Enable systemd service hardening features for default system 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 system 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 the following on a case by case basis:

LockPersonality=yes
ProtectHostname=yes
ProtectClock=yes
SystemCallArchitectures=native
RestrictSUIDSGID=yes
RemoveIPC=yes
MemoryDenyWriteExecute=yes
KeyringMode=private

Certain services have very targeted scope that requires reading or writing from only one directory. We could use more fine grained knobs to restrict access even further in that case. 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 system services as well as some of the most commonly used services such as Nginx or PostgreSQL. All of these settings need to be configured on a per service basis instead of global override to avoid impacting users on upgrades. For a Fedora 39 workstation, we have the following system services installed by default which should considered within the scope of the change (excluding systemd associated ones which already have a number of knobs on). We may also consider doing this for some of the high profile services including say Nginx and PostgreSQL permitting time considerations and other contributors if any joining this effort.

abrtd.service
abrt-journal-core.service
abrt-oops.service
abrt-pstoreoops.service
abrt-vmcore.service
abrt-xorg.service
accounts-daemon.service
alsa-restore.service
alsa-state.service
anaconda-direct.service
anaconda-fips.service
anaconda-nm-config.service
anaconda-nm-disable-autocons.service
anaconda-noshell.service
anaconda-pre.service
anaconda.service
anaconda-sshd.service
arp-ethers.service
auditd.service
auth-rpcgss-module.service
avahi-daemon.service
blivet.service
blk-availability.service
bluetooth.service
bolt.service
brltty.service
canberra-system-bootup.service
canberra-system-shutdown-reboot.service
canberra-system-shutdown.service
chronyd-restricted.service
chronyd.service
chrony-wait.service
colord.service
console-getty.service
cups-browsed.service
cups.service
dbus-broker.service
dbus-daemon.service
dbus-org.freedesktop.hostname1.service
dbus-org.freedesktop.import1.service
dbus-org.freedesktop.locale1.service
dbus-org.freedesktop.login1.service
dbus-org.freedesktop.machine1.service
dbus-org.freedesktop.portable1.service
dbus-org.freedesktop.timedate1.service
~~debug-shell.service~~ (opens a user shell that must be able to do arbitrary stuff)
dm-event.service
dnf-makecache.service
dnf-system-upgrade-cleanup.service
dnf-system-upgrade.service
dnsmasq.service
dracut-cmdline.service
dracut-initqueue.service
dracut-mount.service
dracut-pre-mount.service
dracut-pre-pivot.service
dracut-pre-trigger.service
dracut-pre-udev.service
dracut-shutdown-onfailure.service
dracut-shutdown.service
~~emergency.service~~ (opens a user shell that must be able to do arbitrary stuff)
fedora-third-party-refresh.service
firewalld.service
flatpak-add-fedora-repos.service
flatpak-system-helper.service
fprintd.service
fsidd.service
fstrim.service
fwupd-offline-update.service
fwupd-refresh.service
fwupd.service
gdm.service
geoclue.service
grub-boot-indeterminate.service
gssproxy.service
htcacheclean.service
httpd.service
httpd.service.d
hypervfcopyd.service
hypervkvpd.service
hypervvssd.service
iio-sensor-proxy.service
import-state.service
initrd-cleanup.service
initrd-parse-etc.service
initrd-switch-root.service
initrd-udevadm-cleanup-db.service
instperf.service
ipp-usb.service
iscsid.service
iscsi-init.service
iscsi-onboot.service
iscsi.service
iscsi-shutdown.service
iscsi-starter.service
iscsiuio.service
kdump.service
kmod-static-nodes.service
ldconfig.service
libvirtd.service
libvirt-guests.service
livesys-late.service
livesys.service
loadmodules.service
logrotate.service
low-memory-monitor.service
lvm2-lvmdbusd.service
lvm2-lvmpolld.service
lvm2-monitor.service
man-db-cache-update.service
man-db-restart-cache-update.service
mcelog.service
mdcheck_continue.service
mdcheck_start.service
mdmonitor-oneshot.service
mdmonitor.service
ModemManager.service
ndctl-monitor.service
netavark-dhcp-proxy.service
NetworkManager-dispatcher.service
NetworkManager.service
NetworkManager-wait-online.service
nfs-blkmap.service
nfsdcld.service
nfs-idmapd.service
nfs-mountd.service
nfs-server.service
nfs-utils.service
nftables.service
nis-domainname.service
nm-priv-helper.service
numad.service
nvmefc-boot-connections.service
nvmf-autoconnect.service
ostree-boot-complete.service
ostree-finalize-staged-hold.service
ostree-finalize-staged.service
ostree-prepare-root.service
ostree-remount.service
packagekit-offline-update.service
packagekit.service
pam_namespace.service
pcscd.service
plocate-updatedb.service
plymouth-halt.service
plymouth-kexec.service
plymouth-poweroff.service
plymouth-quit.service
plymouth-quit-wait.service
plymouth-read-write.service
plymouth-reboot.service
plymouth-start.service
plymouth-switch-root-initramfs.service
plymouth-switch-root.service
podman-auto-update.service
podman-clean-transient.service
podman-restart.service
podman.service
polkit.service
power-profiles-daemon.service
psacct.service
qemu-guest-agent.service
qemu-pr-helper.service
quotaon.service
raid-check.service
~~rc-local.service~~ (this can do arbitrary stuff)
realmd.service
rescue.service
rpcbind.service
rpc-gssd.service
rpc-statd-notify.service
rpc-statd.service
rpmdb-migrate.service
rpmdb-rebuild.service
rtkit-daemon.service
saslauthd.service
selinux-autorelabel-mark.service
selinux-autorelabel.service
selinux-check-proper-disable.service
speech-dispatcherd.service
spice-vdagentd.service
spice-webdavd.service
sshd.service
ssh-host-keys-migration.service
sssd-autofs.service
sssd-kcm.service
sssd-nss.service
sssd-pac.service
sssd-pam.service
sssd.service
sssd-ssh.service
sssd-sudo.service
switcheroo-control.service
system-update-cleanup.service
tcsd.service
thermald.service
udisks2.service
unbound-anchor.service
upower.service
uresourced.service
usbmuxd.service
vboxclient.service
vboxservice.service
vgauthd.service
virtinterfaced.service
virtlockd.service
virtlogd.service
virtnetworkd.service
virtnodedevd.service
virtnwfilterd.service
virtproxyd.service
virtqemud.service
virtsecretd.service
virtstoraged.service
vmtoolsd.service
wpa_supplicant.service
zfs-fuse-scrub.service
zfs-fuse.service
zvbid.service

We will encourage package maintainers to follow https://docs.fedoraproject.org/en-US/package-maintainers/Staying_Close_to_Upstream_Projects/ and upstream these changes. Systemd will ignore any of these settings it does not understand on older versions. Hence this should be safe for upstream to merge on any services.

Feedback

Benefit to Fedora

Fedora services will get a significant security boost by default by avoiding or mitigating any unknown security vulnerabilities in default system 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. Current text only recommends a couple of knobs for long running services. 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. If advanced non-default functionality requires overrides default knobs, we can document those in the release notes to provide guidance.

User Experience

This should be largely transparent change for users. 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 will 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.

Dependencies

None. We are merely enabling some of systemd security features by default for default system services and potentially some 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

Release Notes

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

PostgreSQL
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

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