Dm-crypt/特記事項

Dm-crypt に戻る。

暗号化されていない boot パーティションのセキュア化

たとえ root を暗号化したとしても、/boot パーティションと Master Boot Record はディスクの中で暗号化されてない状態で残ります。この2つを暗号化するのは基本的に不可能です。ブートローダーと BIOS が dm-crypt コンテナの暗号化を解除してブートプロセスを続行することが出来なくなってしまいます。例外は GRUB で、LUKS で暗号化した /boot を復号する機能が存在します。GRUB#Boot partition を見て下さい。

警告: Note that securing the /boot partition and MBR can mitigate numerous attacks that occur during the boot process, but systems configured this way may still be vulnerable to BIOS/UEFI/firmware tampering, hardware keyloggers, cold boot attacks, and many other threats that are beyond the scope of this article. For an overview of system-trust issues and how these relate to full-disk encryption, refer to [1].

リムーバルデバイスから起動

Using a separate device to boot a system is a fairly straightforward procedure, and offers a significant security improvement against some kinds of attacks. Two vulnerable parts of a system employing an encrypted root filesystem are

• the Master Boot Record, and
• the /boot partition.

These must be stored unencrypted in order for the system to boot. In order to protect these from tampering, it is advisable to store them on a removable medium, such as a USB drive, and boot from that drive instead of the hard disk. As long as you keep the drive with you at all times, you can be certain that those components have not been tampered with, making authentication far more secure when unlocking your system.

It is assumed that you already have your system configured with a dedicated partition mounted at /boot. If you do not, please follow the steps in dm-crypt/System configuration#Boot loader, substituting your hard disk for a removable drive.

ノート: You must make sure your system supports booting from the chosen medium, be it a USB drive, an external hard drive, an SD card, or anything else.

Prepare the removable drive (/dev/sdx).

# gdisk /dev/sdx #format if necessary. Alternatively, cgdisk, fdisk, cfdisk, gparted...
# mkfs.ext2 /dev/sdx1
# mount /dev/sdx1 /mnt

Copy your existing /boot contents to the new one.

# cp -R -i -d /boot/* /mnt

Mount the new partition. Do not forget to update your fstab file accordingly.

# umount /boot
# umount /mnt
# mount /dev/sdx1 /boot
# genfstab -p -U / > /etc/fstab

Update GRUB. grub-mkconfig should detect the new partition UUID automatically, but custom menu entries may need to be updated manually.

# grub-mkconfig -o /boot/grub/grub.cfg
# grub-install /dev/sdx #install to the removable device, not the hard disk.

Reboot and test the new configuration. Remember to set your device boot order accordingly in your BIOS or UEFI. If the system fails to boot, you should still be able to boot from the hard drive in order to correct the problem.

chkboot

警告: chkboot makes a /boot partition tamper-evident, not tamper-proof. By the time the chkboot script is run, you have already typed your password into a potentially compromised boot loader, kernel, or initrd. If your system fails the chkboot integrity test, no assumptions can be made about the security of your data.

Referring to an article from the ct-magazine (Issue 3/12, page 146, 01.16.2012, [2]) the following script checks files under /boot for changes of SHA-1 hash, inode, and occupied blocks on the hard drive. It also checks the Master Boot Record. The script cannot prevent certain type of attacks, but a lot are made harder. No configuration of the script itself is stored in unencrypted /boot. With a locked/powered-off encrypted system, this makes it harder for some attackers because it is not apparent that an automatic checksum comparison of the partition is done upon boot. However, an attacker who anticipates these precautions can manipulate the firmware to run his own code on top of your kernel and intercept file system access, e.g. to boot, and present the untampered files. Generally, no security measures below the level of the firmware are able to guarantee trust and tamper evidence.

The script with installation instructions is available (Author: Juergen Schmidt, ju at heisec.de; License: GPLv2). There is also package chkboot^AUR to install.

After installation add a service file (the package includes one based on the following) and enable it:

[Unit]
Description=Check that boot is what we want
Requires=basic.target
After=basic.target

[Service]
Type=oneshot
ExecStart=/usr/local/bin/chkboot.sh

[Install]
WantedBy=multi-user.target

There is a small caveat for systemd: At the time of writing, the original chkboot.sh script provided contains an empty space at the beginning of #!/bin/bash which has to be removed for the service to start successfully.

As /usr/local/bin/chkboot_user.sh need to be excuted after login, add it to the autostart (e.g. under KDE -> System Settings -> Startup and Shutdown -> Autostart; GNOME 3: gnome-session-properties).

With Arch Linux, changes to /boot are pretty frequent, for example by new kernels rolling-in. Therefore it may be helpful to use the scripts with every full system update. One way to do so:

#!/bin/bash
#
# Note: Insert your <user>  and execute it with sudo for pacman & chkboot to work automagically
#
echo "Pacman update [1] Quickcheck before updating" & 
sudo -u <user> /usr/local/bin/chkboot_user.sh		# insert your logged on <user> 
/usr/local/bin/chkboot.sh
sync							# sync disks with any results 
sudo -u <user> /usr/local/bin/chkboot_user.sh		# insert your logged on <user> 
echo "Pacman update [2] Syncing repos for pacman" 
pacman -Syu
/usr/local/bin/chkboot.sh
sync	
sudo -u <user> /usr/local/bin/chkboot_user.sh		# insert your logged on <user>
echo "Pacman update [3] All done, let us roll on ..."

mkinitcpio-chkcryptoboot

警告: This hook does not encrypt GRUB's core (MBR) code or EFI stub, nor does it protect against situations where an attacker is able to modify the behaviour of the bootloader to compromise the kernel and/or initramfs at run-time.

mkinitcpio-chkcryptoboot^AUR は初期ユーザー空間で整合性チェックを行ってシステムのセキュリティが破られている場合に root パーティションのパスワードを入力しないようにユーザーに忠告する mkinitcpio フックです。Security is achieved through an encrypted boot partition, which is unlocked using GRUB's cryptodisk.mod module, and a root filesystem partition, which is encrypted with a password different from the former. This way, the initramfs and kernel are secured against offline tampering, and the root partition can remain secure even if the /boot partition password is entered on a compromised machine (provided that the chkcryptoboot hook detects the compromise, and is not itself compromised at run-time).

This hook requires GRUB release >=2.00 to function, and a dedicated, LUKS encrypted /boot partition with its own password in order to be secure.

インストール

Install mkinitcpio-chkcryptoboot^AUR and edit /etc/default/chkcryptoboot.conf. If you want the ability of detecting if your boot partition was bypassed, edit the CMDLINE_NAME and CMDLINE_VALUE variables, with values known only to you. You can follow the advice of using two hashes as is suggested right after the installation. Also, be sure to make the appropriate changes to the kernel command line in /etc/default/grub. Edit the HOOKS= line in /etc/mkinitcpio.conf, and insert the chkcryptoboot hook before encrypt. When finished, rebuild the initramfs.

技術的な概要

mkinitcpio-chkcryptoboot^AUR consists of an install hook and a run-time hook for mkinitcpio. The install hook runs every time the initramfs is rebuilt, and hashes the GRUB EFI stub ($esp/EFI/grub_uefi/grubx64.efi) (in the case of UEFI systems) or the first 446 bytes of the disk on which GRUB is installed (in the case of BIOS systems), and stores that hash inside the initramfs located inside the encrypted /boot partition. When the system is booted, GRUB prompts for the /boot password, then the run-time hook performs the same hashing operation and compares the resulting hashes before prompting for the root partition password. If they do not match, the hook will print an error like this:

CHKCRYPTOBOOT ALERT!
CHANGES HAVE BEEN DETECTED IN YOUR BOOT LOADER EFISTUB!
YOU ARE STRONGLY ADVISED NOT TO ENTER YOUR ROOT CONTAINER PASSWORD!
Please type uppercase yes to continue:

In addition to hashing the boot loader, the hook also checks the parameters of the running kernel against those configured in /etc/default/chkcryptoboot.conf. This is checked both at run-time and after the boot process is done. This allows the hook to detect if GRUB's configuration was not bypassed at run-time and afterwards to detect if the entire /boot partition was not bypassed.

For BIOS systems the hook creates a hash of GRUB's first stage bootloader (installed to the first 446 bytes of the bootdevice) to compare at the later boot processes. The main second-stage GRUB bootloader core.img is not checked.

他の方法

Alternatively to above scripts, a hash check can be set up with AIDE which can be customized via a very flexible configuration file.

While one of these methods should serve the purpose for most users, they do not address all security problems associated with the unencrypted /boot. One approach which endeavours to provide a fully authenticated boot chain was published with POTTS as an academic thesis to implement the STARK authentication framework.

The POTTS proof-of-concept uses Arch Linux as a base distribution and implements a system boot chain with

• POTTS - a boot menu for a one-time authentication message prompt
• TrustedGrub - a GRUB Legacy implementation which authenticates the kernel and initramfs against TPM chip registers
• TRESOR - a kernel patch which implements AES but keeps the master-key not in RAM but in CPU registers during runtime.

As part of the thesis installation instructions based on Arch Linux (ISO as of 2013-01) have been published. If you want to try it, be aware these tools are not in standard repositories and the solution will be time consuming to maintain.

GPG や OpenSSL で暗号化されたキーファイルを使う

The following forum posts give instructions to use two factor authentication, gpg or openssl encrypted keyfiles, instead of a plaintext keyfile described earlier in this wiki article System Encryption using LUKS with GPG encrypted keys:

• GnuPG: Post regarding GPG encrypted keys This post has the generic instructions.
• OpenSSL: Post regarding OpenSSL encrypted keys This post only has the ssldec hooks.
• OpenSSL: Post regarding OpenSSL salted bf-cbc encrypted keys This post has the bfkf initcpio hooks, install, and encrypted keyfile generator scripts.
• LUKS: Post regarding LUKS encrypted keys with a lukskey initcpio hook.

Note that:

• You can follow the above instructions with only two primary partitions one boot partition

(required because of LVM), and one primary LVM partition. Within the LVM partition you can have as many partitions as you need, but most importantly it should contain at least root, swap, and home logical volume partitions. This has the added benefit of having only one keyfile for all your partitions, and having the ability to hibernate your computer (suspend to disk) where the swap partition is encrypted. If you decide to do so your hooks in /etc/mkinitcpio.conf should look like HOOKS=" ... usb usbinput (etwo or ssldec) encrypt(if using openssl) lvm2 resume ... " and you should add "resume=/dev/mapper/<VolumeGroupName>-<LVNameOfSwap>" to your kernel parameters.

• If you need to temporarily store the unecrypted keyfile somewhere, do not store them on an unencrypted disk. Even better make sure to store them to RAM such as /dev/shm.
• If you want to use a GPG encrypted keyfile, you need to use a statically compiled GnuPG version 1.4 or you could edit the hooks and use this AUR package gnupg1^AUR
• It is possible that an update to OpenSSL could break the custom ssldec mentioned in the second forum post.

root などのパーティションのリモート解除

LUKS によって完全に暗号化されたシステムをリモートで再起動したい場合、もしくは Wake-on-LAN サービスを使ってシステムを起動したい場合、起動時に root パーティション/ボリュームのパスフレーズを入力する手段が必要になります。initrd で mkinitcpio の net フックと SSH サーバーを実行することでこれを実現可能です。dropbear_initrd_encrypt^AUR パッケージをインストールしてインストール後の指示に従って下さい:

1. If you do not have an SSH key pair yet, generate one on the client system (the one which will be used to unlock the remote machine).
2. Insert your SSH public key (i.e. the one you usually put onto hosts so that you can ssh in without a password, or the one you just created and which ends with .pub) into the remote machine's /etc/dropbear/root_key file using the method of your choice, e.g.:
   • copy the public key to the remote system
   • then enter the following command (on the remote system):

     # cat /home/<user>/.ssh/authorized_keys > /etc/dropbear/root_key

     ヒント: This method can later be used to add other SSH public keys as needed; in that case verify the content of remote ~/.ssh/authorized_keys contains only keys you agree to be used to unlock the remote machine. When adding additional keys, also regenerate your initrd with mkinitcpio. See also SSH keys#Security.
3. Add the dropbear encryptssh hooks before filesystems within the "HOOKS" array in /etc/mkinitcpio.conf (or replace encrypt with them if it was present). Put the net hook early in the HOOKS array if your DHCP server takes a long time to lease IP addresses, and in any case place it before the dropbear encryptssh hooks (between modconf and block proves functional). Then rebuild the initramfs image.
4. Configure the required cryptdevice= parameter and add the ip= kernel command parameter to your bootloader configuration with the appropriate arguments (see Mkinitcpio#Using_net). For example, if the DHCP server does not attribute a static IP to your remote system, making it difficult to access via SSH accross reboots, you can explicitly state the IP you want to be used:

   ip=192.168.1.1:::::eth0:none

   ノート: Make sure to use kernel device names for the interface name (under the form eth#) and not udev ones, as those will not work.
   Then update the configuration of your bootloader, e.g. for GRUB:

   # grub-mkconfig -o /boot/grub/grub.cfg

5. Finally, restart the remote system and try to ssh to it, explicitly stating the "root" username (even if the root account is disabled on the machine, here it is a special "root" user set by dropbear for the purpose of unlocking the remote system). You may see a warning about host authenticity that you can safely ignore (type yes), then you should be presented with a prompt asking you to enter the passphrase for unlocking the remote root:

$ ssh root@192.168.1.1

Enter passphrase for /dev/disk/by-id/wwn-...-part2:    
Connection to 192.168.1.1 closed.

Afterwards, the system will complete its boot process and you can ssh to it as you normally would (with the remote user of your choice).

ヒント: If you would simply like a nice solution to mount other encrypted partitions (such as /home) remotely, you may want to look at this forum thread.

ソリッドステートドライブ (SSD) の Discard/TRIM のサポート

Solid state drive users should be aware that by default, Linux's full-drive encryption mechanisms will not forward TRIM commands from the filesystem to the underlying drive. The device-mapper maintainers have made it clear that TRIM support will never be enabled by default on dm-crypt devices because of the potential security implications.[3][4]

Most users will still want to use TRIM on their encrypted SSDs. Minimal data leakage in the form of freed block information, perhaps sufficient to determine the filesystem in use, may occur on devices with TRIM enabled. An illustration and discussion of the issues arising from activating TRIM is available in the blog of a cryptsetup developer. As a result encryption schemes that rely on plausible deniability should never be used on a device that utilizes TRIM.

In linux 3.1 and up, support for dm-crypt TRIM pass-through can be toggled upon device creation or mount with dmsetup. Support for this option also exists in cryptsetup version 1.4.0 and up. To add support during boot, you will need to add :allow-discards to the cryptdevice option. The TRIM option may look like this:

cryptdevice=/dev/sdaX:root:allow-discards

For the main cryptdevice configuration options before the :allow-discards see Dm-crypt/System configuration.

Besides the kernel option, it is also required to periodically run fstrim or mount the filesystem (e.g. /dev/mapper/root in this example) with the discard option in /etc/fstab. For details, please refer to the SSD page. For LUKS devices unlocked manually on the console or via /etc/crypttab either discard or allow-discards may be used.

encrypt フックと複数のディスク

The encrypt hook only allows for a single cryptdevice= entry. In system setups with multiple drives this may be limiting, because dm-crypt has no feature to exceed the physical device. For example, take "LVM on LUKS": The entire LVM exists inside a LUKS mapper. This is perfectly fine for a single-drive system, since there is only one device to decrypt. But what happens when you want to increase the size of the LVM? You cannot, at least not without modifying the encrypt hook.

The following sections briefly show alternatives to overcome the limitation. The first deals with how to expand a LUKS on LVM setup to a new disk. The second with modifying the encrypt hook to unlock multiple disks in LUKS setups without LVM. The third section then again uses LVM, but modifies the encrypt hook to unlock the encrypted LVM with a remote LUKS header.

LVM を複数のディスクに拡張

The management of multiple disks is a basic LVM feature and a major reason for its partitioning flexibility. It can also be used with dm-crypt, but only if LVM is employed as the first mapper. In such a LUKS on LVM setup the encrypted devices are created inside the logical volumes (with a separate passphrase/key per volume). The following covers the steps to expand that setup to another disk.

警告: Backup! While resizing filesystems may be standard, keep in mind that operations may go wrong and the following might not apply to a particular setup. Generally, extending a filesystem to free disk space is less problematic than shrinking one. This in particular applies when stacked mappers are used, as it is the case in the following example.

新しいドライブの追加

First, it may be desired to prepare a new disk according to Dm-crypt/Drive preparation. Second, it is partitioned as a LVM, e.g. all space is allocated to /dev/sdY1 with partition type "8E00" (Linux LVM). Third, the new disk/partition is attached to the existing LVM volume group, e.g.:

# pvcreate /dev/sdY1
# vgextend MyStorage /dev/sdY1

論理ボリュームの拡張

For the next step, the final allocation of the new diskspace, the logical volume to be extended has to be unmounted. It can be performed for the cryptdevice root partition, but in this case the procedure has to be performed from an Arch Install ISO.

In this example, it is assumed that the logical volume for /home (lv-name homevol) is going to be expanded with the fresh disk space:

# umount /home
# fsck /dev/mapper/home
# cryptsetup luksClose /dev/mapper/home
# lvextend -l +100%FREE MyStorage/homevol

Now the logical volume is extended and the LUKS container comes next:

# cryptsetup open --type luks /dev/mapper/MyStorage-homevol home
# umount /home      # as a safety, in case it was automatically remounted
# cryptsetup --verbose resize home

Finally, the filesystem itself is resized:

# e2fsck -f /dev/mapper/home
# resize2fs /dev/mapper/home

Done! If it went to plan, /home can be remounted

# mount /dev/mapper/home /home

and now includes the span to the new disk. Note that the cryptsetup resize action does not affect encryption keys, they have not changed.

複数のパーティションの encrypt フックを修正

複数の root パーティション

It is possible to modify the encrypt hook to allow multiple hard drive decrypt root (/) at boot. The cryptsetup-multi^AUR package may be used for it. An alternative way according to an Arch user (benke):

# cp /usr/lib/initcpio/hooks/encrypt  /usr/lib/initcpio/hooks/encrypt2
# cp /usr/lib/initcpio/install/encrypt /usr/lib/initcpio/install/encrypt2
# nano /usr/lib/initcpio/hooks/encrypt2

Change $cryptkey to $cryptkey2, and $cryptdevice to $cryptdevice2. Add cryptdevice2= (e.g. cryptdevice2=/dev/sdb:hdd2) to your boot options (and cryptkey2= if needed).

Change the /etc/fstab flag for root:

/dev/sdb     /mnt    btrfs    device=/dev/sda,device=/dev/sdb, ...    0 0

複数の root 以外のパーティション

Maybe you have a requirement for using the encrypt hook on a non-root partition. Arch does not support this out of the box, however, you can easily change the cryptdev and cryptname values in /lib/initcpio/hooks/encrypt (the first one to your /dev/sd* partition, the second to the name you want to attribute). That should be enough.

The big advantage is you can have everything automated, while setting up /etc/crypttab with an external key file (i.e. the keyfile is not on any internal hard drive partition) can be a pain - you need to make sure the USB/FireWire/... device gets mounted before the encrypted partition, which means you have to change the order of /etc/fstab (at least).

Of course, if the cryptsetup package gets upgraded, you will have to change this script again. Unlike /etc/crypttab, only one partition is supported, but with some further hacking one should be able to have multiple partitions unlocked.

If you want to do this on a software RAID partition, there is one more thing you need to do. Just setting the /dev/mdX device in /lib/initcpio/hooks/encrypt is not enough; the encrypt hook will fail to find the key for some reason, and not prompt for a passphrase either. It looks like the RAID devices are not brought up until after the encrypt hook is run. You can solve this by putting the RAID array in /boot/grub/menu.lst, like

kernel /boot/vmlinuz-linux md=1,/dev/hda5,/dev/hdb5

If you set up your root partition as a RAID, you will notice the similarities with that setup ;-). GRUB can handle multiple array definitions just fine:

kernel /boot/vmlinuz-linux root=/dev/md0 ro md=0,/dev/sda1,/dev/sdb1 md=1,/dev/sda5,/dev/sdb5,/dev/sdc5

リモート LUKS ヘッダーを使ってシステムを暗号化

This example follows the same setup as in Dm-crypt/Encrypting an entire system#Plain dm-crypt, which should be read first before following this guide.

By using a remote header the encrypted blockdevice itself only carries encrypted data, which gives deniable encryption as long as the existence of a header is unknown to the attackers. It is similar to using plain dm-crypt, but with the LUKS advantages such as multiple passphrases for the masterkey and key derivation. Further, using a remote header offers a form of two factor authentication with an easier setup than using GPG or OpenSSL encrypted keyfiles, while still having a built-in password prompt for multiple retries. See Disk encryption#Cryptographic metadata for more information.

See Dm-crypt/Device encryption#Encryption options for LUKS mode for encryption options before performing the first step to setup the encrypted system partition and creating a header file to use with cryptsetup:

# truncate -s 2M header.img
# cryptsetup luksFormat /dev/sdX --header header.img

Open the container:

# cryptsetup open --header header.img --type luks /dev/sdX enc

Now follow the LVM on LUKS setup to your requirements. The same applies for preparing the boot partition on the removable device (because if not, there is no point in having a separate header file for unlocking the encrypted disk). Next move the header.img onto it:

# mv header.img /mnt/boot

Follow the installation procedure up to the mkinitcpio step (you should now be arch-chrooted inside the encrypted system).

There are two options for initramfs to support a detached LUKS header.

systemd フックを使う

ノート: This method requires systemd 219 or later.

First create /etc/crypttab.initramfs and add the encrypted device to it. The syntax is defined in crypttab(5)

/etc/crypttab.initramfs

MyStorage    PARTUUID=00000000-0000-0000-0000-000000000000    none    header=/boot/header.img

Modify /etc/mkinitcpio.conf to use systemd and add the header to FILES.

/etc/mkinitcpio.conf

FILES="/boot/header.img"

HOOKS="... systemd ... block sd-encrypt sd-lvm2 filesystems ..."

Recreate the initramfs and you are done.

ノート:

• No cryptsetup parameters need to be passed to the kernel command line, since/etc/crypttab.initramfs will be added as /etc/crypttab in the initramfs. If you wish to specify them in the kernel command line see systemd-cryptsetup-generator(8) for the supported options.
• Be aware the systemd hook adds further files to the initramfs (e.g. /etc/passwd and /etc/group), in case you consider them sensitive.

encrypt フックを修正する

This method shows how to modify the encrypt hook in order to use a remote LUKS header. Now the encrypt hook has to be modified to let cryptsetup use the separate header (base source and idea for these changes published on the BBS). Make a copy so it is not overwritten on a mkinitcpio update:

# cp /lib/initcpio/hooks/encrypt{,2}
# cp /usr/lib/initcpio/install/encrypt{,2}

/lib/initcpio/hooks/encrypt2 (around line 52)

warn_deprecated() {
    echo "The syntax 'root=${root}' where '${root}' is an encrypted volume is deprecated"
    echo "Use 'cryptdevice=${root}:root root=/dev/mapper/root' instead."
}

local headerFlag=false
for cryptopt in ${cryptoptions//,/ }; do
    case ${cryptopt} in
        allow-discards)
            cryptargs="${cryptargs} --allow-discards"
            ;;  
        header)
            cryptargs="${cryptargs} --header /boot/header.img"
            headerFlag=true
            ;;
        *)  
            echo "Encryption option '${cryptopt}' not known, ignoring." >&2 
            ;;  
    esac
done

if resolved=$(resolve_device "${cryptdev}" ${rootdelay}); then
    if $headerFlag || cryptsetup isLuks ${resolved} >/dev/null 2>&1; then
        [ ${DEPRECATED_CRYPT} -eq 1 ] && warn_deprecated
        dopassphrase=1

Now edit the mkinitcpio.conf to add the encrypt2 and lvm2 hooks, the header.img to FILES and the loop to MODULES, apart from other configuration the system requires:

/etc/mkinitcpio.conf

MODULES="loop"

FILES="/boot/header.img"

HOOKS="... encrypt2 lvm2 ... filesystems ..."

This is required so the LUKS header is available on boot allowing the decryption of the system, exempting us from a more complicated setup to mount another separate USB device in order to access the header. After this set up the initramfs is created.

Next the boot loader is configured to specify the cryptdevice= also passing the new header option for this setup:

cryptdevice=/dev/sdX:enc:header

To finish, following Dm-crypt/Encrypting an entire system#Post-installation is particularly useful with a /boot partition on an USB storage medium.

ヒント: You will notice that since the system partition only has "random" data, it does not have a partition table and by that an UUID or a name. But you can still have a consistent mapping using the disk id under /dev/disk/by-id/