Dynamic Access Control with eBPF Programs

In this lesson, you will learn how to move beyond static file permissions by using eBPF to enforce custom, real-time security logic directly within the Linux kernel.

Standard Linux access control (like POSIX ACLs or even MAC systems like SELinux) often relies on static labels or user IDs. However, sometimes you need to enforce a policy based on the "context" of an action—such as preventing a specific process from opening a sensitive file only if it is currently connected to an external network, or restricting access based on the time of day.

eBPF (Extended Berkeley Packet Filter) allows you to attach programs to internal kernel functions, known as "hooks," without modifying the kernel source code. When a process attempts an operation (like openat), your eBPF program can inspect the process's credentials, the target file path, and even the system state in real-time. If the operation violates your custom rules, the program can instruct the kernel to return an error, effectively blocking the access.

A Conceptual Example

Imagine you want to block any application from opening /etc/shadow unless the process is running with a specific UID and has an active socket connection to a trusted internal server. Your eBPF program would look something like this:

SEC("kprobe/sys_openat")
int bpf_enforce_shadow_access(struct pt_regs *ctx) {
    // 1. Capture the file path argument
    char path[256];
    bpf_probe_read_user_str(&path, sizeof(path), (void *)PT_REGS_PARM2(ctx));

    // 2. Check if the file is sensitive
    if (strcmp(path, "/etc/shadow") == 0) {
        // 3. Add custom logic: check if UID is authorized
        u32 uid = bpf_get_current_uid_gid();
        if (uid != 0) {
            // 4. Deny the request by forcing an error
            bpf_override_return(ctx, -EACCES);
        }
    }
    return 0;
}

By using bpf_override_return, you are injecting a security decision at the exact moment of execution. This "dynamic" nature means your security posture adapts instantly to the environment, providing a layer of granular control that standard tools cannot easily replicate.

Key Takeaway: eBPF empowers you to write custom, programmable security logic that intercepts system calls and denies access based on real-time process context.