Re: [PATCH net] bonding: fix oops during rmmod

[Jay Vosburgh <jay.vosburgh@canonical.com>]

Tony Battersby <tonyb@cybernetics.com> wrote:

>"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.
>
>Fixes: cc317ea3d927 ("bonding: remove redundant NULL check in debugfs function")
>Cc: stable@vger.kernel.org
>Signed-off-by: Tony Battersby <tonyb@cybernetics.com>

Acked-by: Jay Vosburgh <jay.vosburgh@canonical.com>

>---
> drivers/net/bonding/bond_main.c | 13 +++++++------
> 1 file changed, 7 insertions(+), 6 deletions(-)
>
>diff --git a/drivers/net/bonding/bond_main.c b/drivers/net/bonding/bond_main.c
>index 2c5ed0a7cb18..bceda85f0dcf 100644
>--- a/drivers/net/bonding/bond_main.c
>+++ b/drivers/net/bonding/bond_main.c
>@@ -6477,16 +6477,16 @@ static int __init bonding_init(void)
> 	if (res)
> 		goto out;
> 
>+	bond_create_debugfs();
>+
> 	res = register_pernet_subsys(&bond_net_ops);
> 	if (res)
>-		goto out;
>+		goto err_net_ops;
> 
> 	res = bond_netlink_init();
> 	if (res)
> 		goto err_link;
> 
>-	bond_create_debugfs();
>-
> 	for (i = 0; i < max_bonds; i++) {
> 		res = bond_create(&init_net, NULL);
> 		if (res)
>@@ -6501,10 +6501,11 @@ static int __init bonding_init(void)
> out:
> 	return res;
> err:
>-	bond_destroy_debugfs();
> 	bond_netlink_fini();
> err_link:
> 	unregister_pernet_subsys(&bond_net_ops);
>+err_net_ops:
>+	bond_destroy_debugfs();
> 	goto out;
> 
> }
>@@ -6513,11 +6514,11 @@ static void __exit bonding_exit(void)
> {
> 	unregister_netdevice_notifier(&bond_netdev_notifier);
> 
>-	bond_destroy_debugfs();
>-
> 	bond_netlink_fini();
> 	unregister_pernet_subsys(&bond_net_ops);
> 
>+	bond_destroy_debugfs();
>+
> #ifdef CONFIG_NET_POLL_CONTROLLER
> 	/* Make sure we don't have an imbalance on our netpoll blocking */
> 	WARN_ON(atomic_read(&netpoll_block_tx));
>
>base-commit: a38297e3fb012ddfa7ce0321a7e5a8daeb1872b6
>-- 
>2.25.1
>
>