Documentation

Basic-Secure-Kernel-Module

The code is a basic kernel module that demonstrates how to initialize a secure driver, though integrating TPM or HSM functionality would require additional kernel-space libraries, such as tpm_tis or tpm_dev for TPM devices.

How This Works

• Uses the Linux Kernel Crypto API to allocate an AES cipher instance.
• The module is initialized with module_init(secure_module_init);, where it allocates the cipher.
• Cleans up resources in secure_module_exit();.
• Logging uses printk() for kernel-space debugging.

Expanding for TPM/HSM

If you need TPM or HSM support, you might:

• Interface with TPM devices via /dev/tpm0.
• Use kernel TPM drivers (tpm_tis, tpm_dev).
• Implement secure key storage and cryptographic operations.

Interfacing with a Trusted Platform Module (TPM) in your Linux kernel module requires using the TPM kernel API and interacting with TPM devices via /dev/tpm0 or specialized kernel drivers like tpm_tis. Here’s a high-level approach:

1. Ensure Your System Has TPM Enabled

• Run ls /dev/tpm* to check if TPM devices exist.
• Verify TPM status with dmesg | grep -i tpm or cat /sys/class/tpm/tpm0/device/status.

2. Load TPM Kernel Drivers

• The Linux kernel has built-in TPM drivers (tpm_tis for physical TPMs, tpm_crb for newer TPM 2.0, or tpm_vtpm for virtual TPMs).
• Load the driver using:

  modprobe tpm_tis

• Confirm it's loaded:

  lsmod | grep tpm

3. Modify Your Kernel Module to Access TPM

Here’s a basic kernel module that tries to read TPM details:

#include <linux/module.h>
#include <linux/kernel.h>
#include <linux/fs.h>
#include <linux/uaccess.h>

#define TPM_DEVICE "/dev/tpm0"

static int __init tpm_module_init(void) {
    struct file *f;
    char tpm_buf[20];
    mm_segment_t old_fs;

    printk(KERN_INFO "Initializing TPM Interface Module\n");

    old_fs = get_fs();
    set_fs(KERNEL_DS);

    f = filp_open(TPM_DEVICE, O_RDONLY, 0);
    if (IS_ERR(f)) {
        printk(KERN_ERR "Failed to open TPM device\n");
        set_fs(old_fs);
        return PTR_ERR(f);
    }

    // Attempt to read TPM data (simplified for demonstration)
    vfs_read(f, tpm_buf, sizeof(tpm_buf), &f->f_pos);
    printk(KERN_INFO "TPM Data: %s\n", tpm_buf);

    filp_close(f, NULL);
    set_fs(old_fs);

    return 0;
}

static void __exit tpm_module_exit(void) {
    printk(KERN_INFO "Unloading TPM Interface Module\n");
}

module_init(tpm_module_init);
module_exit(tpm_module_exit);

MODULE_LICENSE("MIT");
MODULE_AUTHOR("Your Name");
MODULE_DESCRIPTION("Basic Linux Kernel Module for TPM Interaction");

4. Explanation

• The module opens /dev/tpm0 to interact with the TPM.
• Uses vfs_read() to read TPM data.
• Ensures proper kernel-space file handling with set_fs(KERNEL_DS) to access user-space paths safely.
• Logs the data retrieved for debugging.

5. Expanding This Module

For stronger TPM functionality, consider:

• Using tpm_request_locality() to request secure access.
• Implementing TPM command structures using tpm_transmit().
• Writing a user-space program using libtpm or tpm2-tools for more advanced interaction.

TPM command structures are fundamental to communicating with a Trusted Platform Module (TPM) and executing security-related tasks, such as encryption, authentication, and key management. TPM commands follow a standardized format defined by the TPM 2.0 specification, ensuring compatibility across hardware and software implementations.

1. TPM Command Structure Format

Each TPM command consists of a header and a payload, structured as follows:

Field	Size (bytes)	Description
TPM Header	10-12	Identifies the command type and length
Command Code	4	Specifies the operation (e.g., TPM2_EncryptDecrypt)
Handles (Optional)	Variable	References TPM objects, like keys or sessions
Authorization	Variable	Provides authentication credentials if required
Command Parameters	Variable	Contains data needed for the command execution

2. Example TPM Command: TPM2_GetRandom

This command requests random bytes from the TPM’s hardware-based random number generator.

TPM Request Packet

80 01 00 00 00 0C 00 00 01 7B 00 20

Breakdown:

• 80 01 → TPM_ST_NO_SESSIONS (command structure type)
• 00 00 00 0C → Command size (12 bytes)
• 00 00 01 7B → Command code (TPM2_GetRandom)
• 00 20 → Requesting 32 bytes of random data

TPM Response Packet

80 01 00 00 00 2C 00 00 00 00 <random_bytes>

The TPM returns 32 bytes of cryptographically secure random data.

3. Sending Commands to TPM in Linux

You can send TPM commands using:

• TPM device files (/dev/tpm0) with write() and read()
• TPM user-space libraries (tpm2-tools, libtpm) for simpler interaction
• Kernel TPM API for deeper integration in a secure kernel module

Below is a C program that sends a TPM2_GetRandom command to a TPM device and reads back the random data.

Example: Sending TPM Command in C

#include <stdio.h>
#include <fcntl.h>
#include <unistd.h>
#include <string.h>

#define TPM_DEVICE "/dev/tpm0"

int main() {
    int fd;
    unsigned char command[] = {
        0x80, 0x01,  // TPM_ST_NO_SESSIONS (command structure type)
        0x00, 0x00, 0x00, 0x0C,  // Command size (12 bytes)
        0x00, 0x00, 0x01, 0x7B,  // TPM2_GetRandom command
        0x00, 0x20  // Request 32 bytes of random data
    };
    unsigned char response[64];

    // Open TPM device
    fd = open(TPM_DEVICE, O_RDWR);
    if (fd < 0) {
        perror("Failed to open TPM device");
        return 1;
    }

    // Send TPM command
    if (write(fd, command, sizeof(command)) < 0) {
        perror("Failed to write TPM command");
        close(fd);
        return 1;
    }

    // Read TPM response
    if (read(fd, response, sizeof(response)) < 0) {
        perror("Failed to read TPM response");
        close(fd);
        return 1;
    }

    // Print response bytes
    printf("TPM Response: ");
    for (int i = 0; i < sizeof(response); i++) {
        printf("%02X ", response[i]);
    }
    printf("\n");

    close(fd);
    return 0;
}

How This Works

1. Opens /dev/tpm0 for direct communication with the TPM.
2. Sends the TPM2_GetRandom command, requesting 32 random bytes.
3. Reads the TPM response containing the random data.
4. Prints the received bytes in hexadecimal format.

Expanding This Program

• Integrate Authentication: Add TPM sessions for secure access.
• Use TPM Libraries: Instead of raw byte commands, leverage tpm2-tools or libtpm.
• Perform Cryptographic Operations: Encrypt/decrypt data using TPM keys.