Prototyping Kernel Modules for Process Control

Prototyping Kernel Modules for Process Control

In this lesson, you will learn how to intercept the birth of new processes using a kernel module to monitor fork events in real-time.

While user-space tools like ps or top provide a snapshot of system activity, they often miss transient processes that appear and disappear in milliseconds. To gain absolute control, we move into the kernel. By utilizing the task_struct and the kprobes infrastructure, we can hook into the exact moment the kernel duplicates a process.

Hooking the Fork Event

The kernel uses the do_fork() or kernel_clone() functions (depending on your kernel version) to create a new task. By placing a "probe" at the entry point of this function, we can execute our own code whenever a new process is spawned.

Here is a simplified conceptual example of how you might register a handler to log the PID of a newly created process:

#include <linux/kernel.h>
#include <linux/module.h>
#include <linux/kprobes.h>

static int handler_pre(struct kprobe *p, struct pt_regs *regs) {
    printk(KERN_INFO "New process detected! Tracking potential fork.\n");
    return 0;
}

static struct kprobe kp = {
    .symbol_name = "_do_fork", // Use kernel_clone on newer kernels
    .pre_handler = handler_pre,
};

int init_module(void) {
    register_kprobe(&kp);
    return 0;
}

void cleanup_module(void) {
    unregister_kprobe(&kp);
}

Why This Matters for Process Control

When your module catches the fork event, you have full access to the task_struct of the parent and the child. This allows you to prototype sophisticated control logic directly within the kernel:

1. Validation: You can inspect the child's credentials or namespace attributes before the process is fully scheduled.
2. Resource Throttling: You could immediately apply a cgroup or specific priority class to the child if it matches a forbidden pattern.
3. Prevention: If you determine the fork is malicious, you could theoretically return an error code from the probe to abort the process creation, effectively "killing" it before it even begins execution.

Building these prototypes helps you understand exactly how the kernel manages the lifecycle of tasks before they are exposed to user-space security policies.

Key Takeaway: Kernel modules allow you to move from observing process events to actively intercepting and regulating them at the point of creation.