bonding: fix set mode race conditions

Changing the mode without any locking can result in multiple races (e.g.
upping a bond, enslaving/releasing). Depending on which race is hit the
impact can vary from incosistent bond state to kernel crash.
Use RTNL to synchronize the mode setting with the dangerous races.
Signed-off-by: Nikolay Aleksandrov <nikolay@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/bonding/bond_sysfs.c

@@ -316,6 +316,9 @@ static ssize_t bonding_store_mode(struct device *d,
 	int new_value, ret = count;
 	struct bonding *bond = to_bond(d);
 
+	if (!rtnl_trylock())
+		return restart_syscall();
+
 	if (bond->dev->flags & IFF_UP) {
 		pr_err("unable to update mode of %s because interface is up.\n",
 		       bond->dev->name);
@@ -352,6 +355,7 @@ static ssize_t bonding_store_mode(struct device *d,
 		bond->dev->name, bond_mode_tbl[new_value].modename,
 		new_value);
 out:
+	rtnl_unlock();
 	return ret;
 }
 static DEVICE_ATTR(mode, S_IRUGO | S_IWUSR,