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diff --git a/docs/CONTAINER_INTERFACE.md b/docs/CONTAINER_INTERFACE.md new file mode 100644 index 0000000..7fa8558 --- /dev/null +++ b/docs/CONTAINER_INTERFACE.md @@ -0,0 +1,397 @@ +--- +title: Container Interface +category: Interfaces +layout: default +SPDX-License-Identifier: LGPL-2.1-or-later +--- + +# The Container Interface + +Also consult [Writing Virtual Machine or Container +Managers](https://www.freedesktop.org/wiki/Software/systemd/writing-vm-managers). + +systemd has a number of interfaces for interacting with container managers, +when systemd is used inside of an OS container. If you work on a container +manager, please consider supporting the following interfaces. + +## Execution Environment + +1. If the container manager wants to control the hostname for a container + running systemd it may just set it before invoking systemd, and systemd will + leave it unmodified when there is no hostname configured in `/etc/hostname` + (that file overrides whatever is pre-initialized by the container manager). + +2. Make sure to pre-mount `/proc/`, `/sys/`, and `/sys/fs/selinux/` before + invoking systemd, and mount `/sys/`, `/sys/fs/selinux/` and `/proc/sys/` + read-only (the latter via e.g. a read-only bind mount on itself) in order + to prevent the container from altering the host kernel's configuration + settings. (As a special exception, if your container has network namespaces + enabled, feel free to make `/proc/sys/net/` writable. If it also has user, ipc, + uts and pid namespaces enabled, the entire `/proc/sys` can be left writable). + systemd and various other subsystems (such as the SELinux userspace) have + been modified to behave accordingly when these file systems are read-only. + (It's OK to mount `/sys/` as `tmpfs` btw, and only mount a subset of its + sub-trees from the real `sysfs` to hide `/sys/firmware/`, `/sys/kernel/` and + so on. If you do that, make sure to mark `/sys/` read-only, as that + condition is what systemd looks for, and is what is considered to be the API + in this context.) + +3. Pre-mount `/dev/` as (container private) `tmpfs` for the container and bind + mount some suitable TTY to `/dev/console`. If this is a pty, make sure to + not close the controlling pty during systemd's lifetime. PID 1 will close + ttys, to avoid being killed by SAK. It only opens ttys for the time it + actually needs to print something. Also, make sure to create device nodes + for `/dev/null`, `/dev/zero`, `/dev/full`, `/dev/random`, `/dev/urandom`, + `/dev/tty`, `/dev/ptmx` in `/dev/`. It is not necessary to create `/dev/fd` + or `/dev/stdout`, as systemd will do that on its own. Make sure to set up a + `BPF_PROG_TYPE_CGROUP_DEVICE` BPF program — on cgroupv2 — or the `devices` + cgroup controller — on cgroupv1 — so that no other devices but these may be + created in the container. Note that many systemd services use + `PrivateDevices=`, which means that systemd will set up a private `/dev/` + for them for which it needs to be able to create these device nodes. + Dropping `CAP_MKNOD` for containers is hence generally not advisable, but + see below. + +4. `systemd-udevd` is not available in containers (and refuses to start), and + hence device dependencies are unavailable. The `systemd-udevd` unit files + will check for `/sys/` being read-only, as an indication whether device + management can work. Therefore make sure to mount `/sys/` read-only in the + container (see above). Various clients of `systemd-udevd` also check the + read-only state of `/sys/`, including PID 1 itself and `systemd-networkd`. + +5. If systemd detects it is run in a container it will spawn a single shell on + `/dev/console`, and not care about VTs or multiple gettys on VTs. (But see + `$container_ttys` below.) + +6. Either pre-mount all cgroup hierarchies in full into the container, or leave + that to systemd which will do so if they are missing. Note that it is + explicitly *not* OK to just mount a sub-hierarchy into the container as that + is incompatible with `/proc/$PID/cgroup` (which lists full paths). Also the + root-level cgroup directories tend to be quite different from inner + directories, and that distinction matters. It is OK however, to mount the + "upper" parts read-only of the hierarchies, and only allow write-access to + the cgroup sub-tree the container runs in. It's also a good idea to mount + all controller hierarchies with exception of `name=systemd` fully read-only + (this only applies to cgroupv1, of course), to protect the controllers from + alteration from inside the containers. Or to turn this around: only the + cgroup sub-tree of the container itself (on cgroupv2 in the unified + hierarchy, and on cgroupv1 in the `name=systemd` hierarchy) may be writable + to the container. + +7. Create the control group root of your container by either running your + container as a service (in case you have one container manager instance per + container instance) or creating one scope unit for each container instance + via systemd's transient unit API (in case you have one container manager + that manages all instances. Either way, make sure to set `Delegate=yes` in + it. This ensures that the unit you created will be part of all cgroup + controllers (or at least the ones systemd understands). The latter may also + be done via `systemd-machined`'s `CreateMachine()` API. Make sure to use the + cgroup path systemd put your process in for all operations of the container. + Do not add new cgroup directories to the top of the tree. This will not only + confuse systemd and the admin, but also prevent your implementation from + being "stackable". + +## Environment Variables + +1. To allow systemd (and other programs) to identify that it is executed within + a container, please set the `$container` environment variable for PID 1 in + the container to a short lowercase string identifying your + implementation. With this in place the `ConditionVirtualization=` setting in + unit files will work properly. Example: `container=lxc-libvirt` + +2. systemd has special support for allowing container managers to initialize + the UUID for `/etc/machine-id` to some manager supplied value. This is only + enabled if `/etc/machine-id` is empty (i.e. not yet set) at boot time of the + container. The container manager should set `$container_uuid` as environment + variable for the container's PID 1 to the container UUID. (This is similar + to the effect of `qemu`'s `-uuid` switch). Note that you should pass only a + UUID here that is actually unique (i.e. only one running container should + have a specific UUID), and gets changed when a container gets duplicated. + Also note that systemd will try to persistently store the UUID in + `/etc/machine-id` (if writable) when this option is used, hence you should + always pass the same UUID here. Keeping the externally used UUID for a + container and the internal one in sync is hopefully useful to minimize + surprise for the administrator. + +3. systemd can automatically spawn login gettys on additional ptys. A container + manager can set the `$container_ttys` environment variable for the + container's PID 1 to tell it on which ptys to spawn gettys. The variable + should take a space separated list of pty names, without the leading `/dev/` + prefix, but with the `pts/` prefix included. Note that despite the + variable's name you may only specify ptys, and not other types of ttys. Also + you need to specify the pty itself, a symlink will not suffice. This is + implemented in + [systemd-getty-generator(8)](https://www.freedesktop.org/software/systemd/man/systemd-getty-generator.html). + Note that this variable should not include the pty that `/dev/console` maps + to if it maps to one (see below). Example: if the container receives + `container_ttys=pts/7 pts/8 pts/14` it will spawn three additional login + gettys on ptys 7, 8, and 14. + +4. To allow applications to detect the OS version and other metadata of the host + running the container manager, if this is considered desirable, please parse + the host's `/etc/os-release` and set a `$container_host_<key>=<VALUE>` + environment variable for the ID fields described by the [os-release + interface](https://www.freedesktop.org/software/systemd/man/os-release.html), eg: + `$container_host_id=debian` + `$container_host_build_id=2020-06-15` + `$container_host_variant_id=server` + `$container_host_version_id=10` + +5. systemd supports passing immutable binary data blobs with limited size and + restricted access to services via the `ImportCredential=`, `LoadCredential=` + and `SetCredential=` settings. The same protocol may be used to pass credentials + from the container manager to systemd itself. The credential data should be + placed in some location (ideally a read-only and non-swappable file system, + like 'ramfs'), and the absolute path to this directory exported in the + `$CREDENTIALS_DIRECTORY` environment variable. If the container managers + does this, the credentials passed to the service manager can be propagated + to services via `LoadCredential=` or `ImportCredential=` (see ...). The + container manager can choose any path, but `/run/host/credentials` is + recommended. + +## Advanced Integration + +1. Consider syncing `/etc/localtime` from the host file system into the + container. Make it a relative symlink to the containers's zoneinfo dir, as + usual. Tools rely on being able to determine the timezone setting from the + symlink value, and making it relative looks nice even if people list the + container's `/etc/` from the host. + +2. Make the container journal available in the host, by automatically + symlinking the container journal directory into the host journal directory. + More precisely, link `/var/log/journal/<container-machine-id>` of the + container into the same dir of the host. Administrators can then + automatically browse all container journals (correctly interleaved) by + issuing `journalctl -m`. The container machine ID can be determined from + `/etc/machine-id` in the container. + +3. If the container manager wants to cleanly shutdown the container, it might + be a good idea to send `SIGRTMIN+3` to its init process. systemd will then + do a clean shutdown. Note however, that since only systemd understands + `SIGRTMIN+3` like this, this might confuse other init systems. + +4. To support [Socket Activated + Containers](https://0pointer.de/blog/projects/socket-activated-containers.html) + the container manager should be capable of being run as a systemd + service. It will then receive the sockets starting with FD 3, the number of + passed FDs in `$LISTEN_FDS` and its PID as `$LISTEN_PID`. It should take + these and pass them on to the container's init process, also setting + $LISTEN_FDS and `$LISTEN_PID` (basically, it can just leave the FDs and + `$LISTEN_FDS` untouched, but it needs to adjust `$LISTEN_PID` to the + container init process). That's all that's necessary to make socket + activation work. The protocol to hand sockets from systemd to services is + hence the same as from the container manager to the container systemd. For + further details see the explanations of + [sd_listen_fds(1)](https://0pointer.de/public/systemd-man/sd_listen_fds.html) + and the [blog story for service + developers](https://0pointer.de/blog/projects/socket-activation.html). + +5. Container managers should stay away from the cgroup hierarchy outside of the + unit they created for their container. That's private property of systemd, + and no other code should modify it. + +6. systemd running inside the container can report when boot-up is complete + using the usual `sd_notify()` protocol that is also used when a service + wants to tell the service manager about readiness. A container manager can + set the `$NOTIFY_SOCKET` environment variable to a suitable socket path to + make use of this functionality. (Also see information about + `/run/host/notify` below.) + +## Networking + +1. Inside of a container, if a `veth` link is named `host0`, `systemd-networkd` + running inside of the container will by default run DHCPv4, DHCPv6, and + IPv4LL clients on it. It is thus recommended that container managers that + add a `veth` link to a container name it `host0`, to get an automatically + configured network, with no manual setup. + +2. Outside of a container, if a `veth` link is prefixed "ve-", `systemd-networkd` + will by default run DHCPv4 and DHCPv6 servers on it, as well as IPv4LL. It + is thus recommended that container managers that add a `veth` link to a + container name the external side `ve-` + the container name. + +3. It is recommended to configure stable MAC addresses for container `veth` + devices, for example, hashed out of the container names. That way it is more + likely that DHCP and IPv4LL will acquire stable addresses. + +## The `/run/host/` Hierarchy + +Container managers may place certain resources the manager wants to provide to +the container payload below the `/run/host/` hierarchy. This hierarchy should +be mostly immutable (possibly some subdirs might be writable, but the top-level +hierarchy — and probably most subdirs should be read-only to the +container). Note that this hierarchy is used by various container managers, and +care should be taken to avoid naming conflicts. `systemd` (and in particular +`systemd-nspawn`) use the hierarchy for the following resources: + +1. The `/run/host/incoming/` directory mount point is configured for `MS_SLAVE` + mount propagation with the host, and is used as intermediary location for + mounts to establish in the container, for the implementation of `machinectl + bind`. Container payload should usually not directly interact with this + directory: it's used by code outside the container to insert mounts inside + it only, and is mostly an internal vehicle to achieve this. Other container + managers that want to implement similar functionality might consider using + the same directory. + +2. The `/run/host/inaccessible/` directory may be set up by the container + manager to include six file nodes: `reg`, `dir`, `fifo`, `sock`, `chr`, + `blk`. These nodes correspond with the six types of file nodes Linux knows + (with the exceptions of symlinks). Each node should be of the specific type + and have an all zero access mode, i.e. be inaccessible. The two device node + types should have major and minor of zero (which are unallocated devices on + Linux). These nodes are used as mount source for implementing the + `InaccessiblePath=` setting of unit files, i.e. file nodes to mask this way + are overmounted with these "inaccessible" inodes, guaranteeing that the file + node type does not change this way but the nodes still become + inaccessible. Note that systemd when run as PID 1 in the container payload + will create these nodes on its own if not passed in by the container + manager. However, in that case it likely lacks the privileges to create the + character and block devices nodes (there are fallbacks for this case). + +3. The `/run/host/notify` path is a good choice to place the `sd_notify()` + socket in, that may be used for the container's PID 1 to report to the + container manager when boot-up is complete. The path used for this doesn't + matter much as it is communicated via the `$NOTIFY_SOCKET` environment + variable, following the usual protocol for this, however it's suitable, and + recommended place for this socket in case ready notification is desired. + +4. The `/run/host/os-release` file contains the `/etc/os-release` file of the + host, i.e. may be used by the container payload to gather limited + information about the host environment, on top of what `uname -a` reports. + +5. The `/run/host/container-manager` file may be used to pass the same + information as the `$container` environment variable (see above), i.e. a + short string identifying the container manager implementation. This file + should be newline terminated. Passing this information via this file has the + benefit that payload code can easily access it, even when running + unprivileged without access to the container PID 1's environment block. + +6. The `/run/host/container-uuid` file may be used to pass the same information + as the `$container_uuid` environment variable (see above). This file should + be newline terminated. + +7. The `/run/host/credentials/` directory is a good place to pass credentials + into the container, using the `$CREDENTIALS_DIRECTORY` protocol, see above. + +## What You Shouldn't Do + +1. Do not drop `CAP_MKNOD` from the container. `PrivateDevices=` is a commonly + used service setting that provides a service with its own, private, minimal + version of `/dev/`. To set this up systemd in the container needs this + capability. If you take away the capability, then all services that set this + flag will cease to work. Use `BPF_PROG_TYPE_CGROUP_DEVICE` BPF programs — on + cgroupv2 — or the `devices` controller — on cgroupv1 — to restrict what + device nodes the container can create instead of taking away the capability + wholesale. (Also see the section about fully unprivileged containers below.) + +2. Do not drop `CAP_SYS_ADMIN` from the container. A number of the most + commonly used file system namespacing related settings, such as + `PrivateDevices=`, `ProtectHome=`, `ProtectSystem=`, `MountFlags=`, + `PrivateTmp=`, `ReadWriteDirectories=`, `ReadOnlyDirectories=`, + `InaccessibleDirectories=`, and `MountFlags=` need to be able to open new + mount namespaces and the mount certain file systems into them. You break all + services that make use of these options if you drop the capability. Also + note that logind mounts `XDG_RUNTIME_DIR` as `tmpfs` for all logged in users + and that won't work either if you take away the capability. (Also see + section about fully unprivileged containers below.) + +3. Do not cross-link `/dev/kmsg` with `/dev/console`. They are different things, + you cannot link them to each other. + +4. Do not pretend that the real VTs are available in the container. The VT + subsystem consists of all the devices `/dev/tty[0-9]*`, `/dev/vcs*`, + `/dev/vcsa*` plus their `sysfs` counterparts. They speak specific `ioctl()`s + and understand specific escape sequences, that other ptys don't understand. + Hence, it is explicitly not OK to mount a pty to `/dev/tty1`, `/dev/tty2`, + `/dev/tty3`. This is explicitly not supported. + +5. Don't pretend that passing arbitrary devices to containers could really work + well. For example, do not pass device nodes for block devices to the + container. Device access (with the exception of network devices) is not + virtualized on Linux. Enumeration and probing of meta information from + `/sys/` and elsewhere is not possible to do correctly in a container. Simply + adding a specific device node to a container's `/dev/` is *not* *enough* to + do the job, as `systemd-udevd` and suchlike are not available at all, and no + devices will appear available or enumerable, inside the container. + +6. Don't mount only a sub-tree of the `cgroupfs` into the container. This will not + work as `/proc/$PID/cgroup` lists full paths and cannot be matched up with + the actual `cgroupfs` tree visible, then. (You may "prune" some branches + though, see above.) + +7. Do not make `/sys/` writable in the container. If you do, + `systemd-udevd.service` is started to manage your devices — inside the + container, but that will cause conflicts and errors given that the Linux + device model is not virtualized for containers on Linux and thus the + containers and the host would try to manage the same devices, fighting for + ownership. Multiple other subsystems of systemd similarly test for `/sys/` + being writable to decide whether to use `systemd-udevd` or assume that + device management is properly available on the instance. Among them + `systemd-networkd` and `systemd-logind`. The conditionalization on the + read-only state of `/sys/` enables a nice automatism: as soon as `/sys/` and + the Linux device model are changed to be virtualized properly the container + payload can make use of that, simply by marking `/sys/` writable. (Note that + as special exception, the devices in `/sys/class/net/` are virtualized + already, if network namespacing is used. Thus it is OK to mount the relevant + sub-directories of `/sys/` writable, but make sure to leave the root of + `/sys/` read-only.) + +8. Do not pass the `CAP_AUDIT_CONTROL`, `CAP_AUDIT_READ`, `CAP_AUDIT_WRITE` + capabilities to the container, in particular not to those making use of user + namespaces. The kernel's audit subsystem is still not virtualized for + containers, and passing these credentials is pointless hence, given the + actual attempt to make use of the audit subsystem will fail. Note that + systemd's audit support is partially conditioned on these capabilities, thus + by dropping them you ensure that you get an entirely clean boot, as systemd + will make no attempt to use it. If you pass the capabilities to the payload + systemd will assume that audit is available and works, and some components + will subsequently fail in various ways. Note that once the kernel learnt + native support for container-virtualized audit, adding the capability to the + container description will automatically make the container payload use it. + +## Fully Unprivileged Container Payload + +First things first, to make this clear: Linux containers are not a security +technology right now. There are more holes in the model than in swiss cheese. + +For example: if you do not use user namespacing, and share root and other users +between container and host, the `struct user` structures will be shared between +host and container, and hence `RLIMIT_NPROC` and so of the container users +affect the host and other containers, and vice versa. This is a major security +hole, and actually is a real-life problem: since Avahi sets `RLIMIT_NPROC` of +its user to 2 (to effectively disallow `fork()`ing) you cannot run more than +one Avahi instance on the entire system... + +People have been asking to be able to run systemd without `CAP_SYS_ADMIN` and +`CAP_SYS_MKNOD` in the container. This is now supported to some level in +systemd, but we recommend against it (see above). If `CAP_SYS_ADMIN` and +`CAP_SYS_MKNOD` are missing from the container systemd will now gracefully turn +off `PrivateTmp=`, `PrivateNetwork=`, `ProtectHome=`, `ProtectSystem=` and +others, because those capabilities are required to implement these options. The +services using these settings (which include many of systemd's own) will hence +run in a different, less secure environment when the capabilities are missing +than with them around. + +With user namespacing in place things get much better. With user namespaces the +`struct user` issue described above goes away, and containers can keep +`CAP_SYS_ADMIN` safely for the user namespace, as capabilities are virtualized +and having capabilities inside a container doesn't mean one also has them +outside. + +## Final Words + +If you write software that wants to detect whether it is run in a container, +please check `/proc/1/environ` and look for the `container=` environment +variable. Do not assume the environment variable is inherited down the process +tree. It generally is not. Hence check the environment block of PID 1, not your +own. Note though that this file is only accessible to root. systemd hence early +on also copies the value into `/run/systemd/container`, which is readable for +everybody. However, that's a systemd-specific interface and other init systems +are unlikely to do the same. + +Note that it is our intention to make systemd systems work flawlessly and +out-of-the-box in containers. In fact, we are interested to ensure that the same +OS image can be booted on a bare system, in a VM and in a container, and behave +correctly each time. If you notice that some component in systemd does not work +in a container as it should, even though the container manager implements +everything documented above, please contact us. |