r2053 - trunk

robert at linuxfromscratch.org robert at linuxfromscratch.org
Sun Feb 22 19:07:06 PST 2009

Author: robert
Date: 2009-02-22 20:07:06 -0700 (Sun, 22 Feb 2009)
New Revision: 2053

Added trunk/crypt-rootfs.txt hint.

Added: trunk/crypt-rootfs.txt
--- trunk/crypt-rootfs.txt	                        (rev 0)
+++ trunk/crypt-rootfs.txt	2009-02-23 03:07:06 UTC (rev 2053)
@@ -0,0 +1,412 @@
+AUTHOR: Lars Bamberger <Lars.Bamberger at gmx dot de>
+DATE: 2009-02-15
+LICENSE: GNU Free Documentation License Version 1.2
+SYNOPSIS: How to setup an encrypted file system including the rootfs.
+This describes one possible way of encrypting your hard drive, including the
+root file system. It is intended for experienced users and tries to
+circumnavigate the pitfalls of encrypting your root file system in a
+straightforward way. 
+This hint requires that you have sufficient knowledge of BeyondLinuxFromScratch
+and reasonably up to date software. You must be comfortable building software,
+finding, reading and understanding other pertinent documentation.
+You must know how to set up an initramfs. (See
+'filesystems/ramfs-rootfs-initramfs.txt' in the kernel's documentation.)
+You must be aware why you'd want an encrypted file system and you must
+understand the nature of the threat you're trying to protect yourself against.
+You must also understand shortcomings and security issues if you follow the
+instructions contained in this hint.
+You must have a complete backup of you system somewhere safe! That includes an
+alternative boot device.
+1. What is this about?
+This is about encrypting all but one of your hard drive partitions using LUKS
+for dm-crypt. We'll boot from one small unencrypted partition using initramfs in
+order to to decrypt the rootfs.
+This hint assumes that a small partition from where to boot from is already set
+up. (10 MB should be enough.)
+2. Required software not in the BLFS book
+2.1 devmapper
+Get it from http://packages.debian.org/stable/source/devmapper
+Compile and install it. Required for cryptsetup.
+2.2 cryptsetup with LUKS extension
+Get it from http://luks.endorphin.org/dm-crypt
+Compile and install it. Required to handle encrypted partitions.
+2.3 busybox
+Get it from http://www.busybox.net/
+The minimum required configuration includes:
+* cp
+* hush (interactive shell not required)
+* mount (with support for lots of -o flags) and
+* switch_root.
+Compile it, but DO NOT install it. Keep the binary and name it
+"busybox-minimum". Next, reconfigure busybox for a full-blown desktop system.
+You will need all the standard tools and utilities for the purpose of initially
+encrypting your root partition and for troubleshooting. Name this binary
+"busybox-large" or something similar. Again, it is not required to install it.
+3 Recompiling the kernel
+Decide what algorithm you would like to use to encrypt your hard drive
+with. Note that this is a crucial decision and you should read more background
+information on this. (See ACKNOWLEDGMENTS below.)
+The appropriate modules need to be compiled (hardcoded, not as modules) into
+the kernel.
+As an example you could use the "twofish-cbc-essiv:sha256" method.
+Also, select the 'Device mapper support' from the 'Multiple devices driver
+support' menu in the kernel configuration and the 'crypto target' support as
+Configure 'Initial RAM filesystem and RAM disk' under 'general setup'
+and 'RAM block device support' under 'Block devices'.
+NOTE: You must boot this new kernel before proceeding.
+4. Encrypting partitions partitions other than rootfs and other than swap
+You need to modify your system in order for it to be able to handle encrypted
+partitions. In the first step, we modify the system so that it can handle
+encrypted partitions OTHER than the rootfs. It is strongly suggested that you
+keep a backup of all files you modify in the process.
+4.1 Encrypting the partitions
+NOTE: This document describes how to encrypt every partition separately. If you
+      have more than one HDD in your system, you might consider encrypting the
+      whole device including the partition table. Using the method described in
+      this document leaves the partition table unencrypted and thus may be
+      exposed to an attack. Consider this a potential security risk.
+*** PITFALL ***
+If /usr is a separate partition, cryptsetup and all libraries needed to run
+cryptsetup must be on the root partition. Use 'ldd cryptsetup' to find out.
+It may be necessary to switch to runlevel 1 because you need to be able to
+unmount /usr. Also, make sure that root's shell does not use any libs on that
+partition. If required, compile a static shell for root's use.
+The process is as follows for every partition:
+1) Create as many keys as you like for the partition, for example:
+   head -c 2880 /dev/urandom | uuencode -m - | head -n-1 | tail -n+2 > keyfile
+   or use an easy to remember passphrase.
+2) Make a secure backup of your keys and secure the keyfile by 'chmod 0400' or
+   so. Your backup keys must be absolutely secure (i.e. not on your computer).
+   Remember: If you lose your key, you will absolutely, definitely NOT be able
+   to access you data!
+3) Make a backup of the data on the partition.
+4) Un-mount the partition.
+5) Create an encrypted partition. (All data will be lost on that partition.)
+   Do a
+   cryptsetup -c $cipher-algorithm luksFormat /dev/sd?? $keyfile
+   Replace '$cipher-algorithm', '/dev/sd??' and '$keyfile' with the
+   corresponding values.
+6) Optionally, add more keys to the partition. Do a
+   cryptsetup -y -d $keyfile luksAddKey /dev/sd??
+   Replace '$keyfile' with the same as above and '/dev/sd??' with the
+   corresponding partition.
+7) Open the encrypted partition. Do a
+   cryptsetup -d $keyfile luksOpen /dev/sd?? sd??
+   Replace '$keyfile' and '/dev/sd??' with the corresponding values. Replace
+   'sd??' with a meaningful name. If everything worked out, the partition will
+   appear as '/dev/mapper/sd??' with sd?? being the name you chose.
+8) Create a filesystem on the partitions. Do a
+   mkefs.$WHATEVER /dev/mapper/sd??
+   Replace '$WHATEVER' with the type of filesystem you would like to use
+  (e.g. ext2) and '/dev/mapper/sd??' with the corresponding partition.
+9) Adjust /etc/fstab
+   Because the mountpoints for encrypted partitions have changed, you need to
+   tell the system where to find them. Change the mountpoint by inserting
+   "mapper/" in the device field.
+   Example:
+   /dev/sda4         /home ext2 defaults 1 2
+   becomes
+   /dev/mapper/sda4  /home ext2 defaults 1 2
+10) Mount the filesystem by 'mount /dev/mapper/sd??'
+11) Copy the data back to the partition.
+4.2 Making the system automatically decrypt and mount the partition(s)
+Create a bootscript that will decrypt your encrypted partitions. It is assumed
+that the passphrases are stored in /etc/crypt for example. Note that storing the
+passphrases on disk might pose a security problem! Use the template for
+bootscripts included with BLFS and make it do:
+/sbin/cryptsetup -d /etc/crypt/$PARTITION.key luksOpen \
+for every encrypted partition other than the root partition and the swap
+# Begin $rc_base/init.d/cryptsetup
+# Description : Make encrypted filesystems available for mounting
+#               Ande clean up afterwards
+# Authors     : Lars Bamberger
+# Version     : 00.01
+# Notes       : This should never be automatically called with any
+#               argument other than "start". During shutdown and reboot,
+#               it is sufficient to umount the filesystems. /dev/mapper/*
+#               will be gone when the kernel stops or reboots.
+. /etc/sysconfig/rc
+. ${rc_functions}
+case "${1}" in
+	start)
+		boot_mesg "luksOpen Home..."
+		$PROC -d /etc/crypt/home.key luksOpen /dev/sda4 sda4
+		evaluate_retval
+	stop)
+		boot_mesg "luksClose Home..."
+		$PROC luksClose sda4
+		evaluate_retval
+		;;
+	reload)
+		boot_mesg "Reloading home..."
+		$PROC reload sda4
+		evaluate_retval
+		;;
+	restart)
+		${0} stop
+		sleep 1
+		${0} start
+		;;
+	status)
+		$PROC status sda4
+		;;
+	*)
+		echo "Usage: ${0} {start|stop|reload|restart|status}"
+		exit 1
+		;;
+# End $rc_base/init.d/cryptsetup
+Now, before proceeding, make sure everything works as expected up until now.
+Become familiar with encrypting your partitions this way.
+Make an appropriate softlink so that this script is called at boottime:
+# cd /etc/rc.d/rcsysinit.d
+# ln -s ../init.d/cryptsetup S19cryptsetup
+Double-check everything so that booting, rebooting, shutting down etc. works as
+5. A word about encrypting the swap partition(s)
+Do not omit encrypting your swap partitions. Lot's of interesting data can be
+found on swap spaces. Do not consider you data safe if you don't use encrypted
+swap spaces. 
+In theory, the data on the swap partition(s) does not need to be consistent
+between reboots. This means we could create a swapspace anew during boottime,
+using a random (and thus different) cryptokey every time the system boots. This
+way you don't have to bother with managing swap's cryptokeys and you won't have
+to store them anywhere (except in memory). This can be considered an additional
+security feature.
+However, if you suspend your system (either to RAM or to disk), data in
+swapspace must remain consistent. Therefore you have to treat the swap
+partition(s) just as if they were a regular partition, meaning you should
+encrypt them like explained above.
+6. Encrypting rootfs 
+You can't just encrypt the rootfs the way the other partitions were encrypted
+because the system is running on it. The idea is to create an initramfs
+containing everything you need to encrypt (and decrypt) the rootfs. (Details can
+be found in the kernel's documentation:
+You'll need all the standard directories (bin, sbin, usr/{bin,sbin}, proc, sys,
+dev, lib). In bin we put our busybox-large (rename to busybox) and a softlink to
+busybox named hush. Copy cryptsetup to sbin.
+In dev put some useful devices: console, null, sd?? and a directory
+'mapper' containing 'control'. Then make a copy of dev:
+cp -a dev init-dev
+In lib (and dev) put everything needed to run busybox and cryptsetup.
+The init script is like this:
+/bin/busybox mount -t proc proc /proc
+/bin/busybox mount -t sysfs sysfs /sys
+/bin/busybox mount -t tmpfs tmpfs /dev
+/bin/busybox cp -a /init-dev/* /dev
+/bin/busybox --install -s
+exec /bin/busybox hush
+Put all this into a directory (init goes there as well and not into sbin) and
+create the image using
+find . | cpio --quiet -H newc -o | gzip -9 -n > /boot/imagefile.img
+Pass the appropriate initrd argument to the kernel when booting and this will
+drop you into the hush shell after system boot.
+cryptsetup needs proc and sys mounted. It also requires the dev directory.
+As we want to save dev when we switch_root later, we mount it as tmpfs. This
+means that the devices in dev will be gone, so copy them back into dev. Be aware
+that you need at least 'null' and 'console' in dev before mounting tmpfs on dev.
+Once in the shell, encrypt your rootfs like any other partition as described
+above. Don't forget the backup! ABSOLUTELY, POSITIVELY make certain that you are
+able to mount and access the unencrypted backup of the rootfs from within the
+hush shell!
+Next, create the encrypted rootpartition. Note that the passphrase won't be
+stored anywhere on disk, so do:
+cryptsetup -y -c $cipher-algorithm luksFormat /dev/sd??
+to create the encrypted rootfs. Replace '$cipher-algorithm' and '/dev/sd??' with
+the respective values. Next, open the partition and format it and recover the
+cryptsetup luksOpen /dev/sd?? sd??
+$BAKUROOTFS/mkefs.$TYPE /dev/mapper/sd??
+mkdir /new-root
+mount -t $FSTYPE /dev/mapper/sd?? /new-root
+cp -a $BACKUPROOTFS /new-root
+Next, modify /etc/fstab (on /new-root) to reflect the new device for the rootfs.
+Also modify the cryptsetup script as described below (7. PITFALL).
+7. Decrypting the rootfs on subsequent boots
+Like in 6., create an initramfs. The difference is that now the
+"busybox-minimum" binary is used and you'll need an additional directory
+new-root. Don't forget the 'hush' softlink.
+The init is like this: (Replace 'sd??' with your root-device and adjust for the
+/bin/busybox mount -t proc proc /proc
+/bin/busybox mount -t sysfs sysfs /sys
+/bin/busybox mount -t tmpfs tmpfs /dev
+/bin/busybox cp -a /init-dev/* /dev
+/sbin/cryptsetup luksOpen /dev/sd?? sd??
+/bin/busybox mount -r -t ext2 /dev/mapper/sd?? /new-root
+/bin/busybox mount --move /proc /new-root/proc
+/bin/busybox mount --move /sys /new-root/sys
+/bin/busybox mount --move /dev /new-root/dev 
+exec /bin/busybox switch_root /new-root /sbin/init $@
+You want to keep /proc /sys and /dev after switch_root because cryptsetup uses
+them. Hence the 'mount --move' commands. Note that /dev/mapper/sd?? (the root
+device) will be gone once you mount the true root partition, switch_root and the
+rootfs proper starts udev. That's the reason why this device needs to be
+recreated. So, modify the cryptsetup bootscript to include 
+		if [[ ! -b /dev/mapper/sd?? ]];
+		then
+			boot_mesg "Making device for rootfs..."
+			/bin/mknod -m 0600 /dev/mapper/sd?? b 254 0
+			evaluate_retval;
+		fi
+in the start section of the script.
+8. Making sure security is not compromised
+Once everything works as it should, remove the unencrypted backup of your
+rootfs. Protect your bootloader (and possibly the BIOS) with a password to
+disable fiddling with the boot parameters.
+Create a bootscript (checkbootfs) that makes sure that the unencrypted partition
+we booted from was not compromised. Use something like:
+		boot_mesg "Checking integrity of boot FS..."
+		if
+		[[ $(/bin/md5sum -b /dev/sd??) == \
+		"$whatevermd5sum */dev/sd??" ]] \
+			&& \
+		[[ $(/bin/sha1sum -b /dev/sd??) == \
+		"$whatevensha1sum */dev/sd??" ]];
+		then
+			echo_ok;
+		else
+			echo_failure
+boot_mesg -n "FAILURE:\n\nThe boot file system seems to have been
+altered!\n\n" ${FAILURE}
+        boot_mesg -n " DO NOT TRUST THIS SYSTEM!\n\n"
+        boot_mesg_flush
+Make sure this is the very last thing you implement, as the hashsums will
+change as we go on. The hashsums will also change if you run a fsck on the boot
+  * Various for the wiki at http://de.gentoo-wiki.com/Cryptsetup-luks_initramfs
+    and
+    http://en.gentoo-wiki.com/wiki/SECURITY_System_Encryption_DM-Crypt_with_LUKS
+  * Clemens Fruhwirth (http://clemens.endorphin.org/) 
+    for LUKS for dm-crypt: http://luks.endorphin.org/dm-crypt
+  * Basic rewrite.
+  * Initial hint.

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