How to install VirtualBox Guest additions on Centos7.x
As root execute the following code
yum install -y make gcc kernel-headers kernel-devel perl dkms bzip2
yum update kernel kernel-headers
reboot now
export KERN_DIR=/usr/src/kernels/$(uname -r)
mount -r /dev/cdrom /media
cd /media/
./VBoxLinuxAdditions.run