dm-crypt/Swap encryption

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Notes: This article is currently under heavy restructuring: for its latest stable revision see Dm-crypt with LUKS (Discuss in Talk:Dm-crypt/Swap encryption#)

A swap partition may be added to an encrypted system, if required. The swap partition must be encrypted as well to protect any data swapped out by the system. Depending on the requirements, different methods may be used which are described in the following. A setup where the swap encryption is re-initialised on reboot (with a new encryption) provides higher data protection. However, re-encrypting swap also forbids using a suspend-to-disk feature generally.

Back to Dm-crypt with LUKS/draft.

Without suspend-to-disk support

In systems where suspend to disk is not a desired feature, it is possible to create a swap file that will have a random master key with each boot. This is accomplished by using dm-crypt directly without LUKS extensions.

The /etc/crypttab is well commented and you can basically just uncomment the swap line and change <device> to a persistent symlink.

/etc/crypttab
# <name>       <device>         <password>              <options>
# swap         /dev/hdx4        /dev/urandom            swap,cipher=aes-cbc-essiv:sha256,size=256

Where:

<name>
Represents the name (/dev/mapper/<name>) to list in /etc/fstab.
<device>
Should be the symlink to the actual partition's device file.
<password>
/dev/urandom sets the dm-crypt master key to be randomized on every volume recreation.
<options>
The swap option runs mkswap after cryptographic's are setup.
Warning: You should use persistent block device naming (in example ID's) for <device> because if there are multiple hard drives installed in the system, their naming order (sda, sdb,...) can occasionally be scrambled upon boot and thus the swap would be created over a valuable file system, destroying its content.

Persistent block device naming is implemented with simple symlinks. Using UUID's or filesystem-labels is not possible as plain dm-crypt writes only encrypted data without a persistent header like LUKS. If you are not familar with one of the directories under /dev/disk/ read on in the section on #Preparation for Persistent block device naming

#ls -l /dev/disk/*/* | grep sda2
lrwxrwxrwx 1 root root 10 Oct 12 16:54 /dev/disk/by-id/ata-WDC_WD2500BEVT-22ZCT0_WD-WXE908VF0470-part2 -> ../../sda2

Example line for the /dev/sda2 symlink from above:

/etc/crypttab
# <name>                      <device>                                   <password>     <options>
  swap  /dev/disk/by-id/ata-WDC_WD2500BEVT-22ZCT0_WD-WXE908VF0470-part2  /dev/urandom   swap,cipher=aes-cbc-essiv:sha256,size=256

This will map /dev/sda2 to /dev/mapper/swap as a swap partition that can be added in /etc/fstab like a normal swap.

If the partition chosen for swap was previously a LUKS partition, crypttab will not overwrite the partition to create a swap partition. This is a safety measure to prevent data loss from accidental mis-identification of the swap partition in crypttab. In order to use such a partition the LUKS header must be overwritten once.

With suspend-to-disk support

To be able to resume after suspending the computer to disk (hibernate), it is required to keep the swap filesystem intact. Therefore, it is required to have a pre-existent LUKS swap partition, which can be stored on the disk or input manually at startup. Because the resume takes place before /etc/crypttab can be used, it is required to create a hook in /etc/mkinitcpio.conf to open the swap LUKS device before resuming.

If you want to use a partition which is currently used by the system, you have to disable it first:

# swapoff /dev/<device>

Also make sure you remove any line in /etc/crypttab pointing to this device.

A simple way to realize encrypted swap with suspend-to-disk support is by using LVM ontop the encryption layer, so one encrypted partition can contain infinite filesystems (root, swap, home, ...). Follow the instructions on #Encrypting a LVM setup.

The following setup has the disadvantage of having to insert an additional passphrase for the swap partition manually on every boot.

Warning: Do not use this setup with a key file. Please read about the issue reported here

To format the encrypted container for the swap partition, follow steps similar to those described in #Configuring LUKS above and create keyslot for a user-memorizable passphrase.

Open the partition in /dev/mapper:

# cryptsetup open --type luks /dev/<device> swapDevice

Create a swap filesystem inside the mapped partition:

# mkswap /dev/mapper/swapDevice

Now you have to create a hook to open the swap at boot time.

  • Create a hook file containing the open command:
/lib/initcpio/hooks/openswap
 # vim: set ft=sh:
 run_hook ()
 {
     cryptsetup open --type luks /dev/<device> swapDevice
 }

for opening the swap device by typing your password or

/lib/initcpio/hooks/openswap
 # vim: set ft=sh:
 run_hook ()
 {
     mkdir crypto_key_device
     mount /dev/mapper/<root-device> crypto_key_device
     cryptsetup open --type luks --key-file crypto_key_device/<path-to-the-key> /dev/<device> swapDevice
     umount crypto_key_device
 }

for opening the swap device by loading a keyfile from a crypted root device

Note: If swap is on a Solid State Disk (SSD) and Discard/TRIM is desired the option --allow-discards has to get added to the cryptsetup line in the openswap hook above. See Discard/TRIM support for solid state disks (SSD) or SSD for more information on discard. Additionally you have to add the mount option 'discard' to your fstab entry for the swap device.
  • Then create and edit the hook setup file:
/lib/initcpio/install/openswap
# vim: set ft=sh:
build ()
{
   add_runscript
}
help ()
{
cat<<HELPEOF
  This opens the swap encrypted partition /dev/<device> in /dev/mapper/swapDevice
HELPEOF
}
  • Add the hook openswap in the HOOKS array in /etc/mkinitcpio.conf, before filesystem but after encrypt. Do not forget to add the resume hook after openswap.
HOOKS="... encrypt openswap resume filesystems ..."
  • Regenerate the boot image:
# mkinitcpio -p linux
  • Add the mapped partition to /etc/fstab by adding the following line:
/dev/mapper/swapDevice swap swap defaults 0 0
  • Set up your system to resume from /dev/mapper/swapDevice. For example, if you use GRUB with kernel hibernation support, add resume=/dev/mapper/swapDevice to the kernel line in /boot/grub/grub.cfg. A line with encrypted root and swap partitions can look like this:
kernel /vmlinuz-linux cryptdevice=/dev/sda2:rootDevice root=/dev/mapper/rootDevice resume=/dev/mapper/swapDevice ro

To make the parameter persistent on kernel updates, add it to /etc/default/grub.

At boot time, the openswap hook will open the swap partition so the kernel resume may use it. If you use special hooks for resuming from hibernation, make sure they are placed after openswap in the HOOKS array. Please note that because of initrd opening swap, there is no entry for swapDevice in /etc/crypttab needed in this case.

Using a swap file for suspend-to-disk support

A swap file can be used to reserve swap-space within an existing partition and may also be setup inside an encrypted blockdevice's partition. When resuming from a swapfile the resume hook must be supplied with the passphrase to unlock the device where the swap file is located. To create it:

  • Choose a mapped partition (e.g. /dev/mapper/rootDevice) whose mounted filesystem (e.g. /) contains enough free space to create a swapfile with the desired size.
  • Create the swap file (e.g. /swapfile) inside the mounted filesystem of your chosen mapped partition. Be sure to activate it with swapon and also add it to your /etc/fstab file afterward. Note that the swapfile's previous contents remain transparent over reboots.
  • Set up your system to resume from your chosen mapped partition. For example, if you use GRUB with kernel hibernation support, add resume=your chosen mapped partition and resume_offset=see calculation command below to the kernel line in /boot/grub/grub.cfg. A line with encrypted root partition can look like this:
kernel /vmlinuz-linux cryptdevice=/dev/sda2:rootDevice root=/dev/mapper/rootDevice resume=/dev/mapper/rootDevice resume_offset=123456789 ro

The resume_offset of the swap-file points to the start (extent zero) of the file and can be identified like this:

# filefrag -v /swapfile | awk '{if($1==0){print $3}}'
  • Add the resume hook to your etc/mkinitcpio.conf file and rebuild the image afterward:
HOOKS="... encrypt resume ... filesystems ..."
  • If you use a USB keyboard to enter your decryption password, then the keyboard module must appear in front of the encrypt hook, as shown below. Otherwise, you will not be able to boot your computer because you couldn't enter your decryption password to decrypt your Linux root partition! (If you still have this problem after adding keyboard, try usbinput, though this is deprecated.)
HOOKS="... keyboard encrypt ..."