isRunning()/Signal()/Kill()/joinSandboxNetNs() trust a raw PID with no liveness/identity check, so PID reuse after the VMM exits lets urunc signal (and even network-namespace-join) an unrelated host process

[Anand-240]

Description

urunc records the monitor (VMM) process's PID once, in u.State.Pid, when the container is created, and then uses that raw integer for the rest of the container's lifecycle: Signal(), Kill(), Delete() via isRunning(), and joinSandboxNetNs(). None of these call sites verify that the PID still refers to the same process that was originally launched, they only check whether some process currently holds that PID number.

On Linux, PIDs are recycled. Once the VMM exits and gets reaped, that PID becomes eligible for reuse by any new process on the host. Issue #716 shows urunc containers can sit around for a long time after their VMM has already died, which is exactly the kind of window that makes PID recycling realistic on a busy node.

Relevant code:

pkg/unikontainers/unikontainers.go:1407-1415 (isRunning):

func (u *Unikontainer) isRunning() bool {
	vmmType := hypervisors.VmmType(u.State.Annotations[annotHypervisor])
	if vmmType != hypervisors.HedgeVmm {
		return syscall.Kill(u.State.Pid, syscall.Signal(0)) == nil
	}
	...
}

pkg/unikontainers/unikontainers.go:794-838 (Signal, Kill):

func (u *Unikontainer) Signal(signal unix.Signal) error {
	...
	return vmm.Signal(u.State.Pid, signal)
}

func (u *Unikontainer) Kill() error {
	err := u.joinSandboxNetNs()
	...
	err = vmm.Stop(u.State.Pid)
	...
	err = network.CleanupAllUruncTaps()
	...
}

pkg/unikontainers/unikontainers.go:921-948 (joinSandboxNetNs):

if netNsPath == "" {
	netNsPath = fmt.Sprintf("/proc/%d/ns/net", u.State.Pid)
	err := checkValidNsPath(netNsPath)
	...
}
...
fd, err := unix.Open(netNsPath, unix.O_RDONLY|unix.O_CLOEXEC, 0)
...
err = unix.Setns(int(fd), unix.CLONE_NEWNET)

checkValidNsPath (pkg/unikontainers/utils.go:209-220) only does an os.Lstat(path) existence check, never an identity check.

pkg/unikontainers/hypervisors/utils.go:69-92 (killProcess, used by every hypervisor's Stop):

func killProcess(pid int) error {
	const timeout = 2 * time.Second
	err := unix.Kill(pid, unix.SIGKILL)
	...
}

Sends SIGKILL to whatever process currently owns that PID, with no identity check.

None of these five call sites cross check the PID against anything that proves it is still the original VMM, for example /proc/<pid>/stat starttime or cgroup membership, which is a technique other runtimes such as runc rely on for this exact reason.

If the VMM's PID gets reused by an unrelated host process before urunc kill or urunc delete runs:

• isRunning() reports true for the unrelated process, so Delete() permanently refuses to clean up the already dead container.
• Kill() calls joinSandboxNetNs(), which opens the unrelated process's network namespace and joins it, then calls vmm.Stop(), which SIGKILLs the unrelated process, then runs network.CleanupAllUruncTaps() inside the wrong namespace, removing tap devices that do not belong to this container.

This causes two real failure modes that need no attacker or malicious image author, just ordinary OS PID recycling:

1. Host level collateral damage: an unrelated process gets killed, and unrelated network resources are torn down in the wrong netns.
2. Stuck containers: Delete() can refuse indefinitely to remove an already dead container whose PID was reused, leaving orphaned state and blocking CRI or Kubernetes garbage collection, which compounds known issue Urunc containers hung as Temrinating in kubernetes #716.

Suggested fix: record a lightweight identity token for the VMM PID at Create() time, for example the /proc/<pid>/stat starttime field (which the kernel guarantees changes across PID reuse), and validate it in isRunning(), Signal(), Kill(), and joinSandboxNetNs() before treating the PID as belonging to this container. A mismatch should be treated the same as the process no longer existing.

System info

• Urunc version: main branch
• Arch: any
• VMM: any (Qemu, Firecracker, Cloud Hypervisor, HVT, SPT), any backend going through killProcess
• Unikernel: any

Steps to reproduce

1. Start a urunc container with any hypervisor. u.State.Pid is set to the VMM's host PID, for example 12345.
2. Let the VMM exit without going through urunc kill or urunc delete (guest panic, VMM OOM kill, or normal completion), so PID 12345 gets reaped and freed.
3. On a host with enough process churn, an unrelated process ends up receiving PID 12345. This is normal PID reuse behavior on Linux.
4. Call urunc kill <id> 9 or urunc delete <id> and observe:
   • isRunning() returns true for the unrelated process, so delete is refused.
   • Kill() joins the unrelated process's network namespace, sends it SIGKILL, and cleans up tap devices in the wrong namespace.