Re: [PATCH net v2] ipv6: addrconf: skip autoconf on unregistering devices

[Ido Schimmel]

On Tue, May 12, 2026 at 08:44:10PM +0800, Xu Rao wrote:
> syzbot reports that unregister_netdevice() can wait forever for a
> netdevsim device whose reference count never drops to zero.
> 
> The leaked reference is held by an IPv6 local route created from
> addrconf.  A late NETDEV_CHANGE notification can still reach
> addrconf_notify() after the device has entered NETREG_UNREGISTERING.
> The handler can then run automatic address configuration, add a
> link-local address and install its host route after unregister teardown
> has already started.  The route nexthop takes a netdev reference in
> fib6_nh_init(), and there might not be a later ifdown pass to remove
> the newly created address and route.

Do you have a reproducer?

The kernel repeatedly sends NETDEV_UNREGISTER notifications when it's
waiting for the reference count to drop.

> 
> Do not run MTU, UP or CHANGE based IPv6 autoconfiguration once the
> device is unregistering.  Keep NETDEV_DOWN and NETDEV_UNREGISTER
> handling unchanged so the teardown path can still remove existing IPv6
> state.
> 
> Reported-by: syzbot+e2af46126e0644cbebdd@xxxxxxxxxxxxxxxxxxxxxxxxx
> Closes: https://syzkaller.appspot.com/bug?extid=e2af46126e0644cbebdd
> Signed-off-by: Xu Rao <raoxu@xxxxxxxxxxxxx>
> ---
> v2:
> - Drop READ_ONCE() around dev->reg_state.  addrconf_notify() is called
>   from the netdevice notifier path, so a plain load is sufficient.
> - Do not add a Fixes tag.  The issue does not appear to be caused by a
>   single commit, but by a long-standing unregister-time lifecycle gap.
> 
>  net/ipv6/addrconf.c | 6 ++++++
>  1 file changed, 6 insertions(+)
> 
> diff --git a/net/ipv6/addrconf.c b/net/ipv6/addrconf.c
> index 5476b6536eb7..a517e57cf86a 100644
> --- a/net/ipv6/addrconf.c
> +++ b/net/ipv6/addrconf.c
> @@ -3666,6 +3666,9 @@ static int addrconf_notify(struct notifier_block *this, unsigned long event,
>  		break;
> 
>  	case NETDEV_CHANGEMTU:
> +		if (dev->reg_state == NETREG_UNREGISTERING)
> +			break;
> +
>  		/* if MTU under IPV6_MIN_MTU stop IPv6 on this interface. */
>  		if (dev->mtu < IPV6_MIN_MTU) {
>  			addrconf_ifdown(dev, dev != net->loopback_dev);
> @@ -3691,6 +3694,9 @@ static int addrconf_notify(struct notifier_block *this, unsigned long event,
>  		fallthrough;
>  	case NETDEV_UP:
>  	case NETDEV_CHANGE:
> +		if (dev->reg_state == NETREG_UNREGISTERING)
> +			break;
> +
>  		if (idev && idev->cnf.disable_ipv6)
>  			break;
> 
> --
> 2.50.1