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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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19
internal/daemon/vm_lifecycle_steps.go
View file

@ -14,6 +14,24 @@ import (
"banger/internal/system"
)
// jailerOpts returns the jailer launch options to bundle in the firecracker
// launch request, or nil when the jailer is disabled or misconfigured.
// nil makes the launch fall back to the legacy direct-firecracker path.
func (s *VMService) jailerOpts() *roothelper.JailerLaunchOpts {
if !s.config.JailerEnabled {
return nil
}
if strings.TrimSpace(s.config.JailerBin) == "" || strings.TrimSpace(s.config.JailerChrootBase) == "" {
return nil
}
return &roothelper.JailerLaunchOpts{
Binary: s.config.JailerBin,
ChrootBaseDir: s.config.JailerChrootBase,
UID: os.Getuid(),
GID: os.Getgid(),
}
}
// buildKernelArgs assembles the kernel command line for a start.
// Direct-boot images (no initrd) get kernel-level IP config so the
// network is up before init, plus init= pointing at the universal
@ -344,6 +362,7 @@ func (s *VMService) buildStartSteps(op *operationLog, sc *startContext) []startS
VSockCID: sc.vm.Runtime.VSockCID,
VCPUCount: sc.vm.Spec.VCPUCount,
MemoryMiB: sc.vm.Spec.MemoryMiB,
Jailer: s.jailerOpts(),
}
machineConfig := firecracker.MachineConfig{Drives: launchReq.Drives}
s.capHooks.contributeMachine(&machineConfig, *sc.vm, sc.image)