[octopus@pc3 ~]$ sudo yum install kmod-VirtualBox-$(uname -r) kmod-VirtualBox
[sudo] password for octopus:
Sorry, try again.
[sudo] password for octopus:
Loaded plugins: langpacks
Package kmod-VirtualBox-3.17.7-300.fc21.x86_64-4.3.20-2.fc21.2.x86_64 already installed and latest version
Resolving Dependencies
--> Running transaction check
---> Package kmod-VirtualBox.x86_64 0:4.3.20-2.fc21.2 will be installed
--> Finished Dependency Resolution

Dependencies Resolved

================================================================================
 Package           Arch     Version              Repository                Size
================================================================================
Installing:
 kmod-VirtualBox   x86_64   4.3.20-2.fc21.2      rpmfusion-free-updates    15 k

Transaction Summary
================================================================================
Install  1 Package

Total download size: 15 k
Installed size: 0  
Is this ok [y/d/N]: y
Downloading packages:
kmod-VirtualBox-4.3.20-2.fc21.2.x86_64.rpm                  |  15 kB  00:01     
Running transaction check
Running transaction test
Transaction test succeeded
Running transaction (shutdown inhibited)
  Installing : kmod-VirtualBox-4.3.20-2.fc21.2.x86_64                       1/1
  Verifying  : kmod-VirtualBox-4.3.20-2.fc21.2.x86_64                       1/1

Installed:
  kmod-VirtualBox.x86_64 0:4.3.20-2.fc21.2                                      

Complete!
[octopus@pc3 ~]$ sudo yum install akmod-VirtualBox kernel-devel-$(uname -r); akmods
Loaded plugins: langpacks
Package kernel-devel-3.17.7-300.fc21.x86_64 already installed and latest version
Resolving Dependencies
--> Running transaction check
---> Package akmod-VirtualBox.x86_64 0:4.3.20-2.fc21 will be installed
--> Processing Dependency: VirtualBox-kmodsrc >= 4.3.20-1 for package: akmod-VirtualBox-4.3.20-2.fc21.x86_64
--> Running transaction check
---> Package VirtualBox-kmodsrc.x86_64 0:4.3.20-3.fc21 will be installed
--> Finished Dependency Resolution

Dependencies Resolved

================================================================================
 Package              Arch     Version           Repository                Size
================================================================================
Installing:
 akmod-VirtualBox     x86_64   4.3.20-2.fc21     rpmfusion-free            22 k
Installing for dependencies:
 VirtualBox-kmodsrc   x86_64   4.3.20-3.fc21     rpmfusion-free-updates   653 k

Transaction Summary
================================================================================
Install  1 Package (+1 Dependent package)

Total download size: 675 k
Installed size: 637 k
Is this ok [y/d/N]: y
Downloading packages:
(1/2): akmod-VirtualBox-4.3.20-2.fc21.x86_64.rpm            |  22 kB  00:09     
(2/2): VirtualBox-kmodsrc-4.3.20-3.fc21.x86_64.rpm          | 653 kB  00:54     
--------------------------------------------------------------------------------
Total                                               12 kB/s | 675 kB  00:54     
Running transaction check
Running transaction test
Transaction test succeeded
Running transaction (shutdown inhibited)
  Installing : VirtualBox-kmodsrc-4.3.20-3.fc21.x86_64                      1/2
  Installing : akmod-VirtualBox-4.3.20-2.fc21.x86_64                        2/2
  Verifying  : akmod-VirtualBox-4.3.20-2.fc21.x86_64                        1/2
  Verifying  : VirtualBox-kmodsrc-4.3.20-3.fc21.x86_64                      2/2

Installed:
  akmod-VirtualBox.x86_64 0:4.3.20-2.fc21                                       

Dependency Installed:
  VirtualBox-kmodsrc.x86_64 0:4.3.20-3.fc21                                     

Complete!
Needs to run as root to be able to install rpms.           [FAILED]
[octopus@pc3 ~]$ sudo systemctl restart systemd-modules-load.service
[octopus@pc3 ~]$ sudo yum install akmod-VirtualBox kernel-devel-$(uname -r); akmods
Loaded plugins: langpacks
Package akmod-VirtualBox-4.3.20-2.fc21.x86_64 already installed and latest version
Package kernel-devel-3.17.7-300.fc21.x86_64 already installed and latest version
Nothing to do
Needs to run as root to be able to install rpms.           [FAILED]
[octopus@pc3 ~]$

具体提示信息是：
https://zh.arslanbar.net/post.php?t=21116
把里面的3个命令依次执行就可以解决了。