Forwarded: [syzbot] test [PATCH net v2] ipv6: addrconf: skip autoconf on unregistering devices

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Subject: [syzbot] test [PATCH net v2] ipv6: addrconf: skip autoconf on unregistering devices
Author: raoxu@uniontech.com

#syz test: git://git.kernel.org/pub/scm/linux/kernel/git/netdev/net.git main

From: Xu Rao <raoxu@uniontech.com>
Date: Tue, 12 May 2026 20:44:10 +0800
Subject: [PATCH net v2] ipv6: addrconf: skip autoconf on unregistering devices

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 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@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=e2af46126e0644cbebdd
Signed-off-by: Xu Rao <raoxu@uniontech.com>
---
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