Linux @lemmy.ml The Hobbyist @lemmy.zip 4 wk. ago

Need help: USB unlock LUKS on Alpine Linux

Hi folks,

I have Alpine Linux installed in an encrypted LUKS partition. I came across this tutorial which shows how to setup a key in a USB drive and when the drive is inserted and the computer booted, the LUKS partition auto-unlocks with the key on the USB drive.

https://askubuntu.com/questions/1414617/configure-ubuntu-22-04-zfs-for-automatic-luks-unlock-on-boot-via-usb-drive

I would like to setup the same thing but I do not have Alpine linux installed on ZFS, so I'm looking for ways to adapt the instructions.

So far, what I've done is:

I've setup the key on the usb stick and I can unlock the LUKS partition with that key.
create a /etc/mkinitfs/features.d/usb-unlock.sh script with the following content:

(the echo to /dev/kmesg was to check whether the script did indeed run at boot by trying to print to the kernel messages but I can't find anything in the kernel messages).

#!/bin/sh

echo "usb-unlock script starting..." > /dev/kmsg

USB_MOUNT="/mnt/my-usb-key" # The USB stick mounting point
LUKS_KEY_FILE="awesome.key"  # The name of your keyfile on the USB stick

# Search for the USB stick with the key
for device in $(ls /dev/disk/by-uuid/*); do
    mount $device $USB_MOUNT 2>/dev/null
    if [ -f "$USB_MOUNT/$LUKS_KEY_FILE" ]; then
        # Unlock the LUKS partition
        cryptsetup luksOpen /dev/sda3 cryptroot \
            --key-file "$USB_MOUNT/$LUKS_KEY_FILE" && exit 0
    fi
    umount $USB_MOUNT
done
echo "No USB key found, falling back to password prompt." # this message never appears, despite not having found the key on the usb stick

echo "usb-unlock script ending." > /dev/kmsg

I added usb-unlock to the features in mkinitfs.conf:

mytestalpine:~# cat /etc/mkinitfs/mkinitfs.conf 
features="ata base ide scsi usb virtio ext4 cryptsetup keymap usb-unlock"

run mkinitfs to rebuild the initramfs. Then reboot to test the implementation, which was unsuccessful.

What am I missing / doing wrong? Thank you for your help!

Edit: forgot to add step 4

21 comments

mkinitfs doesn't support running custom shell hooks. mkinitfs is very, very, very bare-bones custom code and the whole features concept exists only to pull extra files and kernel modules into the initramfs, not for extra logic.

You'd either have to customize the init script itself (not impossible, it's 1000 lines) and pass -i/set init= in the .conf, or install Dracut/Booster instead (which should "just work" if you apk add them, but I've had no need to do so).
- It seems you might be right. There is so little documentation for initramfs in Alpine Linux (the wiki page is very barebones), but I did manage to find this open issue:
  
  https://gitlab.alpinelinux.org/alpine/mkinitfs/-/issues/18
  
  So I guess this confirms that it is not yet possible.
  
  Could you expand on your suggestion with customizing the init script? Where is this file located, and would you have some pointers of how to get started to customize it for my use case?
  
  You'd be looking for /usr/share/mkinitfs/initramfs-init . I've never customized that myself, but it looks like there's already some support for a keyfile if you look for KOPT_cryptroot and check that block of code. That looks like it's mostly set up for a keyfile embedded into the initramfs, but I guess it should be possible to replace that code with something that grabs the keyfile off an USB drive.
  
  I suppose you'd make a copy of it, put it somewhere in /etc or whatever and change the mkinitfs.conf to point to it. init="/etc/whatever/myinitramfs-init" should do the trick since the config file just gets sourced in. That said you're definitively heading into unknown territory here. It might be easier to just use Dracut or the like instead.
I think you may want to use for device in /dev/disk/by-uuid/*

That doesn't explain why you aren't seeing messages. I see there is a shebang at the start of the script. Can you confirm that the script has the executable bit set for the root user?
- Yes it does (have the execution bit).
  
  edit: added paranthesis
More of a debugging step, but have you tried running lsinitrd on the initramfs afterwards to verify your script actually got added?

You theoretically could decompress the entire image to look around as well. I don't know the specifics for alpine, but presumably there would be a file present somewhere that should be calling your custom script.

EDIT: Could it also be failing because the folder you are trying to mount to does not exist? Don't you need a mkdir somewhere in your script?
- From my understanding, features always refer to components from within /etc/mkinitfs/features.d/
Seems that the file /etc/mkinitfs/features.d/ is only linux alphine thing so creating it for another linux distro does nothing.

https://wiki.alpinelinux.org/wiki/Initramfs_init

I would create a systemd service instead if your distro is using systemd https://www.slingacademy.com/article/ubuntu-how-to-create-a-custom-systemd-service/#Introduction

Edit: Sorry please ignore my comment. Your entire system is encrypted so that won't work. I'll see if there is another solution and post it

Edit2: Maybe you need to place the file here instead /usr/share/initramfs- tools/scripts/ ? https://manpages.ubuntu.com/manpages/bionic/en/man8/initramfs-tools.8.html
- Could it be? I don't have that directory. Maybe this is Ubuntu specific? Not sure.
  
  would be easier if you tell us which distro are you running mkinitfs on
@TheHobbyist isn't it better to find the plugged USB flash drive by parsing the output of dmesg?
- That may be an option, but for the time being, I'm not even sure how to start debugging this. I have no idea where to start looking. I don't even know if the usb-unlock.sh script is even running at boot. Any thoughts?
  
  Just a sanity check because I've totally done this before: did you make the script file executable?

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