System and Service Credentials

The systemd service manager supports a “credential” concept for securely acquiring and passing credential data to systems and services. The precise nature of the credential data is up to applications, but the concept is intended to provide systems and services with potentially security sensitive cryptographic keys, certificates, passwords, identity information and similar types of information. It may also be used as generic infrastructure for parameterizing systems and services.

Traditionally, data of this nature has often been provided to services via environment variables (which is problematic because by default they are inherited down the process tree, have size limitations, and issues with binary data) or simple, unencrypted files on disk. systemd’s system and service credentials are supposed to provide a better alternative for this purpose. Specifically, the following features are provided:

  1. Service credentials are acquired at the moment of service activation, and released on service deactivation. They are immutable during the service runtime.

  2. Service credentials are accessible to service code as regular files, the path to access them is derived from the environment variable $CREDENTIALS_DIRECTORY.

  3. Access to credentials is restricted to the service’s user. Unlike environment variables the credential data is not propagated down the process tree. Instead each time a credential is accessed an access check is enforced by the kernel. If the service is using file system namespacing the loaded credential data is invisible to any other services.

  4. Service credentials may be acquired from files on disk, specified as literal strings in unit files, acquired from another service dynamically via an AF_UNIX socket, or inherited from the system credentials the system itself received.

  5. Credentials may optionally be encrypted and authenticated, either with a key derived from a local TPM2 chip, or one stored in /var/, or both. This encryption is supposed to just work, and requires no manual setup. (That is besides first encrypting relevant credentials with one simple command, see below.)

  6. Service credentials are placed in non-swappable memory. (If permissions allow it, via ramfs.)

  7. Credentials may be acquired from a hosting VM hypervisor (SMBIOS OEM strings or qemu fw_cfg), a hosting container manager, the kernel command line, or from the UEFI environment and the EFI System Partition (via systemd-stub). Such system credentials may then be propagated into individual services as needed.

  8. Credentials are an effective way to pass parameters into services that run with RootImage= or RootDirectory= and thus cannot read these resources directly from the host directory tree. Specifically, Portable Services may be parameterized this way securely and robustly.

  9. Credentials can be binary and relatively large (though currently an overall size limit of 1M per service is enforced).

Configuring per-Service Credentials

Within unit files, there are four settings to configure service credentials.

  1. LoadCredential= may be used to load a credential from disk, from an AF_UNIX socket, or propagate them from a system credential.

  2. SetCredential= may be used to set a credential to a literal string encoded in the unit file. Because unit files are world-readable (both on disk and via D-Bus), this should only be used for credentials that aren’t sensitive, i.e. public keys/certificates – but not private keys.

  3. LoadCredentialEncrypted= is similar to LoadCredential= but will load an encrypted credential, and decrypt it before passing it to the service. For details on credential encryption, see below.

  4. SetCredentialEncrypted= is similar to SetCredential= but expects an encrypted credential to be specified literally. Unlike SetCredential= it is thus safe to be used even for sensitive information, because even though unit files are world readable, the ciphertext included in them cannot be decoded unless access to TPM2/encryption key is available.

Each credential configured with these options carries a short name (suitable for inclusion in a filename) in the unit file, under which the invoked service code can then retrieve it. Each name should only be specified once.

For details about these four settings see the man page.

It is a good idea to also enable mount namespacing for services that process credentials configured this way. If so, the runtime credential directory of the specific service is not visible to any other service. Use PrivateMounts= as minimal option to enable such namespacing. Note that many other sandboxing settings (e.g. ProtectSystem=, ReadOnlyPaths= and similar) imply PrivateMounts=, hence oftentimes it’s not necessary to set this option explicitly.

Programming Interface from Service Code

When a service is invoked with one or more credentials set it will have an environment variable $CREDENTIALS_DIRECTORY set. It contains an absolute path to a directory the credentials are placed in. In this directory for each configured credential one file is placed. In addition to the $CREDENTIALS_DIRECTORY environment variable passed to the service processes the %d specifier in unit files resolves to the service’s credential directory.

Example unit file:

…
[Service]
ExecStart=/usr/bin/myservice.sh
LoadCredential=foobar:/etc/myfoobarcredential.txt
Environment=FOOBARPATH=%d/foobar
…

Associated service shell script /usr/bin/myservice.sh:

#!/bin/sh

sha256sum $CREDENTIAL_PATH/foobar
sha256sum $FOOBARPATH

A service defined like this will get the contents of the file /etc/myfoobarcredential.txt passed as credential foobar, which is hence accessible under $CREDENTIALS_DIRECTORY/foobar. Since we additionally pass the path to it as environment variable $FOOBARPATH the credential is also accessible as the path in that environment variable. When invoked, the service will hence show the same SHA256 hash value of /etc/myfoobarcredential.txt twice.

In an ideal world, well-behaved service code would directly support credentials passed this way, i.e. look for $CREDENTIALS_DIRECTORY and load the credential data it needs from there. For daemons that do not support this but allow passing credentials via a path supplied over the command line use ${CREDENTIAL_PATH} in the ExecStart= command line to reference the credentials directory. For daemons that allow passing credentials via a path supplied as environment variable, use the %d specifier in the Environment= setting to build valid paths to specific credentials.

Tools

The systemd-creds tool is provided to work with system and service credentials. It may be used to access and enumerate system and service credentials, or to encrypt/decrypt credentials (for details about the latter, see below).

When invoked from service context, systemd-creds passed without further parameters will list passed credentials. The systemd-creds cat xyz command may be used to write the contents of credential xyz to standard output. If these calls are combined with the --system switch credentials passed to the system as a whole are shown, instead of the those passed to the service the command is invoked from.

Example use:

systemd-run -P --wait -p LoadCredential=abc:/etc/hosts systemd-creds cat abc

This will invoke a transient service with a credential abc sourced from the system’s /etc/hosts file. This credential is then written to standard output via systemd-creds cat.

Encryption

Credentials are supposed to be useful for carrying sensitive information, such as cryptographic key material. For this kind of data (symmetric) encryption and authentication is provided to make storage of the data at rest safer. The data may be encrypted and authenticated with AES256-GCM. The encryption key can either be one derived from the local TPM2 device, or one stored in /var/lib/systemd/credential.secret, or a combination of both. If a TPM2 device is available and /var/ resides on persistent storage the default behaviour is to use the combination of both for encryption, thus ensuring that credentials protected this way can only be decrypted and validated on the local hardware and OS installation. Encrypted credentials stored on disk thus cannot be decrypted without access to the TPM2 chip and the aforementioned key file /var/lib/systemd/credential.secret. Moreover, credentials cannot be prepared on another machine than the local one.

The systemd-creds tool provides the commands encrypt and decrypt to encrypt and decrypt/authenticate credentials. Example:

systemd-creds encrypt plaintext.txt ciphertext.cred
shred -u plaintext.txt
systemd-run -P --wait -p LoadCredentialEncrypted=foobar:$(pwd)/ciphertext.cred systemd-creds cat foobar

This will first create an encrypted copy of the file plaintext.txt in the encrypted credential file ciphertext.cred. It then securely removes the source file. It then runs a transient service, that reads the encrypted file and passes it as decrypted credential foobar to the invoked service binary (which here is the systemd-creds tool, which just writes the data it received to standard output).

Instead of storing the encrypted credential as a separate file on disk, it can also be embedded in the unit file. Example:

systemd-creds encrypt -p --name=foobar plaintext.txt -

This will output a SetCredentialEncrypted= line that can directly be used in a unit file. e.g.:

…
[Service]
ExecStart=/usr/bin/systemd-creds cat foobar
SetCredentialEncrypted=foobar: \
        k6iUCUh0RJCQyvL8k8q1UyAAAAABAAAADAAAABAAAAC1lFmbWAqWZ8dCCQkAAAAAgAAAA \
        AAAAAALACMA0AAAACAAAAAAfgAg9uNpGmj8LL2nHE0ixcycvM3XkpOCaf+9rwGscwmqRJ \
        cAEO24kB08FMtd/hfkZBX8PqoHd/yPTzRxJQBoBsvo9VqolKdy9Wkvih0HQnQ6NkTKEdP \
        HQ08+x8sv5sr+Mkv4ubp3YT1Jvv7CIPCbNhFtag1n5y9J7bTOKt2SQwBOAAgACwAAABIA \
        ID8H3RbsT7rIBH02CIgm/Gv1ukSXO3DMHmVQkDG0wEciABAAII6LvrmL60uEZcp5qnEkx \
        SuhUjsDoXrJs0rfSWX4QAx5PwfdFuxPusgEfTYIiCb8a/W6RJc7cMweZVCQMbTARyIAAA \
        AAJt7Q9F/Gz0pBv1Lc4Dpn1WpebyBBm+vQ5N/lSKW2XSm8cONwCopxpDc7wJjXg7OTR6r \
        xGCpIvGXLt3ibwJl81woLya2RRjIvc/R2zNm/yWzZAjiOLPih4SuHthqiX98ey8PUmZJB \
        VGXglCZFjBx+d7eCqTIdghtp5pkDGwMJT6pjw4FfyFK2nJPawFKPAqzw9DK2iYttFeXi5 \
        19xCfLBH9NKS/idlYXrhp+XIEtsr26s4lx5y10Goyc3qDOR3RD2cuZj0gHwV35hhhhcCz \
        JaYytef1X/YL+7fYH5kuE4rxSksoUuA/LhtjszBeGbcbIT+O8SuvBJHLKTSHxPL8FTyk3 \
        L4FSkEHs0rYwUIkKmnGohDdsYrMJ2fjH3yDNBP16aD1+f/Nuh75cjhUnGsDLt9K4hGg== \
…

Inheritance from Container Managers, Hypervisors, Kernel Command Line, or the UEFI Boot Environment

Sometimes it is useful to parameterize whole systems the same way as services, via systemd credentials. In particular, it might make sense to boot a system with a set of credentials that are then propagated to individual services where they are ultimately consumed.

systemd supports four ways to pass credentials to systems:

  1. A container manager may set the $CREDENTIALS_DIRECTORY environment variable for systemd running as PID 1 in the container, the same way as systemd would set it for a service it invokes. systemd-nspawn(1)’s --set-credential= and --load-credential= switches implement this, in order to pass arbitrary credentials from host to container payload. Also see the Container Interface documentation.

  2. Quite similar, VMs can be passed credentials via SMBIOS OEM strings (example qemu command line switch -smbios type=11,value=io.systemd.credential:foo=bar or -smbios type=11,value=io.systemd.credential.binary:foo=YmFyCg==, the latter taking a Base64 encoded argument to permit binary credentials being passed in). Alternatively, qemu VMs can be invoked with -fw_cfg name=opt/io.systemd.credentials/foo,string=bar to pass credentials from host through the hypervisor into the VM via qemu’s fw_cfg mechanism. (All three of these specific switches would set credential foo to bar.) Passing credentials via the SMBIOS mechanism is typically preferable over fw_cfg since it is faster and less specific to the chosen VMM implementation. Moreover, fw_cfg has a 55 character limitation on names passed that way. So some settings may not fit.

  3. Credentials can also be passed into a system via the kernel command line, via the systemd.set-credential= kernel command line option. Note though that any data specified here is visible to any userspace application via /proc/cmdline. This is hence typically not useful to pass sensitive information.

  4. Credentials may also be passed from the UEFI environment to userspace, if the systemd-stub UEFI kernel stub is used. This allows placing encrypted credentials in the EFI System Partition, which are then picked up by systemd-stub and passed to the kernel and ultimately userspace where systemd receives them. This is useful to implement secure parameterization of vendor-built and signed initial RAM disks, as userspace can place credentials next to these EFI kernels, and be sure they can be accessed securely from initrd context.

Credentials passed to the system may be enumerated/displayed via systemd-creds --system. They may also be propagated down to services, via the LoadCredential= setting. Example:

systemd-nspawn --set-credential=mycred:supersecret -i test.raw -b

or

qemu-system-x86_64 \
        -machine type=q35,accel=kvm,smm=on \
        -smp 2 \
        -m 1G \
        -cpu host \
        -nographic \
        -nodefaults \
        -serial mon:stdio \
        -drive if=none,id=hd,file=test.raw,format=raw \
        -device virtio-scsi-pci,id=scsi \
        -device scsi-hd,drive=hd,bootindex=1 \
        -smbios type=11,value=io.systemd.credential:mycred=supersecret

Either of these lines will boot a disk image test.raw, once as container via systemd-nspawn, and once as VM via qemu. In each case the credential mycred is set to supersecret.

Inside of the system invoked that way the credential may then be viewed:

systemd-creds --system cat mycred

Or propagated to services further down:

systemd-run -p LoadCredential=mycred -P --wait systemd-creds cat mycred

Well-Known Credentials

Various services shipped with systemd consume credentials for tweaking behaviour:

In future more services are likely to gain support for consuming credentials.

Example:

systemd-nspawn -i test.raw  \
        --set-credential=passwd.hashed-password.root:$(mkpasswd mysecret) \
        --set-credential=firstboot.locale:C.UTF-8 \
        -b

This boots the specified disk image as systemd-nspawn container, and passes the root password mysecretand default locale C.UTF-8 to use to it. This data is then propagated by default to systemd-sysusers.service and systemd-firstboot.service, where it is applied. (Note that these services will only do so if these settings in /etc/ are so far unset, i.e. they only have an effect on unprovisioned systems, and will never override data already established in /etc/.) A similar line for qemu is:

qemu-system-x86_64 \
        -machine type=q35,accel=kvm,smm=on \
        -smp 2 \
        -m 1G \
        -cpu host \
        -nographic \
        -nodefaults \
        -serial mon:stdio \
        -drive if=none,id=hd,file=test.raw,format=raw \
        -device virtio-scsi-pci,id=scsi \
        -device scsi-hd,drive=hd,bootindex=1 \
        -smbios type=11,value=io.systemd.credential:passwd.hashed-password.root=$(mkpasswd mysecret) \
        -smbios type=11,value=io.systemd.credential:firstboot.locale=C.UTF-8

Relevant Paths

From service perspective the runtime path to find loaded credentials in is provided in the $CREDENTIALS_DIRECTORY environment variable.

At runtime, credentials passed to the system are placed in /run/credentials/@system/ (for regular credentials, such as those passed from a container manager or via qemu) and /run/credentials/@encrypted/ (for credentials that must be decrypted/validated before use, such as those from systemd-stub).

The LoadCredential= and LoadCredentialEncrypted= settings when configured with a relative source path will search for the source file to read the credential from automatically. Primarily, these credentials are searched among the credentials passed into the system. If not found there, they are searched in /etc/credstore/, /run/credstore/, /usr/lib/credstore/. LoadCredentialEncrypted= will also search /etc/credstore.encrypted/ and similar directories. These directories are hence a great place to store credentials to load on the system.

Conditionalizing Services

Sometimes it makes sense to conditionalize system services and invoke them only if the right system credential is passed to the system. use the ConditionCredential= and AssertCredential= unit file settings for that.