User:NetSysFire/systemd sandboxing

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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
`ProtectClock`⁵	Low	Medium⁴
`ProtectControlGroups`⁵	Medium	Low	Highly recommended since no service should write to that
`ProtectHostname`⁵	Low	Low
`ProtectKernelLogs`^5,6	Low	Low
`ProtectKernelModules`⁵	Medium	Low
`ProtectKernelTunables`⁵	Low	Low
`RestrictRealtime`	Low	Low	May prevent denial-of-service situations
`RestrictSUIDSGID`	Medium	Low	Best used with `NoNewPrivileges`

How effective the directive is
How likely the directive is to break something
Can be enhanced with SystemCallFilter
Some users reported smartctl can not run when this is set, but this should be relatively safe.
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
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_HOME`³
	`tmpfs`	Low	Low
	`read-only`	Low	Low	Ideal for backup services

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

Advanced directives

Syscall filter etc

chroot jail

description how to use RootDirectory=, BindPaths=, BindReadOnlyPaths= and maybe also TemporaryFileSystem=/:ro

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.

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.

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

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