Adds IPAddressDeny= to the sandbox unit covering loopback (127/8 + ::1),
link-local (169.254/16 + fe80::/10), multicast (224/4 + ff00::/8), all
RFC1918 v4 (10/8, 172.16/12, 192.168/16), CGNAT (100.64/10), and ULA v6
(fc00::/7). The kernel attaches systemd's sd_fw_egress BPF program to
the unit's cgroup; egress packets matching any of the deny prefixes are
silently dropped at the cgroup boundary.
Important: do NOT pair this with `IPAddressAllow=any`. Documentation
claims "more specific rule wins" but on this systemd 257 + kernel 6.12
combo, having both set causes the allow to win unconditionally — the
deny gets ignored. Empty IPAddressAllow + populated IPAddressDeny is the
correct shape: kernel default "allow all" applies to non-listed
addresses, and the listed prefixes are blocked.
Because the host's resolv.conf typically points at a private-IP DNS
server (10.0.0.1 in the test deploy), blocking RFC1918 also kills DNS.
Adds a static /etc/left4me/sandbox-resolv.conf with public resolvers
(Cloudflare 1.1.1.1, Google 8.8.8.8) and bind-mounts that into the
sandbox at /etc/resolv.conf, replacing the host's resolver inside the
sandbox only.
Smoke-tested on ckn@10.0.4.128:
- public 1.1.1.1:443: CONNECTED
- public HTTPS via DNS (steamcommunity.com): 200
- localhost web app 127.0.0.1:8000: blocked (TimeoutError)
- localhost sshd 127.0.0.1:22: blocked
- private LAN ssh 10.0.4.128:22: blocked
- private DNS 10.0.0.1:53: blocked
AF_UNIX stays in RestrictAddressFamilies — dropping it would risk
breaking NSS / syslog for marginal gain, and the IP-level filter
addresses the primary threat (reaching the host's HTTP/SSH services).
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
Privileged bash helper that wraps user-authored scripts in
systemd-run --scope (cgroup limits + RuntimeMaxSec=3600) inside a
bubblewrap sandbox dropped to the l4d2-sandbox uid. Network is shared
with the host so scripts can fetch from Steam / l4d2center / etc.;
filesystem is RO except for /overlay (rw bind from
/var/lib/left4me/overlays/{id}) and tmpfs /tmp + /run.
Adds a sudoers rule allowing the left4me user to invoke this helper
without restrictions on its arguments. Strict argument validation is
in the helper itself.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
New privileged helper at /usr/local/libexec/left4me/left4me-overlay
(Python, system /usr/bin/python3, stdlib only) takes only the instance
name, parses instance.env for L4D2_LOWERDIRS, validates each lowerdir
against an allowlist (installation/, overlays/, global_overlay_cache/,
workshop_cache/), refuses upperdirs tainted with user.fuseoverlayfs.*
xattrs from the prior fuse era, and execs `nsenter --mount=/proc/1/ns/mnt
-- mount -t overlay ...` so the resulting mount lives in the host
namespace. Mirrors the existing left4me-systemctl / left4me-journalctl
pattern; sudoers entry is verb-constrained.
KernelOverlayFSMounter implements the existing OverlayMounter ABC,
deriving the instance name from the merged path. No call sites use it
yet — that's the next commit.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
