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diff --git a/debian/README.initramfs b/debian/README.initramfs new file mode 100644 index 0000000..25dfbce --- /dev/null +++ b/debian/README.initramfs @@ -0,0 +1,281 @@ +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` 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: + +0. enter another system (like livecd) +1. open luks device with the new name, change the target name to the new one +2. chroot into it (now, the current target name is the same as it in conf) +3. `update-initramfs -u` +4. reboot + +[#671037]: https://bugs.debian.org/671037 + +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 |