141 lines
4.8 KiB
ReStructuredText
141 lines
4.8 KiB
ReStructuredText
=======================================
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How to use dm-crypt and swsusp together
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=======================================
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Author: Andreas Steinmetz <ast@domdv.de>
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Some prerequisites:
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You know how dm-crypt works. If not, visit the following web page:
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http://www.saout.de/misc/dm-crypt/
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You have read Documentation/power/swsusp.rst and understand it.
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You did read Documentation/admin-guide/initrd.rst and know how an initrd works.
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You know how to create or how to modify an initrd.
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Now your system is properly set up, your disk is encrypted except for
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the swap device(s) and the boot partition which may contain a mini
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system for crypto setup and/or rescue purposes. You may even have
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an initrd that does your current crypto setup already.
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At this point you want to encrypt your swap, too. Still you want to
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be able to suspend using swsusp. This, however, means that you
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have to be able to either enter a passphrase or that you read
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the key(s) from an external device like a pcmcia flash disk
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or an usb stick prior to resume. So you need an initrd, that sets
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up dm-crypt and then asks swsusp to resume from the encrypted
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swap device.
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The most important thing is that you set up dm-crypt in such
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a way that the swap device you suspend to/resume from has
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always the same major/minor within the initrd as well as
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within your running system. The easiest way to achieve this is
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to always set up this swap device first with dmsetup, so that
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it will always look like the following::
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brw------- 1 root root 254, 0 Jul 28 13:37 /dev/mapper/swap0
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Now set up your kernel to use /dev/mapper/swap0 as the default
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resume partition, so your kernel .config contains::
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CONFIG_PM_STD_PARTITION="/dev/mapper/swap0"
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Prepare your boot loader to use the initrd you will create or
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modify. For lilo the simplest setup looks like the following
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lines::
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image=/boot/vmlinuz
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initrd=/boot/initrd.gz
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label=linux
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append="root=/dev/ram0 init=/linuxrc rw"
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Finally you need to create or modify your initrd. Lets assume
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you create an initrd that reads the required dm-crypt setup
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from a pcmcia flash disk card. The card is formatted with an ext2
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fs which resides on /dev/hde1 when the card is inserted. The
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card contains at least the encrypted swap setup in a file
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named "swapkey". /etc/fstab of your initrd contains something
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like the following::
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/dev/hda1 /mnt ext3 ro 0 0
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none /proc proc defaults,noatime,nodiratime 0 0
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none /sys sysfs defaults,noatime,nodiratime 0 0
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/dev/hda1 contains an unencrypted mini system that sets up all
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of your crypto devices, again by reading the setup from the
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pcmcia flash disk. What follows now is a /linuxrc for your
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initrd that allows you to resume from encrypted swap and that
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continues boot with your mini system on /dev/hda1 if resume
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does not happen::
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#!/bin/sh
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PATH=/sbin:/bin:/usr/sbin:/usr/bin
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mount /proc
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mount /sys
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mapped=0
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noresume=`grep -c noresume /proc/cmdline`
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if [ "$*" != "" ]
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then
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noresume=1
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fi
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dmesg -n 1
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/sbin/cardmgr -q
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for i in 1 2 3 4 5 6 7 8 9 0
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do
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if [ -f /proc/ide/hde/media ]
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then
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usleep 500000
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mount -t ext2 -o ro /dev/hde1 /mnt
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if [ -f /mnt/swapkey ]
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then
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dmsetup create swap0 /mnt/swapkey > /dev/null 2>&1 && mapped=1
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fi
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umount /mnt
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break
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fi
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usleep 500000
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done
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killproc /sbin/cardmgr
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dmesg -n 6
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if [ $mapped = 1 ]
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then
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if [ $noresume != 0 ]
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then
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mkswap /dev/mapper/swap0 > /dev/null 2>&1
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fi
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echo 254:0 > /sys/power/resume
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dmsetup remove swap0
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fi
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umount /sys
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mount /mnt
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umount /proc
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cd /mnt
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pivot_root . mnt
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mount /proc
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umount -l /mnt
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umount /proc
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exec chroot . /sbin/init $* < dev/console > dev/console 2>&1
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Please don't mind the weird loop above, busybox's msh doesn't know
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the let statement. Now, what is happening in the script?
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First we have to decide if we want to try to resume, or not.
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We will not resume if booting with "noresume" or any parameters
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for init like "single" or "emergency" as boot parameters.
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Then we need to set up dmcrypt with the setup data from the
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pcmcia flash disk. If this succeeds we need to reset the swap
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device if we don't want to resume. The line "echo 254:0 > /sys/power/resume"
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then attempts to resume from the first device mapper device.
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Note that it is important to set the device in /sys/power/resume,
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regardless if resuming or not, otherwise later suspend will fail.
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If resume starts, script execution terminates here.
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Otherwise we just remove the encrypted swap device and leave it to the
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mini system on /dev/hda1 to set the whole crypto up (it is up to
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you to modify this to your taste).
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What then follows is the well known process to change the root
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file system and continue booting from there. I prefer to unmount
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the initrd prior to continue booting but it is up to you to modify
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this.
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