User:NetSysFire/systemd sandboxing

Related articles

• Firejail
• AppArmor

Sandboxing systemd service units

systemd enables users to harden and sandbox systemd system service units. Because of technical limitations, and ironically security reasons, user units can not be hardened or sandboxed properly since this would make privilege escalation issues possible. This does not affect system units which use the User= directive.

Tip: Firejail can sandbox applications run as an unprivileged and explicitely allowed user but uses a SUID binary, which is in theory susceptible to privilege escalation vulnerabilities.

Because of the nature of other unit types, only service units can be hardened/sandboxed in the traditional sense. See systemd.exec(5) for more information.

General

Since hardening/sandboxing effectively restricts an application, it is not possible to use all the sandboxing directives. A webserver for example should not use ProtectNetwork=true since it usually needs network access.

systemd-analyze security unit generates a score for the unit showing all the used directives, which can be helpful to determine what settings to try next.

Warning: The score is slightly misleading. Only a simple Hello world can achieve a near perfect score. No application can use all the sandboxing settings.

Unfortunately, systemd's error messages on misconfigurations relating to sandboxing are sometimes vague and/or misleading. Setting the log level temporarily to debug may help getting actually relevant information.

# systemctl log-level debug

Common directives

Most of these directives can be applied to most applications without causing too many problems.

Note: "Impact" and "Potential for breakage" are all relative since they depend on the environment and the unit.

Without special configuration

Simple boolean settings which can either be enabled or not. They can not be configured.

Directive	Impact1	Breakage2	Notes
LockPersonality	Medium	Low
MemoryDenyWriteExecute	Medium3	Medium
NoNewPrivileges	High	Low
PrivateDevices	Medium	Low	/dev/null and similar will still be there
PrivateNetwork	High	Very high	Disallows any network access.
PrivateTmp	Medium	Low
PrivateUsers	High	High
ProtectClock5	Low	Medium4
ProtectControlGroups5	Medium	Low	Highly recommended since no service should write to that
ProtectHostname5	Low	Low
ProtectKernelLogs5,6	Low	Low
ProtectKernelModules5	Medium	Low
ProtectKernelTunables5	Low	Low
RestrictRealtime	Low	Low	May prevent denial-of-service situations
RestrictSUIDSGID	Medium	Low	Best used with NoNewPrivileges

1. How effective the directive is
2. How likely the directive is to break something
3. Can be enhanced with SystemCallFilter
4. Some users reported smartctl can not run when this is set, but this should be relatively safe.
5. Even when running as another User=, systemd setups seccomp filters, which can e.g catch the application running sudo modprobe when ProtectKernelModules is set to true
6. All official kernels have set SECURITY_DMESG_RESTRICT to y, but this is still defense in depth.

Configurable directives

Directive	Value	Impact1	Breakage2	Notes
ProtectSystem	strict	Very high	Very high	Usually used with ReadWritePaths=
	full	High	Medium	May break e.g webservers using ACME to renew their own keys which may be in /etc
	true	High	Medium	There are in theory few applications which write to /boot and /usr
ProtectHome	true	Medium	Medium	Some applications may need persistent data stored in XDG_CONFIG_HOME3
	tmpfs	Low	Low
	read-only	Low	Low	Ideal for backup services
ProtectProc	invisible	High	Medium	Defaults to the hidepid value of the /proc mount, which is usually 0 (unrestricted)
	noaccess	Medium	Medium

1. How effective the directive is
2. How likely the directive is to break something
3. StateDirectory= can be used to mitigate some of the negative consequences

Advanced directives

Syscall filter etc

chroot jail

It is possible to severely restrict what a process can see by specifying TemporaryFileSystem=/:ro and mounting required paths into this chroot-like jail. RootDirectory requires a directory to be present, whereas TemporaryFileSystem does not and will override / seamlessly. Both, and especially the latter, appear to be secure chroot-like directives, which can not be broken out easily, as they do not use the chroot syscall.

Warning: ProtectSystem and ProtectHome are incompatible with TemporaryFileSystem=/:ro and will cause the latter to be undone, making / visible again. However, these directives are not needed since paths will be whitelisted anyways.

All required paths must be mounted into this jail via BindReadOnlyPaths and BindPaths:

example_jailed_unit.service

[Unit]
Description=Example unit

[Service]
ExecStart=/home/someuser/executable
User=someuser
Group=someuser
TemporaryFileSystem=/:ro
PrivateTmp=true
BindReadOnlyPaths=/usr/lib /lib64 /lib
BindPaths=/home/someuser/executable

This is a minimal example and most application will need more paths whitelisted. Some common paths include:

• /etc/ca-certificates, /etc/ssl
• /etc/resolv.conf
• /usr/share/zoneinfo
• Any sockets you need, e.g /var/run/mysqld/mysqld.sock

It will be likely that debugging is at some point necessary when trying to sandbox a unit for the first time. If a unit can not be started at all and fails with status=203/EXEC, either the executable itself or required libraries are not accessible. Starting with broad paths at first (e.g allowing the entirety of /usr) and narrowing it down later can help, too.

system.conf

Changes to /etc/systemd/system.conf are global, so they will affect every unit. See systemd-system.conf(5)

Disabling non-native syscalls

Non-native binaries, in almost all cases 32-bit binaries, may partially compromise the security of the system because they do not have access to more hardening. There have been some relatively minor vulnerabilities, like CVE-2009-0835, which affected non-native syscalls.

Warning: This will break 32-bit binaries. Trying to execute such a binary will result in a file not found error.

/etc/systemd/system.conf

SystemCallArchitectures=native

This works well on most systems, but it needs to be at least partially disabled if e.g multilib is in use. Especially gaming with Wine may be impacted. Using systemd-run or modifying the session slice to override SystemCallArchitectures can be used to disable restrictions partially.

Enabling more unit statistics

systemd does not track all resource usage of a unit by default. Enable Default*Accounting to get more statistics in the systemctl status output and the journal. This is not strictly a security setting, but it will certainly make debugging easier and can provide useful insights into resource usage.

/etc/systemd/system.conf

DefaultCPUAccounting=yes
DefaultIOAccounting=yes
DefaultIPAccounting=yes
DefaultBlockIOAccounting=yes
DefaultMemoryAccounting=yes
DefaultTasksAccounting=yes