net-sysfs: require net admin in the init ns for setting tx_maxrate

An upcoming change will allow container root to open some /sys/class/net
files for writing. The tx_maxrate attribute can result in changes
to actual hardware devices so err on the side of caution by requiring
CAP_NET_ADMIN in the init namespace in the corresponding attribute store
operation.
Signed-off-by: Tyler Hicks <tyhicks@canonical.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/core/net-sysfs.c

@@ -1087,6 +1087,9 @@ static ssize_t tx_maxrate_store(struct netdev_queue *queue,
 	int err, index = get_netdev_queue_index(queue);
 	u32 rate = 0;
 
+	if (!capable(CAP_NET_ADMIN))
+		return -EPERM;
+
 	err = kstrtou32(buf, 10, &rate);
 	if (err < 0)
 		return err;