bonding: fix oops during rmmod

In the Linux kernel, the following vulnerability has been resolved:

bonding: fix oops during rmmod

"rmmod bonding" causes an oops ever since commit cc317ea3d927 ("bonding:

remove redundant NULL check in debugfs function"). Here are the relevant

functions being called:

bonding_exit()

bond_destroy_debugfs()

debugfs_remove_recursive(bonding_debug_root);

bonding_debug_root = NULL; <--------- SET TO NULL HERE

bond_netlink_fini()

rtnl_link_unregister()

__rtnl_link_unregister()

unregister_netdevice_many_notify()

bond_uninit()

bond_debug_unregister()

(commit removed check for bonding_debug_root == NULL)

debugfs_remove()

simple_recursive_removal()

down_write() -> OOPS

However, reverting the bad commit does not solve the problem completely

because the original code contains a race that could cause the same

oops, although it was much less likely to be triggered unintentionally:

CPU1

rmmod bonding

bonding_exit()

bond_destroy_debugfs()

debugfs_remove_recursive(bonding_debug_root);

CPU2

echo -bond0 > /sys/class/net/bonding_masters

bond_uninit()

bond_debug_unregister()

if (!bonding_debug_root)

CPU1

bonding_debug_root = NULL;

So do NOT revert the bad commit (since the removed checks were racy

anyway), and instead change the order of actions taken during module

removal. The same oops can also happen if there is an error during

module init, so apply the same fix there.