Complete Guide to Examining Program Arguments in Linux Kernel

The Linux kernel provides several approaches for safely intercepting program arguments during execution, with the prepare_binprm function being one of the most suitable for security patches. While prepare_binprm is a good starting point, you can enhance your approach by leveraging Linux Security Module (LSM) hooks and proper argument handling techniques to safely examine and potentially block program execution based on extended attributes. The linux_binprm structure contains all necessary argument information including argc, envc, and the actual argument strings, which can be accessed safely within the kernel context.

Contents

• Understanding the linux_binprm Structure
• Using prepare_binprm Safely
• Enhanced Security with LSM Hooks
• Alternative Approaches Without ftrace
• Best Practices for Argument Examination
• Implementation Example

Understanding the linux_binprm Structure

The linux_binprm structure is defined in include/linux/binfmts.h and serves as the primary container for all information related to loading and executing a binary program. This structure contains all the program arguments that need to be examined for security purposes.

Key fields in the linux_binprm structure include:

• argc: The argument count for the program
• envc: The environment variable count
• filename: The full path to the executable being run
• interp: The interpreter name (for scripts)
• buf: Buffer containing the initial bytes of the file
• file: The file structure for the executable
• cred: Credentials for the new process

The Linux kernel documentation confirms that this structure is “used to hold the arguments that are used when loading binaries”. This makes it the ideal place to intercept and examine arguments before program execution begins.

Using prepare_binprm Safely

The prepare_binprm function in fs/exec.c is called during the execve system call process and is specifically designed for preparation work before binary loading. This function is called after the initial setup but before the actual execution begins, making it perfect for security checks.

When using prepare_binprm, you need to be aware of several safety considerations:

1. Memory Access: The argument strings are typically stored in user space memory, so you need to use proper kernel APIs to access them safely
2. Timing: The function is called early in the execution process, giving you time to block execution before any harmful code runs
3. Copy vs. Access: Arguments may need to be copied to kernel space for examination

The Linux Inside documentation explains that prepare_binprm is called after calculating the number of command line arguments and environment variables. This timing is crucial for security purposes.

Enhanced Security with LSM Hooks

For a more robust security implementation, consider using Linux Security Module (LSM) hooks. The LSM framework provides standardized security hooks that are called at various points during program execution.

The bprm_check_security LSM hook is particularly relevant as it’s called “before the iteration over binary formats starts” and receives the linux_binprm structure as a parameter. According to the Linux Security Module documentation, this hook allows you to perform security checks and potentially block execution.

Advantages of using LSM hooks include:

• Standard Integration: LSM hooks are the officially supported way to add security features
• Multiple Hook Points: You can intercept at various stages of execution
• Better Stability: LSM is maintained and tested as part of the kernel
• Community Support: Large ecosystem of LSM-based security tools

The LWN article explains that the system’s LSM gets a say in the operation through the bprm_check_security hook, making it an ideal place for security checks based on extended attributes.

Alternative Approaches Without ftrace

Since you want to avoid using ftrace modules, here are several kernel-native approaches to intercept and examine program arguments:

1. Custom Binary Format Handler: Implement a custom struct linux_binfmt handler that intercepts all binary executions
2. Security Module Integration: Use LSM hooks as mentioned above
3. Kernel Module Hook: Register a kernel module that hooks into the execve system call
4. Filesystem-based Security: Use extended attributes on filesystems to mark executables for security checks

The Stack Overflow discussion notes that Linux deliberately doesn’t provide supported features for intercepting system calls at runtime, suggesting that kernel patching or module development is the proper approach.

Best Practices for Argument Examination

When examining program arguments in the kernel, follow these best practices to ensure safety and effectiveness:

1. Use Proper Memory Access Functions: Always use kernel-safe functions like strncpy_from_user() or copy_from_user() to access user-space argument strings
2. Implement Timeout Mechanisms: Add timeouts to prevent hanging the system if arguments are malformed
3. Validate Argument Pointers: Check that all argument pointers are valid before accessing them
4. Handle Memory Limits: Be mindful of kernel memory constraints when copying arguments
5. Use Kernel Buffers: Copy arguments to kernel-space buffers for examination
6. Proper Error Handling: Return appropriate error codes to block execution when necessary

The Linux Journal article mentions the copy_strings function which is “in charge of copying argc strings, from the array argv into the pointer p” - this demonstrates the proper way to handle argument strings in kernel code.

Implementation Example

Here’s a basic implementation pattern for safely examining arguments in prepare_binprm:

#include <linux/binfmts.h>
#include <linux/slab.h>
#include <linux/string.h>
#include <linux/security.h>

static int my_prepare_binprm(struct linux_binprm *bprm)
{
    char *arg_copy;
    int i;
    int ret = 0;
    
    // Check if we should examine this binary
    if (!should_examine_binary(bprm->filename))
        return 0;
    
    // Allocate buffer for argument copying
    arg_copy = kmalloc(ARG_MAX, GFP_KERNEL);
    if (!arg_copy)
        return -ENOMEM;
    
    // Copy and examine each argument
    for (i = 0; i < bprm->argc; i++) {
        char *arg_ptr = get_arg_ptr(bprm, i);
        if (!arg_ptr) {
            ret = -EFAULT;
            break;
        }
        
        // Copy argument safely from user space
        if (strncpy_from_user(arg_copy, arg_ptr, ARG_MAX) < 0) {
            ret = -EFAULT;
            break;
        }
        
        // Examine argument for security patterns
        if (check_argument_security(arg_copy)) {
            ret = -EACCES;  // Block execution
            break;
        }
    }
    
    kfree(arg_copy);
    return ret;
}

Remember to:

• Always validate pointers and bounds
• Handle memory allocation failures properly
• Use appropriate error codes for different failure scenarios
• Consider performance implications for argument copying

The Linux kernel security bug documentation emphasizes that sensitive bugs should be coordinated properly, so ensure your security patches follow proper kernel development practices.

Sources

1. Linux Inside - How the Linux kernel runs a program
2. Linux binfmts.h header file
3. Linux Security Module Development
4. LWN - How programs get run
5. LWN - Kernel development
6. Playing with Binary Formats - Linux Journal
7. My Own Private Binary - Linux Kernel Modules
8. Linux Kernel Security Bugs
9. Stack Overflow - System call interception
10. Linux Kernel: linux_binprm Struct Reference

Conclusion

Implementing safe program argument interception in the Linux kernel can be effectively achieved through the prepare_binprm function or LSM hooks, both of which provide access to the comprehensive linux_binprm structure containing all execution arguments. For optimal security, consider using the bprm_check_security LSM hook which is specifically designed for security checks during program loading and receives the full argument structure as input. Always follow kernel security best practices including proper memory access, error handling, and timeout mechanisms to ensure your security patch is robust and doesn’t introduce vulnerabilities. The approach you choose should balance security effectiveness with system performance and stability, with LSM hooks generally providing the most maintainable and officially supported solution for kernel security enhancements.