Oracle Linux 7.6 (UEK5) / VirtualBox Shared Folder Issue

It appears that there’s an incompatibility between the latest UEK Release 5 kernel releases for Oracle Linux 7 and version 5.2.20 of the VirtualBox Guest Additions kernel module.  This issue prevents VirtualBox shared folders from working.  If you have an Oracle Linux 7.6 VM that’s affected by this problem, one workaround is to downgrade the kernel.

There are many things that can cause VirtualBox shared folders to stop working.  To see whether your VM might be affected by this specific issue, check 3 things:

1. From a terminal or ssh session, run cat /etc/oracle-release.  If the output says “Oracle Linux Server release 7.6”, your VM might be affected.
    
2. Next, run uname -r.  This will show the version of the kernel that's running.  If the running kernel is one of the following, your VM might be affected:
   
   

   Kernel version
   4.14.35-1818.4.5.el7uek.x86_64
   4.14.35-1818.4.6.el7uek.x86_64
   4.14.35-1818.4.7.el7uek.x86_64
   4.14.35-1818.5.4.el7uek.x86_64

    
3. Finally, run yum list installed *vbox*.  If the version of the VirtualBox Guest Additions kernel module is “kmod-vboxguest-uek5.x86_64 5.2.20-1.el7”, your VM might be affected.

If ALL 3 of these conditions are true, you may be able to restore shared folder functionality by downgrading the kernel.  To do that, run sudo yum -y install kernel-uek-4.14.35-1818.3.3.el7uek.x86_64, then reboot the VM.  Version 4.14.35-1818.3.3.el7uek.x86_64 of the UEK Release 5 kernel seems to be the latest version that works properly with version 5.2.20 of the VirtualBox Guest Additions kernel module.

I’m sure this issue will be resolved soon by updates to the Guest Additions kernel module and/or the kernel itself.  It’s helpful to have a workaround in the meantime, though.

[Updated on December 11, 2018 to add 4.14.35-1818.5.4.el7uek.x86_64 to list of affected kernel versions.]