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

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Date: Tue, 26 Mar 2024 18:07:34 -0700
From: Alexei Starovoitov <alexei.starovoitov@...il.com>
To: Martin KaFai Lau <martin.lau@...ux.dev>
Cc: Alexei Starovoitov <ast@...nel.org>, Andrii Nakryiko <andrii@...nel.org>, 
	Daniel Borkmann <daniel@...earbox.net>, Yonghong Song <yonghong.song@...ux.dev>, 
	syzbot <syzbot+1a3cf6f08d68868f9db3@...kaller.appspotmail.com>, 
	bpf <bpf@...r.kernel.org>, "David S. Miller" <davem@...emloft.net>, Eddy Z <eddyz87@...il.com>, 
	Hao Luo <haoluo@...gle.com>, Jesper Dangaard Brouer <hawk@...nel.org>, 
	John Fastabend <john.fastabend@...il.com>, Jiri Olsa <jolsa@...nel.org>, 
	KP Singh <kpsingh@...nel.org>, Jakub Kicinski <kuba@...nel.org>, 
	LKML <linux-kernel@...r.kernel.org>, Network Development <netdev@...r.kernel.org>, 
	Stanislav Fomichev <sdf@...gle.com>, Song Liu <song@...nel.org>, 
	syzkaller-bugs <syzkaller-bugs@...glegroups.com>
Subject: Re: [syzbot] [bpf?] [net?] KMSAN: uninit-value in dev_map_lookup_elem

On Tue, Mar 26, 2024 at 5:54 PM Martin KaFai Lau <martin.lau@...ux.dev> 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@...kaller.appspotmail.com
> >
> > =====================================================
> > 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 ?