (07) r1 += -8
            ^^^^^^^^

    8: (bf) r4 = r10
    9: (07) r4 += -16
   10: (b7) r2 = 8
   11: (18) r3 = map[id:68][0]+0
   13: (b7) r3 = 0
   14: (85) call bpf_strtol#896752
   15: (95) exit

#syz fix: bpf: Mark bpf prog stack with kmsan_unposion_memory in interpreter mode