doc: add new markdown docs for credentials

[thirdparty/systemd.git] / docs / CREDENTIALS.md

---
title: Credentials
category: Concepts
layout: default
SPDX-License-Identifier: LGPL-2.1-or-later
---

# 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 invisble 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 (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](https://systemd.io/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](https://www.freedesktop.org/software/systemd/man/systemd.exec.html#Credentials).

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`:

```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 variabe, use the `%d` specifier in the `Environment=`
setting to build valid paths to specific credentials.

## Tools

The
[`systemd-creds`](https://www.freedesktop.org/software/systemd/man/systemd-creds.html)
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:

```sh
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:

```sh
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)`](https://www.freedesktop.org/software/systemd/man/systemd-nspawn.html#Credentials)'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](https://systemd.io/CONTAINER_INTERFACE)
   documentation.

2. Quite similar, 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. (This specific switch would set
   credential `foo` to `bar`.)

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`](https://www.freedesktop.org/software/systemd/man/systemd-stub.html)
   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 userpace 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 \
        -fw_cfg name=opt/io.systemd.credentials/mycred,string=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:

```sh
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:

* [`systemd-sysusers(8)`](https://www.freedesktop.org/software/systemd/man/systemd-sysusers.html)
  will look for the credentials `passwd.hashed-password.<username>`,
  `passwd.plaintext-password.<username>` and `passwd.shell.<username>` to
  configure the password (either in UNIX hashed form, or plaintext) or shell of
  system users created. Replace `<username>` with the system user of your
  choice, for example `root`.

* [`systemd-firstboot(1)`](https://www.freedesktop.org/software/systemd/man/systemd-firstboot.html)
  will look for the credentials `firstboot.locale`, `firstboot.locale-messages`,
  `firstboot.keymap`, `firstboot.timezone`, that configure locale, keymap or
  timezone settings in case the data is not yet set in `/etc/`.

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 `mysecret`and 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 \
        -fw_cfg name=opt/io.systemd.credentials/passwd.hashed-password.root,string=$(mkpasswd mysecret) \
        -fw_cfg name=opt/io.systemd.credentials/firstboot.locale,string=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.