unregister_netdevice: waiting for DEV to become free (4)

syzbot

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Aug 19, 2020, 9:54:24 AM8/19/20

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Hello,

syzbot found the following issue on:

HEAD commit: 18445bf4 Merge tag 'spi-fix-v5.9-rc1' of git://git.kernel...
git tree: upstream
console output: https://syzkaller.appspot.com/x/log.txt?x=1710d97a900000
kernel config: https://syzkaller.appspot.com/x/.config?x=bb68b9e8a8cc842f
dashboard link: https://syzkaller.appspot.com/bug?extid=df400f2f24a1677cd7e0
compiler: clang version 10.0.0 (https://github.com/llvm/llvm-project/ c2443155a0fb245c8f17f2c1c72b6ea391e86e81)
syz repro: https://syzkaller.appspot.com/x/repro.syz?x=15859986900000
C reproducer: https://syzkaller.appspot.com/x/repro.c?x=1228fea1900000

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: syzbot+df400f...@syzkaller.appspotmail.com

unregister_netdevice: waiting for lo to become free. Usage count = 1

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Dmitry Vyukov

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Aug 19, 2020, 10:03:45 AM8/19/20

to syzbot, LKML, syzkaller-bugs, bpf

On Wed, Aug 19, 2020 at 3:54 PM syzbot
<syzbot+df400f...@syzkaller.appspotmail.com> wrote:
>
> Hello,
>
> syzbot found the following issue on:
>
> HEAD commit: 18445bf4 Merge tag 'spi-fix-v5.9-rc1' of git://git.kernel...
> git tree: upstream
> console output: https://syzkaller.appspot.com/x/log.txt?x=1710d97a900000
> kernel config: https://syzkaller.appspot.com/x/.config?x=bb68b9e8a8cc842f
> dashboard link: https://syzkaller.appspot.com/bug?extid=df400f2f24a1677cd7e0
> compiler: clang version 10.0.0 (https://github.com/llvm/llvm-project/ c2443155a0fb245c8f17f2c1c72b6ea391e86e81)
> syz repro: https://syzkaller.appspot.com/x/repro.syz?x=15859986900000
> C reproducer: https://syzkaller.appspot.com/x/repro.c?x=1228fea1900000
>
> IMPORTANT: if you fix the issue, please add the following tag to the commit:
> Reported-by: syzbot+df400f...@syzkaller.appspotmail.com
>
> unregister_netdevice: waiting for lo to become free. Usage count = 1

Based on the repro, it looks bpf/bpf link related:

syz_emit_ethernet(0x86, &(0x7f0000000000)={@local, @empty=[0x2],
@void, {@ipv4={0x800, @udp={{0x5, 0x4, 0x0, 0x0, 0x78, 0x0, 0x0, 0x0,
0x11, 0x0, @empty, @empty}, {0x0, 0x1b59, 0x64, 0x0,
@wg=@response={0x5, 0x0, 0x0, "020000010865390406030500000000010900",
"9384bbeb3018ad591b661fe808b21b77",
{"694c875dfb1be5d2a0057a62022a1564",
"a329d3a73b8268129e5fa4316a5d8c69"}}}}}}}, 0x0)
mkdirat(0xffffffffffffff9c, &(0x7f0000000000)='./file0\x00', 0x0)
mount(0x0, &(0x7f0000000080)='./file0\x00',
&(0x7f0000000040)='cgroup2\x00', 0x0, 0x0)
r0 = openat$cgroup_root(0xffffffffffffff9c, &(0x7f0000000000), 0x200002, 0x0)
r1 = bpf$PROG_LOAD(0x5, &(0x7f0000000080)={0x9, 0x4,
&(0x7f0000000000)=@framed={{}, [@alu={0x8000000201a7f19, 0x0, 0x6,
0x2, 0x1}]}, &(0x7f0000000100)='GPL\x00', 0x0, 0x0, 0x0, 0x0, 0x0, [],
0x0, 0x0, 0xffffffffffffffff, 0x8, 0x0, 0x0, 0x10, 0x0}, 0x70)
bpf$BPF_LINK_CREATE(0x1c, &(0x7f0000000100)={r1, r0, 0x2}, 0x10)

syzbot

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Aug 19, 2020, 10:51:07 AM8/19/20

to a...@kernel.org, b...@vger.kernel.org, da...@davemloft.net, dvy...@google.com, linux-...@vger.kernel.org, linux-...@vger.kernel.org, mcg...@kernel.org, syzkall...@googlegroups.com, vi...@zeniv.linux.org.uk

syzbot has bisected this issue to:

commit 449325b52b7a6208f65ed67d3484fd7b7184477b
Author: Alexei Starovoitov <a...@kernel.org>
Date: Tue May 22 02:22:29 2018 +0000

umh: introduce fork_usermode_blob() helper

bisection log: https://syzkaller.appspot.com/x/bisect.txt?x=11f86186900000
start commit: 18445bf4 Merge tag 'spi-fix-v5.9-rc1' of git://git.kernel...
git tree: upstream
final oops: https://syzkaller.appspot.com/x/report.txt?x=13f86186900000
console output: https://syzkaller.appspot.com/x/log.txt?x=15f86186900000

kernel config: https://syzkaller.appspot.com/x/.config?x=bb68b9e8a8cc842f
dashboard link: https://syzkaller.appspot.com/bug?extid=df400f2f24a1677cd7e0

syz repro: https://syzkaller.appspot.com/x/repro.syz?x=15859986900000
C reproducer: https://syzkaller.appspot.com/x/repro.c?x=1228fea1900000

Reported-by: syzbot+df400f...@syzkaller.appspotmail.com
Fixes: 449325b52b7a ("umh: introduce fork_usermode_blob() helper")

For information about bisection process see: https://goo.gl/tpsmEJ#bisection

Andrii Nakryiko

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Aug 20, 2020, 1:07:12 PM8/20/20

to Dmitry Vyukov, syzbot, LKML, syzkaller-bugs, bpf

The only place where BPF link-related code is bumping refcount for
net_device is in bpf_xdp_link_attach(), but both success and failure
code paths always do dev_put() in the end. bpf_link itself has a
pointer on net_device, but it's protected by rtnl_lock() only, no
refcnt associated with it. So I don't see how bpf_link can cause this.
I also couldn't reproduce this locally, using the provided C
reproducer.

Dmitry Vyukov

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Aug 20, 2020, 1:15:29 PM8/20/20

to Andrii Nakryiko, syzbot, LKML, syzkaller-bugs, bpf

I was able to reproduce this in qemu the first time.

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