dm-crypt/Swap encryption

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Back to Dm-crypt.

Depending on requirements, different methods may be used to encrypt the swap partition 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, because it avoids sensitive file fragments which may have been swapped out a long time ago without being overwritten. However, re-encrypting swap also forbids using a suspend-to-disk feature generally.

Without suspend-to-disk support

In systems where suspend-to-disk is not a desired feature, it is possible to encrypt the swap partition with a random key at boot-time, thus destroying the contents of the named partition during every boot. This is accomplished by using plain dm-crypt and configuring /etc/crypttab to call mkswap: see point 2.3 of cryptsetup FAQ and "swap" option description in man (5) crypttab.

Default /etc/crypttab already contains a line for swap encryption so you can basically just uncomment it and change the <device> parameter to the persistent name of your swap device.

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

Where:

<name>
Represents the name to state in the first column of /etc/fstab (as "/dev/mapper/<name>").
<device>
Should be the persistent device name for the swap device.
<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: Make sure to use either by-id, by-path or LVM logical volumes' persistent device naming for the <device> array (especially if there are multiple storage drives in the system), as it might happen that their usual kernel naming order (sda, sdb,...) changes upon boots and thus the swap would be created over a valuable file system, destroying all its content. Because of the recreation and re-encryption of the swap device on every boot with mkswap, labels and UUIDs cannot be used (see naming by UUID and point 2.3 of cryptsetup FAQ).

For example, by-id persistent device naming is first identified for the chosen device:

# ls -l /dev/disk/*/* | grep sdaX
lrwxrwxrwx 1 root root 10 Oct 12 16:54 /dev/disk/by-id/ata-WDC_WD2500BEVT-22ZCT0_WD-WXE908VF0470-partX -> ../../sdaX
lrwxrwxrwx 1 root root 10 Oct 12 16:54 /dev/disk/by-id/wwn-0x60015ee0000b237f-partX -> ../../sdaX

and then used as a persistent reference for the /dev/sdaX example partition (if two results are returned as above, choose either one of them):

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

This will map /dev/sdaX 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

The following three methods are alternatives for setting up an encrypted swap for resume-from-disk. If you apply any of them, be aware that critical data swapped out by the system may potentially stay in the swap over a long period (i.e. until it is overwritten). To reduce this risk consider setting up a system job which re-encrypts swap, e.g. each time the system is going into a regular shut-down, along with the method of your choice.

LVM on LUKS

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 Dm-crypt/Encrypting an entire system#LVM on LUKS and then just configure the required kernel parameters.

Assuming you have setup LVM on LUKS with a swap logical volume (at /dev/MyStorage/swap for example), all you need to do is add the resume mkinitcpio hook, and add the resume=/dev/MyStorage/swap kernel parameter to your boot loader. For GRUB, this can be done by appending it to the GRUB_CMDLINE_LINUX_DEFAULT variable in /etc/default/grub.

/etc/default/grub
GRUB_CMDLINE_LINUX_DEFAULT="... resume=/dev/MyStorage/swap"

then run grub-mkconfig -o /boot/grub/grub.cfg to update GRUB's configuration file. To add the mkinitcpio hook, edit the following line in mkinitcpio.conf

/etc/mkinitcpio.conf
HOOKS="... encrypt lvm2 resume ... filesystems ..."

then run mkinitcpio -p linux to update the initramfs image.

mkinitcpio hook

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.

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. Alternatively, use a gnupg-encrypted keyfile as per https://bbs.archlinux.org/viewtopic.php?id=120181

To format the encrypted container for the swap partition, create a 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
 run_hook ()
 {
     cryptsetup open --type luks /dev/<device> swapDevice
 }

for opening the swap device by typing your password or

/lib/initcpio/hooks/openswap
 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
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

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, first 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.

Now 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 $4}}'

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 could not 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 ..."