How do I install VirtualBox guest additions for CentOS 7?

vagrant init centos/7
vagrant up; vagrant halt

• Add a CDROM to the virtual machine and select the VBoxGuestAdditions.iso file to insert in the drive.

• Manually add a shared folder in VirtualBox with name vagrant and path to the directory of your Vagrantfile. Vagrant doesn't set up /vagrant properly.

• Edit Vagrantfile and add this to the bottom to provision the shared folder each time you start.

  config.vm.provision "shell", run: "always", inline: <<-SHELL
      mount -t vboxsf -o uid=`id -u vagrant`,gid=`id -g vagrant` vagrant /vagrant
  SHELL
  

• Continue with

  vagrant up
  vagrant ssh
  sudo yum update
  sudo yum install kernel-devel gcc
  exit
  vagrant halt; vagrant up; vagrant ssh
  export KERN_DIR=/usr/src/kernels/`uname -r`
  sudo mount /dev/sr0/ /mnt
  cd /mnt
  sudo ./VBoxLinuxAdditions.run