Debian Cryptsetup Initramfs integration

1. Introduction

Kernels more recent than 2.6.12 have dropped support for devfs, which means that initrd-tools can no longer be used to boot into an encrypted root partition. Instead, a similar functionality has been developed for use with an initramfs-image.

2. A fresh installation

If you plan to perform a completely new installation of Debian onto a machine and to do so using an encrypted root partition, you might want to consider using a version of Debian Installer with partman-crypto (see https://wiki.debian.org/DebianInstaller/PartmanCrypto).

The installation will then take care of all the details and perform the necessary configuration for you, meaning that you should not have to read the rest of this document to get a machine with an encrypted root filesystem up and running.

However, if you are not planning to perform a new installation from scratch, the following information might be useful to you.

3. Requirements

In order to boot from an encrypted root filesystem, you need an initramfs-image which includes the necessary kernel modules and scripts to setup the root device after the kernel has been initialized, but before the rest of the operating system is booted.

To do so, you need two partitions: * an unencrypted /boot partition * an encrypted / partition

In addition, you need to have both initramfs-tools and busybox installed.

NOTE: You should make sure that your swap partition is either encrypted, or that you are using a swap file on an encrypted partition, as crypto keys and other sensitive information might otherwise be written out to the swap partition in unencrypted form.

4. Setup (regular dm-crypt)

First of all, you must edit /etc/crypttab and add a line describing your root device, for example:

cryptroot /dev/sda2 none cipher=aes-xts-plain64,size=256,hash=sha1

This will allow cryptsetup to create /dev/mapper/cryptroot from the encrypted partition /dev/sda2 during boot.

In addition, you must also make sure that the root device is listed in /etc/fstab, for example:

/dev/mapper/cryptroot / ext4 defaults 0 1

This will allow the initramfs support scripts to know which of the devices in the crypttab that is the root device.

After doing these changes, you should regenerate the initramfs by running update-initramfs -u, then make sure that your boot loader is configured to feed the initramfs to the kernel when booting. The kernel root argument should also be changed to /dev/mapper/cryptroot.

Now, reboot the machine, and if everything is correctly configured, you should be given a prompt to type in the passphrase for the encrypted root partition before the boot can continue.

NOTE: In order to ensure that the crypto setup works in a consistent manner, you should make sure that the hash function is specified in the /etc/crypttab file if you are using regular dm-crypt (with LUKS the hash function to use is stored in the LUKS header).

5. Setup (using LUKS)

If you are using the LUKS feature of cryptsetup, the above setup recipe should still apply, but since most options can be derived from the information stored in the LUKS header on-disk, the line to add to /etc/crypttab should look something like this:

cryptroot /dev/sda2 none luks,discard

6. Exotic key types

The above examples assume that you use a regular passphrase as the key to the encrypted filesystem. However, if you wish to make use of more complex setups (such as root-key-on-usb-memory), you can create a script which does all the steps necessary to retrieve the key and then prints it to stdout.

Then add a keyscript=/path/to/your/script.sh to the options (fourth column) in the above mentioned /etc/crypttab line, so that it looks something like this:

cryptroot /dev/sda2 none luks,discard,keyscript=/usr/local/sbin/cryptkey

Next, regenerate your initramfs image. This will copy the script into the initramfs image under the /lib/cryptsetup/keyscripts/ directory.

NOTE: there is a limited set of tools available when the script is executing as part of the initramfs bootup, you have to make sure that you do not use any tools which are not available or your script, and therefore boot, will fail.

7. “cryptopts” boot argument

In general, you should use the above approach with a line describing your root partition in /etc/crypttab and /etc/fstab. However, if for some reason you wish to override the settings that are derived from these files and stored in the initramfs image, you can use the “cryptopts” boot argument (this only works for the root partition).

The format of cryptopts is:

cryptopts=<opt1>=<value1>,<opt2>=<value2>...

Beside the hash, size, cipher and lvm options that correspond to the same options in the fourth field of /etc/crypttab, the options target, source and key are also supported. They correspond to the first, second and third field of /etc/crypttab, respectively. See the crypttab man page for further details.

Several cryptopts boot arguments can also be specified in case more than one mapping needs to be setup in the initramfs stage of the boot.

Example boot arguments:

root=/dev/mapper/crypt0 cryptopts=target=crypt0,source=/dev/sda1,cipher=aes-xts-plain64,size=256,hash=sha1

In particular, if all cryptopts boot arguments have an empty value then no mapping is setup. This can be used to disable the cryptsetup initramfs scripts for a particular boot.

8. Resume device support

The initramfs scripts will also try to automatically determine the devices, if any, that are used for software suspend (swsusp, suspend2 or uswsusp) and to set them up during the initramfs stage in order to allow suspend and resume in combination with encryption to keep the resume image safe from potential attackers.

If your resume device and your root partition use two different cryptsetup mappings, you might want to use the decrypt_derived keyscript as described below.

9. The decrypt_derived keyscript

Assume that you have two entries in /etc/crypttab:

cryptroot /dev/sda1 none luks,discard
cryptswap /dev/sda2 none luks

If cryptswap is used as your suspend/resume device, you’d normally need to enter two different passphrases during the boot, but the decrypt_derived script can generate the key for the second mapping using a hash of the key for the first mapping.

In short, you’ll need to do something like the following to take advantage of the decrypt_derived script:

  1. swapoff -a
  2. cryptsetup luksClose cryptswap
  3. edit /etc/crypttab and change the cryptswap line to e.g.: cryptswap /dev/sda2 cryptroot cipher=aes-xts-plain65,size=256,hash=sha1,keyscript=decrypt_derived,swap
  4. cryptdisks_start cryptswap
  5. Make sure that /dev/mapper/cryptswap has been created
  6. swapon -a
  7. (optional) update-initramfs -u

After you’ve followed the above steps, your swap device should be setup automatically after the root device has been setup during the boot stage.

WARNING: If you use the decrypt_derived keyscript for devices with persistent data (i.e. not swap or temp devices), then you will lose access to that data permanently if something damages the LUKS header of the LUKS device you derive from. The same applies if you luksFormat the device, even if you use the same passphrase(s). A LUKS header backup, or better a backup of the data on the derived device may be a good idea. See the Cryptsetup FAQ on how to do this right.

Note: The decrypt_derived keyscript won’t work when the volume key of the device being derived from is offloaded to the kernel keyring service (thus not readable by userspace). That behavior is the default for LUKS2 devices (unless opened with the --disable-keyring option) since Linux 4.10. For such devices, an alternative is to use the same passphrase and unlock the source device using the decrypt_keyctl keyscript.

Note: If you don’t use suspend device support, it’s better to use completely random keys for your encrypted swap device. See the section ‘2. Encrypted swap partition(s)’ in /usr/share/doc/cryptsetup/README.Debian.gz for information on how to setup this.

10. The passdev keyscript

If you have a keyfile on a removable device (e.g. a USB-key), you can use the passdev keyscript. It will wait for the device to appear, mount it read-only, read the key and then unmount the device.

The key part of /etc/crypttab will be interpreted as <device>:<path>[:<timeout>], it is strongly recommended that you use one of the persistent device names from /dev/disk/*, e.g. /dev/disk/by-label/myusbkey.

This is an example of a suitable line in cryptsetup:

cryptroot /dev/sda2 /dev/disk/by-label/myusbkey:/keys/root.key discard,cipher=aes-xts-plain64,size=256,hash=sha1,keyscript=passdev

The above line would cause the boot to pause until /dev/disk/by-label/myusbkey appears in the fs, then mount that device and use the file /keys/root.key on the device as the key (without any hashing) as the key for the fs.

The timeout option has to be in seconds.

If any modules are required in order to mount the filesystem on the removable device, then initramfs-tools needs to be configured to add these modules to the initramfs. This can be done by listing the required modules in /etc/initramfs-tools/modules.

11. Limitation: renaming of target name for encrypted root device

As spotted by Adam Lee in bug report #671037, it’s not possible to simply rename the target name for encrypted root devices. It breaks the initramfs creation process. The bug report submitter found a solution to work around this limitation:

  1. enter another system (like livecd)
  2. open luks device with the new name, change the target name to the new one
  3. chroot into it (now, the current target name is the same as it in conf)
  4. update-initramfs -u
  5. reboot

12. Storing keyfiles directly in the initrd

Normally devices using a keyfile are ignored (with a loud warning), and the key file itself is not included in the initrd, because the initramfs image typically lives on an unencrypted /boot partition. However in some cases it is desirable to include the key file in the initrd; for instance recent versions of GRUB support booting from encrypted block devices, allowing an encrypted /boot partition.

Among the key files listed in the crypttab(5), those matching the value of the environment variable KEYFILE_PATTERN (interpreted as a shell pattern) will be included in the initramfs image. For instance if /etc/crypttab lists two key files /etc/keys/{root,swap}.key, you can add the following to /etc/cryptsetup-initramfs/conf-hook to add them to the initrd.

KEYFILE_PATTERN="/etc/keys/*.key"

Furthermore if the initramfs image is to include private key material, you’ll want to create it with a restrictive umask in order to keep non-privileged users at bay. This can be achieved by adding the following to /etc/initramfs-tools/initramfs.conf.

UMASK=0077

– David Härdeman david@hardeman.nu

– Jonas Meurer mejo@debian.org Thu, 01 Nov 2012 13:44:31 +0100

– Guilhem Moulin guilhem@debian.org Wed, 09 Dec 2015 04:53:41 +0100