validateManagedPath was textual-only: filepath.Clean + dest-prefix
match. That stopped `..` escapes but not the symlink-bypass attack
that motivated this fix — a daemon-UID attacker can write into
StateDir/RuntimeDir (it's their UID), so they can plant
`<StateDir>/redirect -> /etc` and any helper RPC that then operates
on `<StateDir>/redirect/...` resolves through the symlink at the
kernel and lands at /etc/... on the host.
Concretely the leaks this closed:
* priv.create_dm_snapshot: rootfs/cow paths fed to losetup —
losetup follows the symlink and attaches a host block device.
* priv.launch_firecracker: kernel/initrd paths hard-linked into
the chroot via `ln -f` — link(2) on Linux follows source
symlinks, hard-linking host files into the jail.
* priv.read_ext4_file / priv.write_ext4_files: image paths fed
to debugfs / e2cp as root.
* validateLaunchDrivePath: drive paths mknod'd or hard-linked.
* validateJailerOpts: chroot base.
Fix: after the existing prefix match, walk every component below
the matched root with Lstat. Any existing symlink — leaf or
intermediate — fails the validator. ENOENT is tolerated because
several callers pass paths firecracker/the helper materialise
later (sockets, log files, kernel hard-link targets); whoever
materialises them goes through the same validation when the
helper-side primitive runs.
Subsumes most of validateNotSymlink's coverage but the explicit
call sites (methodEnsureSocketAccess, methodCleanupJailerChroot)
keep their belt-and-braces check — those paths must EXIST and
not be symlinks, which validateNotSymlink enforces strictly while
the broadened validateManagedPath tolerates ENOENT.
Race-free in practice: helper RPCs are short and the validator
fires on the same kernel state the next syscall sees. The helper
loop processes RPCs serially per-connection, and the validator
plus the syscall both run as root within microseconds of each
other.
Four new tests cover symlink leaf, symlink intermediate, missing
leaf (must pass), and the plain happy path. Smoke at JOBS=4 still
green — every legitimate daemon-supplied path passes the walk.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
Defence-in-depth pass over every helper method that touches the host
as root. Each fix narrows what a compromised owner-uid daemon could
ask the helper to do; many close concrete file-ownership and DoS
primitives that the previous validators didn't reach.
Path / identifier validation:
* priv.fsck_snapshot now requires /dev/mapper/fc-rootfs-* (was
"is the string non-empty"). e2fsck -fy on /dev/sda1 was the
motivating exploit.
* priv.kill_process and priv.signal_process now read
/proc/<pid>/cmdline and require a "firecracker" substring before
sending the signal. Killing arbitrary host PIDs (sshd, init, …)
is no longer a one-RPC primitive.
* priv.read_ext4_file and priv.write_ext4_files now require the
image path to live under StateDir or be /dev/mapper/fc-rootfs-*.
* priv.cleanup_dm_snapshot validates every non-empty Handles field:
DM name fc-rootfs-*, DM device /dev/mapper/fc-rootfs-*, loops
/dev/loopN.
* priv.remove_dm_snapshot accepts only fc-rootfs-* names or
/dev/mapper/fc-rootfs-* paths.
* priv.ensure_nat now requires a parsable IPv4 address and a
banger-prefixed tap.
* priv.sync_resolver_routing and priv.clear_resolver_routing now
require a Linux iface-name-shaped bridge name (1–15 chars, no
whitespace/'/'/':') and, for sync, a parsable resolver address.
Symlink defence:
* priv.ensure_socket_access now validates the socket path is under
RuntimeDir and not a symlink. The fcproc layer's chown/chmod
moves to unix.Open(O_PATH|O_NOFOLLOW) + Fchownat(AT_EMPTY_PATH)
+ Fchmodat via /proc/self/fd, so even a swap of the leaf into a
symlink between validation and the syscall is refused. The
local-priv (non-root) fallback uses `chown -h`.
* priv.cleanup_jailer_chroot rejects symlinks at both the leaf
(os.Lstat) and intermediate path components (filepath.EvalSymlinks
+ clean-equality). The umount sweep was rewritten from shell
`umount --recursive --lazy` to direct unix.Unmount(MNT_DETACH |
UMOUNT_NOFOLLOW) per child mount, deepest-first; the findmnt
guard remains as the rm-rf safety net. Local-priv mode falls
back to `sudo umount --lazy`.
Binary validation:
* validateRootExecutable now opens with O_PATH|O_NOFOLLOW and
Fstats through the resulting fd. Rejects path-level symlinks and
narrows the TOCTOU window between validation and the SDK's exec
to fork+exec time on a healthy host.
Daemon socket:
* The owner daemon now reads SO_PEERCRED on every accepted
connection and refuses any UID that isn't 0 or the registered
owner. Filesystem perms (0600 + ownerUID) already enforced this;
the check is belt-and-braces in case the socket FD is ever
leaked to a non-owner process.
Docs:
* docs/privileges.md walked end-to-end. Each helper RPC's
Validation gate row reflects what the code actually enforces.
New section "Running outside the system install" calls out the
looser dev-mode trust model (NOPASSWD sudoers, helper hardening
bypassed) so users don't deploy that path on shared hosts.
Trust list updated to include every new validator.
Tests added: validators (DM-loop, DM-remove-target, DM-handles,
ext4-image-path, iface-name, IPv4, resolver-addr, not-symlink,
firecracker-PID, root-executable variants), the daemon's authorize
path (non-unix conn rejection + unix conn happy path), the umount2
ordering contract (deepest-first + --lazy on the sudo branch), and
positive/negative cases for the chown-no-follow fallback.
Verified end-to-end via `make smoke JOBS=4` on a KVM host.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
Move the supported systemd path to two services: an owner-user bangerd for
orchestration and a narrow root helper for bridge/tap, NAT/resolver, dm/loop,
and Firecracker ownership. This removes repeated sudo from daily vm and image
flows without leaving the general daemon running as root.
Add install metadata, system install/status/restart/uninstall commands, and a
system-owned runtime layout. Keep user SSH/config material in the owner home,
lock file_sync to the owner home, and move daemon known_hosts handling out of
the old root-owned control path.
Route privileged lifecycle steps through typed privilegedOps calls, harden the
two systemd units, and rewrite smoke plus docs around the supported service
model.
Verified with make build, make test, make lint, and make smoke on the
supported systemd host path.
