Dm-verity uses a tree of sha256 hashes to verify blocks as they are read from a block device. Consequently, this ensures files have not changed between reboots or during runtime. This is useful for extending trust to the OS by mitigating zero days and unauthorized changes to root, as well as enforcing security policies, encryption and userspace security. Verity devices are regular block devices which can be accessed in
This article mainly describes setting up a verity-protected read-only root partition.
A dm-verity root setup setup consists of the following:
/root filesystem image or partition,
- verity hash tree
- the root hash of the verity tree
- verity kernel command line options,
- veritysetup (part of ),
- a unified kernel image which contains a stub EFI loader, kernel, initramfs, kernel command line, and microcode:
- Secure Boot.
The unified kernel image and Secure Boot are recommended but not required. Verity is intended to be used as one of the last steps in a boot process that protects the OS and the kernel from changes. It is easily defeated without Secure Boot and unified kernel images.
To enable dm-verity, you must have a working system already installed and configured. See Installation guide for the details.
Typically, it is necessary to have a separate partition or logical volume to store the verity hash data.
The recommended disk layout is similar to this:
- EFI system partition for the boot loader (
- XBOOTLDR partition (
LABEL=XBOOT)Note: Using systemd's XBOOTLDR partition type allows you to keep the boot loader and kernels separate. This can be useful for creating images used to install or update embedded systems and servers, but it requires you to use systemd-boot. It is recommended to also put the Type 1 boot loader entries into this partition.
- Root partition (
LABEL=OS, optionally with encryption, see Data-at-rest encryption).
- Verity partition (
LABEL=VERITY), should require 8-10% the size of Root
- Home (optional if you want write access for a user)
- Var (many programs will not run if
/varis not writable, so it should be separate from the root partition depending on use case)
- Note: Using file system labels instead of UUIDs simplifies deploying images on embedded devices. Two devices will have differing partition UUIDs making bundling cmdline into a UKI difficult.
/var should be writable filesystems. On a server that just has one purpose this may be optional, since e.g. a wireguard server needs no write access to the disk.
offers an attractive alternative to ext4 or squashfs on the root partition. EROFS, like squashfs, does not allow writes by design and has better performance in many cases than comparable filesystems on flash and solid-state media. It uses lz4 compression by default and was designed for Android phones by Huawei, which make extensive use of dm-verity.
Possible issues with boot and runtime
Any files that need to be written to during init or changed during runtime must be made writable by some method otherwise the program will not function as expected.
Many programs need write access to
etc. You can use a separate
/etc partition but this will make all of these configuration files writable. Create a folder
/var/etc and move the files that need write access into it than symlink into
etc, as with the example with NetworkManager below.
Some programs will expect these folders and files to still exist (even read-only) on the root filesystem for early init. For instance, systemd-journald will break if
/etc/machine-id does not exist or is a symlink. Bind mounts can be useful for this.
One way to find out which files will change when the system is running is to enable the
dracut-overlayroot module, use the system, and check the files in
/run/overlayroot/u to see what you may need to address. Any files in this folder were written to the tmpfs overlaid on top of root. Place the module into
overlayroot to the dracut modules list, and
overlayroot=1 to your kernel command line and regenerate the initramfs. The module can be found at https://github.com/TylerHelt0/dracut-overlayroot[dead link 2023-05-06 ⓘ].
Since the root filesystem will be mounted read-only and
/var should be mounted read-write in most cases, the path to the pacman database should be changed to
/usr/lib/pacman. This will ensure the rootfs always has the correct list of installed packages.
cp /var/lib/pacman /usr/lib
DBPath = /usr/lib/pacman
- To be able to sync lists and check updates, move
/var/lib/pacmanand symlink it.
- If you wish to be able to change mirrorlist without modifying the root file system, move it to
/var/etcand symlink it as well.
To setup connections with NetworkManager, you need write access to
/etc/NetworkManager/system-connections. Move the
system-connections folder to
/var/etc/NetworkManager/system-connections and symlink to it on the root filesystem.
# ln -sf /etc/NetworkManager/system-connections /var/etc/NetworkManager/system-connections
Setting up verity
- Boot from a live medium
- Mount your root filesystem as read-only
- Make sure all your changes are perfect
veritysetup format <root device> <verity device> | grep Root | cut -f2 >> roothash.txt
You will now have the rootfs, the verity hash tree, and the roothash. Alternatively you can save the hashes to a file by replacing the
<verity device> path and write it to the device later.
To test it you can use
veritysetup open <root device> root <verity device> $(cat roothash.txt). The verity device can be mounted from
Configuring the kernel command line
Add the following options to your kernel command line:
systemd.verity_root_data=PATH-TO-ROOT, e.g. LABEL=Root
systemd.verity_root_hash=PATH-TO-VERITY-PARTITION, e.g. LABEL=Verity
panic-on-corruption(the default behavior will just print an error to dmesg and will not prevent untrusted code from running)
If the roothash changes you must also edit the cmdline/rebuild the Unified Kernel Image with the new value. Failure to do so can result in an unbootable system.
Additional recommended options
roto prevent changes to root if not using erofs or squashfs
rd.emergency=rebootto prevents access to a shell if the root is corrupt
rd.shell=0to prevents access to a shell if boot fails
lsm=lockdownenables kernel lockdown mode, requires signed kernel modules
lockdown=confidentialityprevents users from accessing kernel memory
Devices other than root
The use of dm-verity is not limited to the root device. Other devices that need to be verified at boot can be put into
/etc/veritytab and will be assembled by
systemd-veritysetup@.service. See for more information.
Be aware that it is much easier to remount a non-root partition as RW while the system is running. Integrity violations also will not trigger a reboot. Even if
has verity enabled, it is trivial for a user with root privileges to disable verity on a non-root partition.
dm-verity does not provide an all-in-one solution but should be used alongside other methods of securing the system when the disk is removed and when the system is fully booted.
It is recommended to enable Secure Boot with custom keys after verity is setup.
Verity protection is useless if a virus or attacker can replace the
kernel.efi containing the embedded roothash which would allow any root filesystem to be booted. Signing the kernel image for Secure Boot will prevent the kernel image from being replaced and ensure integrity of the root filesystem as long as the firmware is secure.
AUR or can be used to maintain your Unified kernel images and keep your bootloader signed. AUR will also handle your kernel command line. can be used to create Secure Boot keys.
Unified Kernel Image
UKIs bundle together at minimum the linux kernel, an initramfs, CPU microcode, and a cmdline. The advantage to using an UKI is that it prevents changes to both the kernel, initramfs and cmdline when the UKI is signed and used with secureboot. If the cmdline section of the UKI is left blank, it can be supplied by a bootloader like systemd-boot. Otherwise it can only be changed by rebuilding and resigning a new UKI.
UKIs can be directly booted by UEFI if kernel efistub is enabled or if shim/preloader is used.
Signing kernel modules/DKMS
The default kernels ship with pre-signed native modules. If DKMS is used, one must create a custom kernel to enable signing and loading of DKMS modules when secureboot is enabled which activates lockdown mode. If you skip this step DKMS modules will refuse to load.
More information about signed kernel modules can be found here: Signed kernel modules
Although the verity root device will be tamper-resistant, it provides no confidentiality. It could be located on an unencrypted partition if it contains no secret data. If the kernel is protected by Secure Boot, it would be impossible to replace the data in the root or verity devices without replacing the kernel.
The verity root device can be used to unlock other encrypted devices. If done with keyfiles, the verity root should be encrypted. If using a TPM and systemd-cryptenroll to store keys, the verity root could be unencrypted.
A TPM 2.0 can be used to protect encryption keys for the LUKS device containing root. After Secure Boot is enabled, you can use
systemd-cryptenroll to bind keys to PCRs. Recommended PCRs are 0,1,5,7. This will stop decryption if the firmware, firmware options, GPT layout or secure boot state is changed, respectively.
The reason for binding on 0,1, and 5 is to ensure attackers cannot replace the motherboard firmware to disable secureboot and consequently disable verity.
You must pass this kernel option:
You may also need to add tpm2 support to your initramfs or include the module if using dracut. See Trusted Platform Module#systemd-cryptenroll for more information.
If you use systemd-boot as your bootloader, it will measure the
kernel.efi into PCR 4. This can be used to prevent decryption of root if the kernel image, initramfs, or kernel command line is changed.
Mandatory access control
During runtime, methods such as OverlayFS, tmpfs, and bind mounts can still be used to get write access on the folders within
root. For this reason, it is important to still harden the OS. Apparmor, SELinux and other access control mechanisms are useful for this.
A dm-verity read-only root should not be updated the traditional way with pacman. Verity is intended mostly for embedded devices and others which value code-integrity over the rolling release model. This has the primary benefits of extending trust to the OS and ensuring a device always boots the same way. E.g. an emulator box for normies or a secure web server. dm-verity, combined with other security methods like selinux, eliminate entire classes of zero-days and consequently the need to update is less frequent unless new features are desired. Think about a router running linux: many routers are compromised by malware without the user ever knowing. If the router was verity-protected in a secure way, it would prevent viruses from gaining persistence.
You could use ext4 for
/ which enables you to mount it rw. You can then do updates, rehash the filesystem, and change the roothash in the cmdline, but it would be better to release incrementally updated images of the filesystems. Disabling verity to do updates on a writable filesystem is as simple as omitting the systemd.verity=1 cmdline option.
A VM can be used to maintain a 'rolling' system and imaged when updates are needed. A chroot could work as well. Setup all the partitioning and boot logic the way it is expected to work on the target system, than reboot the VM into a live media and make images of the partitions. If you image the xboot partition, it can be flashed directly to a partition to update the UKI/kernel/initramfs/cmdline. If paired with an image for the root filesystem this is more or less a complete system update.
Systemd already has the logic to retrieve images from an update server (Or local dir) and flash them to partitions which may or may not already exist. It can also install and remove files into existing partitions.
A/B update scheme
Another way to handle updates would be to use a system similar to Android's A/B partition system. This entails having two sets of the root and verity partitions. When an update is necessary the active partition could be copied to the inactive one. The inactive partition could than be updated as normal from chroot or with pacman and 'sealed' with dm-verity. On next boot, the inactive partition becomes the active partition.
If using UKI the UKI must be updated with the root and verity partitions. At minimum the kernel cmdline must be updated with new roothash.
If the user wants a system that has optional persistence or can install packages which are reverted at reboot, an overlay can be mounted as root with the verity root as the lower dir. The upper dir could be a persistent block device or a tmpfs. If using A/B, one could remount
/ as a writable OverlayFS and use normal update methods, than copy the contents of the overlayfs into the inactive partition and rehash verity.
If the user requires temporary persistence (for example, the ability to install packages that are reset at boot),
systemd.volatile=overlay can be passed on the kernel command line.
Flatpak can be used to install and update apps within
home without write access to
/. Flatpak would be ideal to solve most user's needs for installing applications and updating them in a verity-protected desktop PC. Flatpak works on
/var by default.
Tips and tricks
The above steps can be automated with the packageAUR. It will build a squashfs rootfs and sign the roothash with the kernel and the initramfs. On boot, you can decide to boot a persistent system, where changes on the overlayfs are saved, or to boot a volatile system. It also keeps the last rootfs as a backup, so you can decide to boot the last working rootfs.