[Unit]
Description=restic-manager agent
Documentation=https://gitea.dcglab.co.uk/steve/restic-manager
After=network-online.target
Wants=network-online.target

[Service]
Type=simple
ExecStart=/usr/local/bin/restic-manager-agent -config /etc/restic-manager/agent.yaml
Restart=always
RestartSec=5

# The agent runs as root. A fleet-backup tool needs to read every
# file on the system regardless of DAC permissions; running as a
# dedicated unprivileged user means either silent skips on /home,
# /root, /var/lib/<other-daemons>, or operators having to add the
# service user to every group whose files they want backed up. Both
# are worse than the threat model already implies (the agent holds
# repo credentials, executes arbitrary restic, and runs operator-
# defined hooks — its blast radius is already large).
#
# The mitigation is aggressive systemd sandboxing of the root
# process: drop all capabilities except the few we need, deny
# writes outside our state dirs, and forbid privilege escalation.
User=root
Group=root

# CAP_DAC_READ_SEARCH lets us read any file regardless of DAC perms
# (the "backup everything" capability). CAP_DAC_OVERRIDE is needed
# during restore for chown/chmod to recreate ownership. Drop the
# rest — root in this process means "can read", not "can do".
CapabilityBoundingSet=CAP_DAC_READ_SEARCH CAP_DAC_OVERRIDE CAP_FOWNER CAP_CHOWN
AmbientCapabilities=CAP_DAC_READ_SEARCH CAP_DAC_OVERRIDE CAP_FOWNER CAP_CHOWN

# Hardening — blocks privilege escalation even from root, and
# confines kernel / namespace / privilege surface. Filesystem reads
# stay open (that's the whole job) and restore writes are
# unrestricted: a backup tool whose entire purpose is "put files
# back where they belong" can't have ProtectHome=read-only or
# ProtectSystem=strict without breaking on the first cross-user
# restore. ProtectSystem=full keeps /usr, /boot, /efi read-only so a
# compromised agent can't swap out /usr/bin/restic or drop a kernel
# module, while leaving /home, /root, /var, /opt, /srv, /tmp etc.
# writable for arbitrary restore targets. The agent is treated as a
# high-trust component (it runs operator hooks as root and holds
# repo credentials); the residual hardening is about kernel + privesc
# protection, not write confinement.
NoNewPrivileges=true
ProtectSystem=full
# ProtectSystem=full mounts /usr, /boot, /efi *and* /etc read-only.
# The agent rewrites /etc/restic-manager/agent.yaml on enrolment and
# whenever a new SecretsKey is minted, so we need a targeted
# write-exemption for that dir. No exemption for the rest of /etc:
# the agent has no business editing /etc/passwd, /etc/sudoers, etc.
#
# /usr/local/bin is writable so the self-update flow (P6-01) can
# atomic-rename a fresh binary over the running one. Permitting the
# whole directory (rather than just the binary path) is required
# because os.Rename takes a write lock on the parent dir.
ReadWritePaths=/etc/restic-manager /usr/local/bin
ProtectHostname=true
ProtectKernelTunables=true
ProtectKernelModules=true
ProtectKernelLogs=true
ProtectControlGroups=true
ProtectClock=true
PrivateTmp=true
RestrictAddressFamilies=AF_UNIX AF_INET AF_INET6
RestrictRealtime=true
RestrictSUIDSGID=true
RestrictNamespaces=true
LockPersonality=true
MemoryDenyWriteExecute=true
SystemCallArchitectures=native
# (No SystemCallFilter — the cap drop above already constrains what
# root can do; an allow-list filter killed restic with SIGSYS during
# init because @system-service excludes some of the syscalls Go's
# runtime + restic's file scanner reach for. The Protect*/Restrict*
# toggles still cover network / kernel / mount / namespace.)

[Install]
WantedBy=multi-user.target