Encrypting More: /boot Joins The Party

Typically when installing major linux distros they make it easy to select encryption as an option to have encrypted block devices. This is great! The not so great part is the linux kernel and the initial ramdisk aren't typically invited to the party; they are left sitting in a separate and unencrypted /boot partition. Historically it has been necessary to leave /boot unencrypted because bootloaders didn't support decrypting block devices. However, there are some dangers to leaving the bootloader and ramdisks unencrypted (see this post).

Newer versions of GRUB do support booting from encrypted block devices (a reference here). This means that we can theoretically boot from a device that is encrypted. And the theory is right!

While the installers don't make it easy to actually install in this setup (without a separate boot partition) it is actually pretty easy to convert an existing system to use this setup. I'll step through doing this on a Fedora 22 system (I have done this one Fedora 21 in the past).

The typical disk configuration (with crypto selected) from a vanilla install of Fedora 22 looks like this:

[root@localhost ~]# lsblk -i -o NAME,TYPE,MOUNTPOINT
NAME                                          TYPE  MOUNTPOINT
sda                                           disk
|-sda1                                        part  /boot
`-sda2                                        part
  `-luks-cb85c654-7561-48a3-9806-f8bbceaf3973 crypt
    |-fedora-swap                             lvm   [SWAP]
    `-fedora-root                             lvm   /

What we need to do is copy the files from the /boot partition and into the /boot directory on the root filesystem. We can do this easily with a bind mount like so:

[root@localhost ~]# mount --bind / /mnt/
[root@localhost ~]# cp -a /boot/* /mnt/boot/
[root@localhost ~]# cp -a /boot/.vmlinuz-* /mnt/boot/
[root@localhost ~]# diff -ur /boot/ /mnt/boot/
[root@localhost ~]# umount /mnt

This copied the files over and verified the contents matched. The final step is to unmount the partition and to remove the mount from /etc/fstab. Since we'll no longer be using that partition we don't want kernel updates to be written to the wrong place:

[root@localhost ~]# umount /boot
[root@localhost ~]# sed -i -e '/\/boot/d' /etc/fstab

The next step is to write out a new grub.cfg that loads the appropriate modules for loading from the encrypted disk:

[root@localhost ~]# cp /boot/grub2/grub.cfg /boot/grub2/grub.cfg.backup
[root@localhost ~]# grub2-mkconfig > /boot/grub2/grub.cfg
Generating grub configuration file ...
Found linux image: /boot/vmlinuz-4.0.4-301.fc22.x86_64
Found initrd image: /boot/initramfs-4.0.4-301.fc22.x86_64.img
Found linux image: /boot/vmlinuz-0-rescue-3f9d22f02d854d9a857066570127584a
Found initrd image: /boot/initramfs-0-rescue-3f9d22f02d854d9a857066570127584a.img
done
[root@localhost ~]# cat /boot/grub2/grub.cfg | grep cryptodisk
        insmod cryptodisk
        insmod cryptodisk

And finally we need to reinstall the GRUB bootloader with GRUB_ENABLE_CRYPTODISK=y set in /etc/default/grub:

[root@localhost ~]# echo GRUB_ENABLE_CRYPTODISK=y >> /etc/default/grub
[root@localhost ~]# cat /etc/default/grub
GRUB_TIMEOUT=5
GRUB_DISTRIBUTOR="$(sed 's, release .*$,,g' /etc/system-release)"
GRUB_DEFAULT=saved
GRUB_DISABLE_SUBMENU=true
GRUB_TERMINAL_OUTPUT="console"
GRUB_CMDLINE_LINUX="rd.lvm.lv=fedora/swap rd.lvm.lv=fedora/root rd.luks.uuid=luks-cb85c654-7561-48a3-9806-f8bbceaf3973 rhgb quiet"
GRUB_DISABLE_RECOVERY="true"
GRUB_ENABLE_CRYPTODISK=y
[root@localhost ~]# grub2-install /dev/sda
Installing for i386-pc platform.
Installation finished. No error reported.

After a reboot you now get your grub prompt:

http://dustymabe.com/content/2015-07-06/grubdecrypt.png

Unfortunately this does mean that you have to type your password twice on boot but at least your system is more encrypted than it was before. This may not completely get rid of the attack vector described in this post as there is still part of the bootloader that isn't encrypted, but at least the grub stage2 and the kernel/ramdisk are encrypted and should make it much harder to attack.

Happy Encrypting!

Dusty