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firecracker: adopt firecracker-jailer for VM launch (Phase B)

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Each VM's firecracker now runs inside a per-VM chroot dropped to the
registered owner UID via firecracker-jailer. Closes the broad ambient-
sudo escalation surface that survived Phase A: the helper still needs
caps for tap/bridge/dm/loop/iptables, but the VMM itself no longer
runs as root in the host root filesystem.

The host helper stages each chroot up front: hard-links the kernel
and (optional) initrd, mknods block-device drives + /dev/vhost-vsock,
copies in the firecracker binary (jailer opens it O_RDWR so a ro bind
fails with EROFS), and bind-mounts /usr/lib + /lib trees read-only so
the dynamic linker can resolve. Self-binds the chroot first so the
findmnt-guarded cleanup can recurse safely.

AF_UNIX sun_path is 108 bytes; the chroot path easily blows past that.
Daemon-side launch pre-symlinks the short request socket path to the
long chroot socket before Machine.Start so the SDK's poll/connect
sees the short path while the kernel resolves to the chroot socket.
--new-pid-ns is intentionally disabled — jailer's PID-namespace fork
makes the SDK see the parent exit and tear the API socket down too
early.

CapabilityBoundingSet for the helper expands to add CAP_FOWNER,
CAP_KILL, CAP_MKNOD, CAP_SETGID, CAP_SETUID, CAP_SYS_CHROOT alongside
the existing CAP_CHOWN/CAP_DAC_OVERRIDE/CAP_NET_ADMIN/CAP_NET_RAW/
CAP_SYS_ADMIN.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
This commit is contained in:
Thales Maciel 2026-04-28 14:38:07 -03:00
parent d73efe6fbc
commit 6b543cb17f
No known key found for this signature in database
GPG key ID: 33112E6833C34679
12 changed files with 864 additions and 56 deletions
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19
internal/config/config.go
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@ -22,6 +22,9 @@ import (
type fileConfig struct {
LogLevel string `toml:"log_level"`
FirecrackerBin string `toml:"firecracker_bin"`
JailerBin string `toml:"jailer_bin"`
JailerEnabled *bool `toml:"jailer_enabled"`
JailerChrootBase string `toml:"jailer_chroot_base"`
SSHKeyPath string `toml:"ssh_key_path"`
DefaultImageName string `toml:"default_image_name"`
AutoStopStaleAfter string `toml:"auto_stop_stale_after"`
@ -75,6 +78,13 @@ func load(layout paths.Layout, home string, ensureDefaultSSHKey bool) (model.Dae
DefaultDNS: model.DefaultDNS,
DefaultImageName: "debian-bookworm",
HostHomeDir: home,
JailerBin: model.DefaultJailerBinary,
JailerEnabled: true,
// Chroot lives under StateDir (ext4) — not RuntimeDir (tmpfs).
// Hard-linking the kernel and any file-backed drives into the
// chroot requires same-filesystem; images already live under
// StateDir, so colocating the chroot avoids EXDEV.
JailerChrootBase: filepath.Join(layout.StateDir, "jail"),
}
var file fileConfig
@ -99,6 +109,15 @@ func load(layout paths.Layout, home string, ensureDefaultSSHKey bool) (model.Dae
} else if path, err := system.LookupExecutable("firecracker"); err == nil {
cfg.FirecrackerBin = path
}
if value := strings.TrimSpace(file.JailerBin); value != "" {
cfg.JailerBin = value
}
if file.JailerEnabled != nil {
cfg.JailerEnabled = *file.JailerEnabled
}
if value := strings.TrimSpace(file.JailerChrootBase); value != "" {
cfg.JailerChrootBase = value
}
if value := strings.TrimSpace(file.DefaultImageName); value != "" {
cfg.DefaultImageName = value
}