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diff --git a/docs/CREDENTIALS.md b/docs/CREDENTIALS.md new file mode 100644 index 0000000..c0a4dfa --- /dev/null +++ b/docs/CREDENTIALS.md @@ -0,0 +1,430 @@ +--- +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 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](PORTABLE_SERVICES.md) 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, + e.g. public keys or 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 $CREDENTIALS_DIRECTORY/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 +`${CREDENTIALS_DIRECTORY}` 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`](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](CONTAINER_INTERFACE.md) + 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`](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 userspace where systemd receives them. This is + useful to implement secure parameterization of vendor-built and signed + initrds, 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: + +```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/`. + +* [`tmpfiles.d(5)`](https://www.freedesktop.org/software/systemd/man/tmpfiles.d.html) + will look for the credentials `tmpfiles.extra` with arbitrary tmpfiles.d lines. + Can be encoded in base64 to allow easily passing it on the command line. + +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 \ + -smbios type=11,value=io.systemd.credential:passwd.hashed-password.root=$(mkpasswd mysecret) \ + -smbios type=11,value=io.systemd.credential:firstboot.locale=C.UTF-8 +``` + +This boots the specified disk image via qemu, provisioning public key SSH access +for the root user from the caller's key: + +``` +qemu-system-x86_64 \ + -machine type=q35,accel=kvm,smm=on \ + -smp 2 \ + -m 1G \ + -cpu host \ + -nographic \ + -nodefaults \ + -serial mon:stdio \ + -drive if=none,id=hd,file=test.raw,format=raw \ + -device virtio-scsi-pci,id=scsi \ + -device scsi-hd,drive=hd,bootindex=1 \ + -smbios type=11,value=io.systemd.credential.binary:tmpfiles.extra=$(echo "f~ /root/.ssh/authorized_keys 600 root root - $(ssh-add -L | base64 -w 0)" | base64 -w 0) +``` +## Relevant Paths + +From *service* perspective the runtime path to find loaded credentials in is +provided in the `$CREDENTIALS_DIRECTORY` environment variable. For *system +services* the credential directory will be `/run/credentials/<unit name>`, but +hardcoding this path is discouraged, because it does not work for *user +services*. Packagers and system administrators may hardcode the credential path +as a last resort for software that does not yet search for credentials relative +to `$CREDENTIALS_DIRECTORY`. + +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. |