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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
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GPG key ID: 33112E6833C34679
12 changed files with 864 additions and 56 deletions
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10
internal/daemon/daemon.go
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@ -9,6 +9,8 @@ import (
"log/slog"
"net"
"os"
"path/filepath"
"strings"
"sync"
"time"
@ -88,6 +90,14 @@ func OpenSystem(ctx context.Context) (*Daemon, error) {
if err != nil {
return nil, err
}
// config.Load fills JailerChrootBase from the layout it sees. In
// system mode that's the owner's layout (no privileged StateDir) so
// the value lands under the owner home — wrong for the helper, which
// validates paths against the system StateDir. Override unconditionally
// here so both daemon and helper see /var/lib/banger/jail.
if strings.TrimSpace(cfg.JailerChrootBase) == "" || !filepath.IsAbs(cfg.JailerChrootBase) || strings.HasPrefix(cfg.JailerChrootBase, ownerLayout.StateDir) {
cfg.JailerChrootBase = filepath.Join(layout.StateDir, "jail")
}
helper := newHelperPrivilegedOps(roothelper.NewClient(installmeta.DefaultRootHelperSocketPath), cfg, layout)
return openWithConfig(ctx, layout, ownerLayout, cfg, -1, -1, false, helper)
}