A debug patch in linux-next-20260121
( https://git.kernel.org/pub/scm/linux/kernel/git/next/linux-next.git/commit?id=fc0f090e41e652d158f946c616cdd82baed3c8f4 )
has demonstrated that calling xfrm_dev_state_flush()/xfrm_dev_policy_flush()
when (dev->features & NETIF_F_HW_ESP) == 0 helps releasing "struct net_device" refcount.

  unregister_netdevice: waiting for netdevsim0 to become free. Usage count = 2
  ref_tracker: netdev@ffff88805d3c0628 has 1/1 users at
       xfrm_dev_state_add+0x6f4/0xc40 net/xfrm/xfrm_device.c:316
       xfrm_state_construct net/xfrm/xfrm_user.c:986 [inline]
       xfrm_add_sa+0x34ca/0x4230 net/xfrm/xfrm_user.c:1022
       xfrm_user_rcv_msg+0x746/0xb20 net/xfrm/xfrm_user.c:3507
       netlink_rcv_skb+0x232/0x4b0 net/netlink/af_netlink.c:2550
       xfrm_netlink_rcv+0x79/0x90 net/xfrm/xfrm_user.c:3529
       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
       sock_sendmsg_nosec+0x18f/0x1d0 net/socket.c:737
       __sock_sendmsg net/socket.c:752 [inline]
       ____sys_sendmsg+0x589/0x8c0 net/socket.c:2610
       ___sys_sendmsg+0x2a5/0x360 net/socket.c:2664
       __sys_sendmsg net/socket.c:2696 [inline]
       __do_sys_sendmsg net/socket.c:2701 [inline]
       __se_sys_sendmsg net/socket.c:2699 [inline]
       __x64_sys_sendmsg+0x1bd/0x2a0 net/socket.c:2699
       do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
       do_syscall_64+0xe2/0xf80 arch/x86/entry/syscall_64.c:94
       entry_SYSCALL_64_after_hwframe+0x77/0x7f
  
  infiniband: balance for netdevsim0@ib_gid_table_entry is 0
  ***** Releasing 1 refcount on 0000000000000000
  ***** Refcount for netdevsim0 changed from 2 to 1

The bond_master_netdev_event(NETDEV_UNREGISTER) case is already calling
xfrm_dev_state_flush() without checking (dev->features & NETIF_F_HW_ESP) != 0.
Therefore, I assume that (dev->features & NETIF_F_HW_ESP) != 0 check in
xfrm_dev_down() is wrong, and I would like to propose

 static int xfrm_dev_down(struct net_device *dev)
 {
- 	if (dev->features & NETIF_F_HW_ESP) {
-		xfrm_dev_state_flush(dev_net(dev), dev, true);
-		xfrm_dev_policy_flush(dev_net(dev), dev, true);
- 	}
+	xfrm_dev_state_flush(dev_net(dev), dev, true);
+	xfrm_dev_policy_flush(dev_net(dev), dev, true);
 
 	return NOTIFY_DONE;
 }

change as a fix for "unregister_netdevice: waiting for netdevsim0 to become free. Usage count = 2"
problem.



But I have a question regarding security_xfrm_state_delete()/security_xfrm_policy_delete().

xfrm_dev_state_flush_secctx_check() calls security_xfrm_state_delete() which can make
xfrm_dev_state_flush() no-op by returning an error value.
xfrm_dev_policy_flush_secctx_check() calls security_xfrm_policy_delete() which can make
xfrm_dev_policy_flush() no-op by returning an error value.

Since xfrm_dev_state_flush()/xfrm_dev_policy_flush() are called by NETDEV_UNREGISTER
event (which is a signal for releasing all resources that prevent "struct net_device"
references from dropping), making xfrm_dev_state_flush()/xfrm_dev_policy_flush() no-op (by
allowing security_xfrm_state_delete()/security_xfrm_policy_delete() to return an error) is
a denial-of-service bug.

Therefore, I wonder what are security_xfrm_state_delete() and security_xfrm_policy_delete()
for. Can I kill xfrm_dev_state_flush_secctx_check() and xfrm_dev_policy_flush_secctx_check() ?