Encrypting Data Disks on Linux with Azure Disk Encryption


There are two main ways to encrypt data disks using Azure Disk Encryption.  You can choose to thoroughly encrypt the existing disk block-by-block using EnableEncryption, or you can choose to rapidly format and encrypt the data disk using EncryptFormatAll.

Both of these techniques have a common setup requirement that will signal to the ADE solution which data disks are to be encrypted:

Prerequisite Data Disk Setup​

First, prior to enabling encryption, the data disks to be encrypted need to be properly listed in /etc/fstab. Take care to use a persistent block device name for this entry, as device names in the "/dev/sdX" format cannot be relied to be associated with the same disk across reboots particularly after encryption is applied. For more detail on this behavior, see:  https://docs.microsoft.com/en-us/azure/virtual-machines/linux/troubleshoot-device-names-problems

 

Next, ensure that the /etc/fstab settings are configured properly for mounting.  To do this, run the mount -a command or reboot the VM and trigger the remount that way.  Once that is complete, check the output of the lsblk command to verify that the desired drive is still mounted.  The reason for this test is simple. If the /etc/fstab file does not mount the drive properly prior to enabling encryption, ADE will not able to mount it properly either.  The ADE process will be moving the mount information out of /etc/fstab and into its own configuration file as part of the encryption process.  Do not be surprised to see the entry missing from /etc/fstab after data drive encryption completes.  Also be sure to give time after reboot for the ADE process to mount the newly encrypted disks.  They will not be immediately available after a reboot, the ADE process will need time to start, unlock, and then mount the encrypted drives prior to their being available for other processes to access.  This may take more than a minute after reboot depending on the system characteristics.

 

​A simple example of commands that can be used to mount the data disks and create the necessary /etc/fstab entries can be found here:

https://github.com/ejarvi/ade-cli-getting-started/blob/master/validate.sh#L197-L205

 

Once the data disk is ready, you can choose to either encrypt the existing data block-by-block using EnableEncryption, or you can choose to rapidly format and encrypt the data disks using EncryptFormatAll.

EnableEncryption (encrypt data drive contents, preserving content)

This process can be very time consuming depending on the size of the drive contents to encrypt, but it allows existing content to be encrypted.

Powershell:

To use this process, prepare the drive as above and then use the Powershell cmdlet Set-AzureRmVmDiskEncryptionExtension:
https://docs.microsoft.com/en-us/powershell/module/azurerm.compute/set-azurermvmdiskencryptionextension?view=azurermps-5.4.0
Encryption may take several hours if the "OS" or "All" VolumeType is selected.  If the "Data" VolumeType is selected then the time required will be proportional to the size of the data volume(s) to be encrypted.

CLI:

Prepare the drive as above, and then use the az vm encryption enable command.
An example bash script of how to enable encryption end to end that uses this command is available here: https://github.com/ejarvi/ade-cli-getting-started/blob/master/validate.sh
Please note that on successful completion of this script, the script will automatically delete the resources that it just created.  If you would like to preserve the resource, make sure to comment out or remove the last line of the script prior to running.

ARM Template:

Using an ARM template to encrypt a running Linux VM is possible.  Ensure the EncryptionOperation value is set to "EnableEncryption” - I don't have a pointer to a template that uses this yet, but for now modifying this template may be sufficient:

 

EncryptFormatAll

This process is very fast but is only for scenarios where there is no existing content on the mounted data drives.
After running this command any drives that were mounted previously will be formatted and then the encryption layer will be started on top of that now empty drive.
Additionally, when this option is selected, the "ephemeral" resource disk attached to the VM will also be encrypted.  If the ephemeral drive is reset, it will be reformatted and re-encrypted by the ADE solution in the VM at the next opportunity.
Some additional documentation of the EncryptFormatAll mode including powershell and ARM template examples is available at:

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