1======================================= 2How to use dm-crypt and swsusp together 3======================================= 4 5Author: Andreas Steinmetz <ast@domdv.de> 6 7 8 9Some prerequisites: 10You know how dm-crypt works. If not, visit the following web page: 11http://www.saout.de/misc/dm-crypt/ 12You have read Documentation/power/swsusp.rst and understand it. 13You did read Documentation/admin-guide/initrd.rst and know how an initrd works. 14You know how to create or how to modify an initrd. 15 16Now your system is properly set up, your disk is encrypted except for 17the swap device(s) and the boot partition which may contain a mini 18system for crypto setup and/or rescue purposes. You may even have 19an initrd that does your current crypto setup already. 20 21At this point you want to encrypt your swap, too. Still you want to 22be able to suspend using swsusp. This, however, means that you 23have to be able to either enter a passphrase or that you read 24the key(s) from an external device like a pcmcia flash disk 25or an usb stick prior to resume. So you need an initrd, that sets 26up dm-crypt and then asks swsusp to resume from the encrypted 27swap device. 28 29The most important thing is that you set up dm-crypt in such 30a way that the swap device you suspend to/resume from has 31always the same major/minor within the initrd as well as 32within your running system. The easiest way to achieve this is 33to always set up this swap device first with dmsetup, so that 34it will always look like the following:: 35 36 brw------- 1 root root 254, 0 Jul 28 13:37 /dev/mapper/swap0 37 38Now set up your kernel to use /dev/mapper/swap0 as the default 39resume partition, so your kernel .config contains:: 40 41 CONFIG_PM_STD_PARTITION="/dev/mapper/swap0" 42 43Prepare your boot loader to use the initrd you will create or 44modify. For lilo the simplest setup looks like the following 45lines:: 46 47 image=/boot/vmlinuz 48 initrd=/boot/initrd.gz 49 label=linux 50 append="root=/dev/ram0 init=/linuxrc rw" 51 52Finally you need to create or modify your initrd. Lets assume 53you create an initrd that reads the required dm-crypt setup 54from a pcmcia flash disk card. The card is formatted with an ext2 55fs which resides on /dev/hde1 when the card is inserted. The 56card contains at least the encrypted swap setup in a file 57named "swapkey". /etc/fstab of your initrd contains something 58like the following:: 59 60 /dev/hda1 /mnt ext3 ro 0 0 61 none /proc proc defaults,noatime,nodiratime 0 0 62 none /sys sysfs defaults,noatime,nodiratime 0 0 63 64/dev/hda1 contains an unencrypted mini system that sets up all 65of your crypto devices, again by reading the setup from the 66pcmcia flash disk. What follows now is a /linuxrc for your 67initrd that allows you to resume from encrypted swap and that 68continues boot with your mini system on /dev/hda1 if resume 69does not happen:: 70 71 #!/bin/sh 72 PATH=/sbin:/bin:/usr/sbin:/usr/bin 73 mount /proc 74 mount /sys 75 mapped=0 76 noresume=`grep -c noresume /proc/cmdline` 77 if [ "$*" != "" ] 78 then 79 noresume=1 80 fi 81 dmesg -n 1 82 /sbin/cardmgr -q 83 for i in 1 2 3 4 5 6 7 8 9 0 84 do 85 if [ -f /proc/ide/hde/media ] 86 then 87 usleep 500000 88 mount -t ext2 -o ro /dev/hde1 /mnt 89 if [ -f /mnt/swapkey ] 90 then 91 dmsetup create swap0 /mnt/swapkey > /dev/null 2>&1 && mapped=1 92 fi 93 umount /mnt 94 break 95 fi 96 usleep 500000 97 done 98 killproc /sbin/cardmgr 99 dmesg -n 6 100 if [ $mapped = 1 ] 101 then 102 if [ $noresume != 0 ] 103 then 104 mkswap /dev/mapper/swap0 > /dev/null 2>&1 105 fi 106 echo 254:0 > /sys/power/resume 107 dmsetup remove swap0 108 fi 109 umount /sys 110 mount /mnt 111 umount /proc 112 cd /mnt 113 pivot_root . mnt 114 mount /proc 115 umount -l /mnt 116 umount /proc 117 exec chroot . /sbin/init $* < dev/console > dev/console 2>&1 118 119Please don't mind the weird loop above, busybox's msh doesn't know 120the let statement. Now, what is happening in the script? 121First we have to decide if we want to try to resume, or not. 122We will not resume if booting with "noresume" or any parameters 123for init like "single" or "emergency" as boot parameters. 124 125Then we need to set up dmcrypt with the setup data from the 126pcmcia flash disk. If this succeeds we need to reset the swap 127device if we don't want to resume. The line "echo 254:0 > /sys/power/resume" 128then attempts to resume from the first device mapper device. 129Note that it is important to set the device in /sys/power/resume, 130regardless if resuming or not, otherwise later suspend will fail. 131If resume starts, script execution terminates here. 132 133Otherwise we just remove the encrypted swap device and leave it to the 134mini system on /dev/hda1 to set the whole crypto up (it is up to 135you to modify this to your taste). 136 137What then follows is the well known process to change the root 138file system and continue booting from there. I prefer to unmount 139the initrd prior to continue booting but it is up to you to modify 140this. 141