Re: [PATCH] ipv4: don't call inetdev_init() if device is no longer registered

On Sun, Jan 25, 2026 at 02:11:16PM +0900, Tetsuo Handa wrote:

syzbot is reporting

  unregister_netdevice: waiting for netdevsim0 to become free. Usage count = 3
  ref_tracker: netdev@ffff88807dcf8618 has 1/2 users at
       __netdev_tracker_alloc include/linux/netdevice.h:4400 [inline]
       netdev_hold include/linux/netdevice.h:4429 [inline]
       inetdev_init+0x201/0x4e0 net/ipv4/devinet.c:286
       inetdev_event+0x251/0x1610 net/ipv4/devinet.c:1600
       notifier_call_chain+0x19d/0x3a0 kernel/notifier.c:85
       call_netdevice_notifiers_mtu net/core/dev.c:2318 [inline]
       netif_set_mtu_ext+0x5aa/0x800 net/core/dev.c:9886
       netif_set_mtu+0xd7/0x1b0 net/core/dev.c:9907
       dev_set_mtu+0x126/0x260 net/core/dev_api.c:248
       team_port_del+0xb07/0xcb0 drivers/net/team/team_core.c:1333
       team_del_slave drivers/net/team/team_core.c:1936 [inline]
       team_device_event+0x207/0x5b0 drivers/net/team/team_core.c:2929
       notifier_call_chain+0x19d/0x3a0 kernel/notifier.c:85
       call_netdevice_notifiers_extack net/core/dev.c:2281 [inline]
       call_netdevice_notifiers net/core/dev.c:2295 [inline]
       __dev_change_net_namespace+0xcb7/0x2050 net/core/dev.c:12592
       do_setlink+0x2ce/0x4590 net/core/rtnetlink.c:3060
       rtnl_changelink net/core/rtnetlink.c:3776 [inline]
       __rtnl_newlink net/core/rtnetlink.c:3935 [inline]
       rtnl_newlink+0x15a9/0x1be0 net/core/rtnetlink.c:4072
       rtnetlink_rcv_msg+0x7d5/0xbe0 net/core/rtnetlink.c:6958
       netlink_rcv_skb+0x232/0x4b0 net/netlink/af_netlink.c:2550
       netlink_unicast_kernel net/netlink/af_netlink.c:1318 [inline]
       netlink_unicast+0x80f/0x9b0 net/netlink/af_netlink.c:1344
       netlink_sendmsg+0x813/0xb40 net/netlink/af_netlink.c:1894

problem, for commit 06770843c2f0 ("ipv: Re-enable IP when MTU > 68")
did not expect that NETDEV_CHANGEMTU event is fired as a part of
NETDEV_UNREGISTER event.

Reported-by: syzbot+881d65229ca4f9ae8c84@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=881d65229ca4f9ae8c84
Fixes: 06770843c2f0 ("ipv: Re-enable IP when MTU > 68")
Signed-off-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
---
Since there is no reproducer for this problem, I can't test this change.
But I think that ref_tracker indicates that NETDEV_CHANGEMTU event from
NETDEV_UNREGISTER event is causing netdev_hold() on NETREG_UNREGISTERING
device.

Thanks for the report and fix. You can reproduce with [1], but the
proposed fix doesn't help. Problem is that NETDEV_UNREGISTER is first
caught by inetdev_event(), which calls inetdev_destroy(), but then the
team driver restores dummy1's MTU and initializes the idev again.

[1]
ip link add name team1 type team
ip link add name dummy1 mtu 1499 master team1 type dummy
ip netns add bla
ip link set dev dummy1 netns bla
ip -n bla link del dev dummy1