Encrypting /home in place

Raison d’être

When I first installed this system, I was oblivious to disk encryption and didn’t factor it in. This resulted in a non-LVM system with some haphazard partitions from subsequent resizing and nuking of Windows in the past few years. One good decision I took when installing was to separate / and /home so this turned out to be easier than I thought.

Preparation

I was apprehensive of ending up with a dead system before today’s DGPLUG session, so I managed to get weechat up and running inside tmux in a virtual console. Next I had the wiki article opened on elinks, a TUI browser. I don’t want to repeat any commands that are already available in the wiki. I would like to make a more cogent story mentioning some caveats in the process. First the resize2fs command to shrink /home took some time. This was done to make space for LUKS header which would be written during encryption. I wish there was warning in the wiki about how long this would take. At least I could’ve enabled the progress option for resize2fs 😀

Encrypting the disk device

After resize2fs completes we can start encrypting the partition. It is interesting to note that cryptsetup-reencrypt is quite recent and I found another project that was developed in parallel; luiksipc, which as the name suggests does luks in-place. But since this functionality is available upstream I didn’t explore the alternative.

cryptsetup-reencrypt doesn’t emit any warnings like the wiki alludes to and this gives me hope that the functionality was stabilized recently. For the passphrase I took Kushal’s suggestion to use XKCD style passwords. gopass includes an option to do it quite easily and it generated a strong yet easy to remember password.

password comic from xkcd.com

This step took more than 4 hours, but I was attending the class and doing something productive in the meantime. The next steps were to grow the filesystem again and then test it out.

Encrypted swap

Since swap holds the data when hibernating the system we have to secure this as well. A swapfile is the easiest approach in my case. Follow the wiki article to create swap but do it under /home since that is what we have encrypted.

Integrating it with Systemd and Syslinux

I would like to skip all the mistakes and go straight to the wonderful solution. The mistakes in this case would only confuse the reader. Thanks to Khorne from #archlinux-newbie I was able to recover quickly after a few reboots.

Add the appropriate hooks

I assume you are using a systemd based initramfs, if not switch to it for the fancy systemd-analyze output.

HOOKS=(systemd autodetect keyboard sd-vconsole modconf block sd-encrypt sd-lvm2 resume filesystems fsck)

Things to lookout for:

  1. keyboard for early userspace functionality to enter the password.
  2. sd-vconsole for different keymaps and fonts which is the systemd equivalent for the keymap hook.
  3. sd-encrpyt and sd-lvm2 are the respective equivalents for encrpyt and lvm2. The former is what prompts for password when booting early on.

Modify kernel parameters in syslinux.cfg

This should be boot loader agnostic but I just like syslinux better after being scarred by grub-mkconfig. If you use grub, I highly suggest using your own grub.cfg like so which was taken from #archlinux. Append the following to kernel parameters,

rd.luks.name=UIID=home resume=/dev/mapper/home resume_offset=OFFSET

resume_offset is obtained from

$ sudo filefrag -v /home/swapfile | awk '{if($1=="0:"){print $4}}'

taken from the wiki article on swap encryption.

fstab changes

The previous section will add the device /dev/mapper/home which we will use in /etc/fstab.

# mounting home
/dev/mapper/home  /home       ext4        rw,relatime,lazytime,data=ordered,nodev,nosuid  0 2

# swapfile
/home/swapfile    none        swap        defaults                                        0 0

Most of these settings are opinionated, so I would suggest looking at fstab(5) and ext4(5) for explanations. This completes the loop to mount the encrypted device opened by sd-encrypt hook. Like most other things systemd takes care of dependencies and generates appropriate .mount and .device files.

Last step

Generate initramfs, reboot and rejoice!

In Arch Linux,

$ sudo mkinitcpio -P

to get initramfs and initrd.

At the end I would say my understanding of all the different systems made it difficult to get it running quickly. Systemd probably made some of this bearable by integrating disparate systems and providing much-needed dependency tracking.

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