Ubuntu 18.04 on kernel 4.15.0-34-generic boots to BusyBox

The problem is that /sbin/cryptsetup is not in initramfs file /boot/initrd.img-4.15.0-36-generic.

You need to add CRYPTSETUP=y in /etc/cryptsetup-initramfs/conf-hook. You also need to add the initramfs option in /etc/crypttab. Then, you have to run update-initramfs, which solves this problem.

Detailed solution:

1. In an other Ubuntu machine, use "Startup Disk Creator" to create a bootable Ubuntu 18.04 USB drive
2. Plug in this USB drive in a USB port of the computer that boots to BusyBox

3. When the live USB Ubuntu is ready, connect to Internet (WiFi or wired)

4. Open a terminal

   ubuntu@ubuntu:~$ sudo su
   

5. Find the encrypted block device.

   root@ubuntu:~# blkid|grep LUKS
   /dev/nvme0n1p3: UUID="4b206e76-1531-48ae-95be-ae0ed7a244c1" TYPE="crypto_LUKS" PARTUUID="21db499d-b87b-41c6-864f-04d1531cb083"
   

6. Decrypt the device

   root@ubuntu:~# cryptsetup open UUID="4b206e76-1531-48ae-95be-ae0ed7a244c1" nvme0n1p3_crypt
   Enter passphrase for /dev/disk/by-uuid/4b206e76-1531-48ae-95be-ae0ed7a244c1: 
   

7. List mapped devices. control is a file used to send command to the device mapper. nvme0n1p3_crypt is the decrypted device. ubuntu--vg-root is a LVM logical volume in the LVM ubuntu-vg volume group.

   root@ubuntu:~# ls /dev/mapper/*
   /dev/mapper/control  /dev/mapper/ubuntu--vg-root  /dev/mapper/nvme0n1p3_crypt
   

8. Create mount point. This is where we will mount the / of our system that won't boot.

   root@ubuntu:~# mkdir -p /mnt/ubuntu-root
   

9. Mount root logical volume

   root@ubuntu:~# mount /dev/mapper/ubuntu--vg-root /mnt/ubuntu-root/
   

10. Mount pseudo file systems

    root@ubuntu:~# mount -o bind /sys /mnt/ubuntu-root/sys
    root@ubuntu:~# mount -o bind /proc /mnt/ubuntu-root/proc
    root@ubuntu:~# mount -o bind /dev /mnt/ubuntu-root/dev
    

11. Copy DNS information

    root@ubuntu:~# cp /etc/resolv.conf /mnt/ubuntu-root/etc/
    

12. Change root

    root@ubuntu:~# chroot /mnt/ubuntu-root/
    

13. Mount /boot, which contains the initramfs file. This partition is unencrypted.

    root@ubuntu:/# mount /boot/
    

14. Install binwalk (to see the content of the init ram file system)

    root@ubuntu:~# apt update
    root@ubuntu:~# apt install binwalk
    

15. Find offset of gzipped initramfs content

    root@ubuntu:~# binwalk /boot/initrd.img-4.15.0-36-generic | grep gzip
    1605632       0x188000        gzip compressed data, from Unix, last modified: 2018-10-18 13:00:32
    

16. The problem is that the initramfs file system does not contain cryptsetup. So, that is why there is no LUKS password prompt.

    root@ubuntu:/# cd /root/
    root@ubuntu:~# mkdir initramfs-4.15.0-36-generic
    root@ubuntu:~# cd initramfs-4.15.0-36-generic
    root@ubuntu:~/initramfs-4.15.0-36-generic# dd if=/boot/initrd.img-4.15.0-36-generic bs=1605632 skip=1 | gunzip | cpio -i
    root@ubuntu:~/initramfs-4.15.0-36-generic# ls sbin/cryptsetup
    ls: cannot access 'sbin/cryptsetup': No such file or directory
    

17. To decrypt root at boot, the initramfs needs to contain:

    sbin/cryptsetup
    lib/modules/4.15.0-36-generic/kernel/drivers/md/dm-crypt.ko
    

18. Add the initramfs option in /etc/crypttab

    root@ubuntu:/# cat /etc/crypttab 
    nvme0n1p3_crypt UUID=4b206e76-1531-48ae-95be-ae0ed7a244c1 none luks,discard,initramfs
    

19. Add CRYPTSETUP=y in /etc/cryptsetup-initramfs/conf-hook

20. Run update-initramfs:

    root@ubuntu:~# update-initramfs -k 4.15.0-36-generic -c -v &> update-initramfs-4.15.0-36-generic.cryptsetup.log
    

21. Now, we have cryptsetup in initramfs and the Linux kernel module dm-crypt.ko too:

    root@ubuntu:~# grep /sbin/cryptsetup update-initramfs-4.15.0-36-generic.cryptsetup.log
    Adding binary /sbin/cryptsetup
    
    root@ubuntu:~# grep dm-crypt.ko update-initramfs-4.15.0-36-generic.cryptsetup.log
    Adding module /lib/modules/4.15.0-36-generic/kernel/drivers/md/dm-crypt.ko
    

22. Now, run update-initramfs, without -c (new) and without -v (verbose):

    root@ubuntu:~# update-initramfs -k 4.15.0-36-generic -u
    

23. Verify that the initramfs is actually correct

    root@ubuntu:~# binwalk /boot/initrd.img-4.15.0-36-generic | grep gzip
    1605632       0x188000        gzip compressed data, from Unix, last modified: 2018-10-18 14:26:29
    
    root@ubuntu:~# dd if=/boot/initrd.img-4.15.0-36-generic bs=1605632 skip=1 2> /dev/null | gunzip | cpio -t 2> /dev/null |grep sbin/crypt 
    sbin/cryptsetup
    
    root@ubuntu:~# dd if=/boot/initrd.img-4.15.0-36-generic bs=1605632 skip=1 2> /dev/null | gunzip | cpio -t 2> /dev/null |grep dm-crypt.ko
    lib/modules/4.15.0-36-generic/kernel/drivers/md/dm-crypt.ko
    

24. Now, reboot.