Encrypted Debian GNU+Linux


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This guide is written for the Debian distribution, but it should also work for Devuan with the net installer. Other Debian based GNU+Linux distributions may also work, using these instructions.

This guide assumes that you are using the GNU GRUB bootloader as a coreboot payload. In this configuration, GNU GRUB runs on bare metal instead of relying on BIOS or UEFI. GNU GRUB has support for directly reading LUKS volumes and it can directly boot your Linux kernel this way. With GRUB already in the boot flash, this means that your /boot/ directory (containing your Linux kernel) can be fully encrypted. The same cannot be said for most other systems, and no other coreboot payload provides this functionality.

If booting in text mode

Libreboot ROM images are provided, which will either boot the system in classic text mode, or with a framebuffer implemented by coreboot for video display initialization (not to be confused with int10h VGA modes).

Text mode is the default video mode on most x86 platforms, using INT 10H functions. It’s an interrupt service that text-mode applications use, a hangover from the days of CS/M and DOS. In this mode, no framebuffer exists and Libreboot currently does not implement VGA modes. The Debian net installer will attempt to use VGA modes that most implementations of INT 10H provide. Therefore, you must force Debian’s installation program to operate in text mode.

To boot the Debian net installer, make sure to specify fb=false on the linux kernel parameters in GRUB. This will boot the installer in text mode instead of using a framebuffer. By default, the netinstaller will try to switch to a high resolution framebuffer. Due to lack of INT10H video BIOS services and mode switching support in libgfxinit, this will fail.

In some setups, you don’t need this. For example, if you’re using an add-on PCIe GPU on a desktop/server board (e.g. ASUS KGPE-D16/KCMA-D8, Gigabyte GA-G41M-ES2L), you would use SeaBIOS payload with text-mode startup, but the Video BIOS option ROM in your add-on graphics card would be executed, and it would presumably handle INT10H VGA modes.

Boot the installer

Libreboot on x86 can use the GNU GRUB bootloader as a bare metal coreboot payload if you wish, which means that the GRUB configuration file (where your GRUB menu comes from) is stored directly alongside Libreboot and its GRUB payload executable, inside the flash chip. In context, this means that installing distributions and managing them is handled slightly differently compared to traditional BIOS or UEFI systems.

On most systems, the /boot/ partition has to be left unencrypted while the others are encrypted. This is so that GRUB, and therefore the kernel, can be loaded and executed since the firmware can’t open a LUKS volume. Not so with Libreboot! Since GRUB is already included directly as a payload, even /boot/ can be encrypted. This protects /boot from tampering by someone with physical access to the system.

This guide is written for Debian net installer. You can download the ISO from the homepage on debian.org. Use this on the GRUB terminal to boot it from USB (for 64-bit Intel or AMD):

set root='usb0'
linux /install.amd/vmlinuz
initrd /install.amd/initrd.gz
boot

If you are on a 32-bit system (e.g. X60):

set root='usb0'
linux /install.386/vmlinuz
initrd /install.386/initrd.gz
boot

This guide shows how to create a boot USB drive with the Debian ISO image.

This guide is only for the GRUB payload. If you use the depthcharge payload, ignore this section entirely.

Note: on some thinkpads, a faulty DVD drive can cause the cryptomount -a step during boot to fail. If this happens to you, try removing the drive.

Set a strong user password (lots of lowercase/uppercase, numbers and symbols).

Use of the diceware method is recommended, for generating secure passphrases (instead of passwords).

When the installer asks you to set up encryption (ecryptfs) for your home directory, select ‘Yes’ if you want to: LUKS is already secure and performs well. Having ecryptfs on top of it will add noticeable performance penalty, for little security gain in most use cases. This is therefore optional, and not recommended. Choose ‘no’.

Your user password should be different from the LUKS password which you will set later on. Your LUKS password should, like the user password, be secure.

Partitioning

Choose ‘Manual’ partitioning:

Further partitioning

Now you are back at the main partitioning screen. You will simply set mountpoints and filesystems to use.

Kernel

Installation will ask what kernel you want to use. linux-generic is fine, but you can choose whatever you want here.

Tasksel

For Debian, use the MATE option, or one of the others if you want. The Libreboot project recommends MATE, unless you’re saavy enough to choose something else.

If you want debian-testing, then you should only select barebones options here and change the entries in /etc/apt/sources.list after install to point to the new distro, and then run apt-get update and apt-get dist-upgrade as root, then reboot and run tasksel as root. This is to avoid downloading large packages twice.

NOTE: If you want the latest up to date version of the Linux kernel, Debian’s kernel is sometimes outdated, even in the testing distro. You might consider using this repository instead, which contains the most up to date versions of the Linux kernel. These kernels are also deblobbed, like Debian’s kernels, so you can be sure that no binary blobs are present.

Postfix configuration

If asked, choose No Configuration here (or maybe you want to select something else. It’s up to you.)

Install the GRUB boot loader to the master boot record

Choose No, and then it will still ask you what HDD to install GRUB on. Select your HDD/SSD from the automatically generated list.

The installer will provide GRUB on your HDD/SSD, but not try to install it to an MBR section. However, the /boot/grub/grub.cfg on your system will be maintained automatically by apt-get when handling kernel packages.

Clock UTC

Just say ‘Yes’.

At this point, your Debian system is installed. Shut down when the installer tells you to.

Booting your system

If you didn’t install GRUB during the net installation process, don’t worry. You can boot your installed system manually, using the terminal in GRUB on your boot flash (the version that Libreboot gives you).

At this point, you will have finished the installation. At your GRUB payload, press C to get to reach the GRUB terminal and enter these commands:

cryptomount -a
set root='lvm/matrix-rootvol'
linux /vmlinuz root=/dev/mapper/matrix-rootvol cryptdevice=/dev/mapper/matrix-rootvol:root
initrd /initrd.img
boot

If you did install GRUB, ignore the above. Just select the default Load Operating System menu option and it should fully boot into your system.

When you type your encryption passphrase in GRUB, it will seem like the process has stalled. The same will be true when you load your linux kernel in Debian. Just be patient and it will boot. If you see errors, just press enter to skip them until you see the Debian GRUB menu.

ecryptfs

If you didn’t encrypt your home directory, then you can safely ignore this section.

Immediately after logging in, do that:

sudo ecryptfs-unwrap-passphrase

This will be needed in the future if you ever need to recover your home directory from another system, so write it down and keep the note somewhere secret. Ideally, you should memorize it and then burn the note (or not even write it down, and memorize it still)>

LUKSv2

LUKSv2 is fully supported nowadays, in recent Libreboot releases. The old Libreboot release, version 20160907 (and earlier releases), did not support LUKSv2 in GNU GRUB. By default, modern Debian distributions will use LUKSv2.

You do not need to downgrade LUKSv2 to v1, but you shouldn’t use any of the special features that LUKSv2 offers. Basically, the partitioning should be done exactly the same way as with LUKSv1 (but with newer encryption/hashing algorithms used by LUKSv2 partitions). This is because of limitations in the implementation of LUKSv2 in GNU GRUB. GRUB uses its own custom implementation, instead of directly adapting the Linux kernel implementation.

Generate distro’s grub.cfg

If /boot/grub/grub.cfg already exists, ignore this step.

Now you need to set it up so that the system will automatically boot, without having to type a bunch of commands.

Install grub-coreboot if not already installed:

apt-get install grub-coreboot

Modify or add following lines to /etc/default/grub

GRUB_CMDLINE_LINUX="cryptdevice=/dev/mapper/matrix-rootvol:root"
GRUB_ENABLE_CRYPTODISK=y

Copy fonts/backgrounds to /boot/grub and generate grub.cfg using following command:

grub-install --target=i386-coreboot

Refer to this guide for further guidance on hardening your GRUB configuration, for security purposes.

Troubleshooting

A user reported issues when booting with a docking station attached on an X200, while decrypting the disk in GRUB. The error AHCI transfer timed out was observed. The workaround was to remove the docking station or remove the CD/DVD drive.

Here is the information on that DVD drive, which said user had:

"sudo wodim -prcap" shows information about the drive:
Device was not specified. Trying to find an appropriate drive...
Detected CD-R drive: /dev/sr0
Using /dev/cdrom of unknown capabilities
Device type    : Removable CD-ROM
Version        : 5
Response Format: 2
Capabilities   : 
Vendor_info    : 'HL-DT-ST'
Identification : 'DVDRAM GU10N    '
Revision       : 'MX05'
Device seems to be: Generic mmc2 DVD-R/DVD-RW.

Drive capabilities, per MMC-3 page 2A:

  Does read CD-R media
  Does write CD-R media
  Does read CD-RW media
  Does write CD-RW media
  Does read DVD-ROM media
  Does read DVD-R media
  Does write DVD-R media
  Does read DVD-RAM media
  Does write DVD-RAM media
  Does support test writing

  Does read Mode 2 Form 1 blocks
  Does read Mode 2 Form 2 blocks
  Does read digital audio blocks
  Does restart non-streamed digital audio reads accurately
  Does support Buffer-Underrun-Free recording
  Does read multi-session CDs
  Does read fixed-packet CD media using Method 2
  Does not read CD bar code
  Does not read R-W subcode information
  Does read raw P-W subcode data from lead in
  Does return CD media catalog number
  Does return CD ISRC information
  Does support C2 error pointers
  Does not deliver composite A/V data

  Does play audio CDs
  Number of volume control levels: 256
  Does support individual volume control setting for each channel
  Does support independent mute setting for each channel
  Does not support digital output on port 1
  Does not support digital output on port 2

  Loading mechanism type: tray
  Does support ejection of CD via START/STOP command
  Does not lock media on power up via prevent jumper
  Does allow media to be locked in the drive via PREVENT/ALLOW command
  Is not currently in a media-locked state
  Does not support changing side of disk
  Does not have load-empty-slot-in-changer feature
  Does not support Individual Disk Present feature

  Maximum read  speed:  4234 kB/s (CD  24x, DVD  3x)
  Current read  speed:  4234 kB/s (CD  24x, DVD  3x)
  Maximum write speed:  4234 kB/s (CD  24x, DVD  3x)
  Current write speed:  4234 kB/s (CD  24x, DVD  3x)
  Rotational control selected: CLV/PCAV
  Buffer size in KB: 1024
  Copy management revision supported: 1
  Number of supported write speeds: 4
  Write speed # 0:  4234 kB/s CLV/PCAV (CD  24x, DVD  3x)
  Write speed # 1:  2822 kB/s CLV/PCAV (CD  16x, DVD  2x)
  Write speed # 2:  1764 kB/s CLV/PCAV (CD  10x, DVD  1x)
  Write speed # 3:   706 kB/s CLV/PCAV (CD   4x, DVD  0x)

Supported CD-RW media types according to MMC-4 feature 0x37:
  Does write multi speed       CD-RW media
  Does write high  speed       CD-RW media
  Does write ultra high speed  CD-RW media
  Does not write ultra high speed+ CD-RW media

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