Changes/SystemdSecurityHardening

Enable systemd service hardening features for default and high profile services

This is a proposed Change for Fedora Linux.
This document represents a proposed Change. As part of the Changes process, proposals are publicly announced in order to receive community feedback. This proposal will only be implemented if approved by the Fedora Engineering Steering Committee.

Summary

Improve security by enabling some of the high level systemd security hardening settings that isolate and sandbox default and high profile services.

Owner

• Name: Rahul Sundaram
• Email: metherid@gmail.com

• Targeted release: Fedora 40
• Last updated: 2023-11-16
• [<will be assigned by the Wrangler> devel thread]
• FESCo issue: <will be assigned by the Wrangler>
• Tracker bug: <will be assigned by the Wrangler>
• Release notes tracker: <will be assigned by the Wrangler>

Detailed Description

systemd provides a number of knobs that can harden security for services. We are selecting a few high level ones to enable by default.

• PrivateTmp=yes
• ProtectSystem=yes/full/strict
• ProtectHome=yes/read-only
• PrivateDevices=yes
• ProtectKernelTunables=yes
• ProtectKernelModules=yes
• ProtectKernelLogs=yes
• ProtectControlGroups=yes
• NoNewPrivileges=yes
• PrivateNetwork=yes

If we want to go further, we could consider:

• LockPersonality=yes
• ProtectHostname=yes
• ProtectClock=yes
• SystemCallArchitectures=native
• RestrictSUIDSGID=yes
• RemoveIPC=yes

We will enable as many of these as feasible for the services but not every knob is going to be applicable to every service. For example, ProtectHome=yes wouldn't work for any of the systemd user services, but ProtectHome=read-only by default is ok and PrivateNetwork=yes can only be used for services that work purely locally. We will aim to cover all the default services as well as some of the most commonly used services such as Nginx or PostgreSQL. The goal is to have the services work as expected with the default functionality but to potentially require tweaking the settings if the configuration is changed by users. For instance, if we add ProtectHome=yes to Apache httpd.service and the user wishes to serve files out of their home directory, they may need to override the systemd setting to 'ProtectHome=read-only' to allow for the service to read from the user home directory in addition to changing the service specific configuration files to enable this feature. All of these settings need to be configured on a per service basis instead of global override to avoid impacting users on upgrades and retain appropriate scope.

Benefit to Fedora

Fedora services will get a significant security boost by default by avoiding or mitigating any unknown security vulnerabilities in these services.

Scope

• Proposal owners: Individual per service pull requests to enable various security features as applicable.
• Other developers: Review PRs as needed
• Release engineering: https://pagure.io/releng/issue/11785
• Policies and guidelines:

Packaging guidelines will have to be modified to add recommendations to use more of the systemd security features by default. In particular, we should add a security settings section in https://fedoraproject.org/wiki/Packaging:Systemd. Sample text:

Systemd services included in Fedora are recommended to use as many of the following security settings as applicable while maintaining the default functionality of the service.

• PrivateTmp=yes
• ProtectSystem=yes/full/strict
• ProtectHome=yes
• PrivateDevices=yes
• ProtectKernelTunables=yes
• ProtectKernelModules=yes
• ProtectKernelLogs=yes
• ProtectControlGroups=yes
• NoNewPrivileges=yes
• PrivateNetwork=yes

The full list of sandboxing features are available in https://www.freedesktop.org/software/systemd/man/latest/systemd.exec.html#Sandboxing. Note that if you are submitting changes to upstream as recommended, systemd will warn and ignore any of these features it doesn't support. So it should be safe for upstream to enable as many of these features as applicable and not worry about distribution support for ones using older versions of systemd.

• Trademark approval: N/A

Upgrade/compatibility impact

Packages will automatically get additional security features enabled by default transparently.

How To Test

You can use tools like systemd-analyze security and systemctl cat to verify that specific security features are enabled by default. Default services with the default features should have no adverse impact and users shouldn't have to do anything beyond using the software as intended and report any regressions. High profile services not installed by default that gain these security features would benefit from more targeting testing to spot any unintended consequences especially for niche or advanced functionality.

User Experience

This should be a fully transparent change for users.

Dependencies

None. We are merely enabling some long supported systemd features by default for default and high profile services.

Contingency Plan

• Contingency mechanism: These settings can be enabled/disabled at a per service level. No wholesale reverts is necessary. If we don't finish the work for all the services, we can follow up in future releases.
• Contingency deadline: N/A
• Blocks release? No

Documentation

• https://www.freedesktop.org/software/systemd/man/latest/systemd.exec.html#Sandboxing
• https://www.redhat.com/sysadmin/systemd-secure-services
• https://www.redhat.com/sysadmin/mastering-systemd

Release Notes

systemd security hardening features are enabled for default services and following high profile services.

• Postgres
• Apache Httpd
• Nginx
• MariaDB

....

If you wish to turn off any particular settings, you can follow the standard systemd method of overriding the config. For example,

$ cat /etc/systemd/system/httpd.service.d/override.conf

[Service]

ProtectHome=no

$ sudo systemctl daemon-reload

$ sudo systemctl restart httpd.service

$ systemctl status httpd.service

● httpd.service - The Apache HTTP Server

    Loaded: loaded (/usr/lib/systemd/system/httpd.service; enabled; vendor preset: disabled)
   Drop-In: /etc/systemd/system/httpd.service.d
            └─override.conf
    Active: active (running) since Mon 2023-11-15 18:29:25 EST; 3min 30s ago