Driver Security Best Practices

Developing secure drivers is paramount to the stability and security of the Windows operating system. This document outlines essential best practices for driver developers to minimize vulnerabilities and protect the system from malicious attacks.

1. Input Validation and Sanitization

Never trust data coming from user mode or other kernel-mode components without proper validation. Assume all input is potentially malicious.

• Validate all buffer sizes: Ensure that any buffer received from user mode is large enough to hold the data and does not exceed system-defined limits or driver-specific allocations.
• Validate pointers: Use kernel-mode functions like ProbeForRead, ProbeForWrite, and MmIsValidMdlSequence to ensure pointers are valid and point to accessible memory.
• Sanitize data: Remove or neutralize any potentially harmful characters or sequences from input data before processing it.

2. Secure Memory Management

Proper memory management is critical to prevent memory corruption and leaks.

• Minimize kernel pool allocations: Frequent and large allocations can lead to memory fragmentation and denial-of-service conditions.
• Use appropriate memory pools: Use NonPagedPoolNx for data that must remain resident in physical memory and cannot be paged out, and PagedPoolNx for data that can be paged out. Avoid using NonPagedPool directly unless absolutely necessary and with careful consideration.
• Zero-initialize allocated memory: Always initialize allocated memory to zero to prevent leakage of sensitive information from previous uses.
• Handle exceptions gracefully: Implement robust error handling and exception handling mechanisms to catch and recover from memory access violations.

3. Principle of Least Privilege

Drivers should operate with the minimum privileges necessary to perform their intended functions.

• Avoid unnecessary elevation: Do not run code in kernel mode that could be safely executed in user mode.
• Limit access to sensitive kernel structures: Restrict direct access to critical operating system data structures unless absolutely required.
• Use I/O Control (IOCTL) codes carefully: Define IOCTLs with appropriate access control checks in user-mode applications.

4. Secure Communication with User Mode

All communication between kernel mode (drivers) and user mode must be secured.

• Use well-defined IOCTLs: Design and implement IOCTLs that have clearly defined inputs, outputs, and error codes.
• Validate IOCTL parameters: Perform rigorous validation of all parameters passed through IOCTLs.
• Avoid passing sensitive information directly: If sensitive information needs to be transferred, consider secure channels or encryption.
• Be mindful of access masks: Ensure that user-mode applications have the necessary access rights to call your driver's IOCTLs.

5. Error Handling and Logging

Robust error handling and logging are essential for debugging and security auditing.

• Handle all potential errors: Ensure that your driver handles all possible error conditions, including device errors, memory errors, and communication errors.
• Log security-relevant events: Log critical events such as failed access attempts or security policy violations. Use the Windows Event Log for this purpose.
• Avoid exposing sensitive information in logs: Ensure that logs do not contain passwords, keys, or other personally identifiable information.

6. Driver Signing and Verification

All drivers must be signed with a valid digital signature. Windows enforces driver signature verification to ensure that drivers are authentic and have not been tampered with.

• Obtain a Windows Hardware Developer Program (WHCP) certificate: This is required for submitting drivers to the Windows Hardware Compatibility program and for obtaining a certificate for driver signing.
• Sign your driver package: Use tools like SignTool.exe to sign your driver binaries and catalog files.
• Test signature validation: Ensure your driver installs correctly on systems with enforcement enabled.

7. Secure Coding Practices

Adhere to general secure coding principles.

• Follow coding standards: Use consistent and clear coding styles.
• Minimize complexity: Simpler code is easier to review and less prone to errors.
• Use static analysis tools: Employ tools like PREfast for Drivers to identify potential bugs and security vulnerabilities early in the development cycle.
• Conduct code reviews: Have experienced developers review your code for potential security issues.
• Be aware of compiler warnings: Treat all compiler warnings as errors and fix them before releasing your driver.

8. Handling User-Mode Exceptions

Drivers might need to handle exceptions originating from user-mode code.

• Use __try/__except blocks: These constructs are crucial for catching exceptions originating from user-mode code accessing driver-provided memory or services.
• Validate data within exception handlers: Even within an exception handler, it's good practice to re-validate any critical data before acting upon it.

9. Device Security

Consider the security implications of the hardware your driver controls.

• Secure device configurations: Ensure that device configurations are stored securely and are not easily accessible by unauthorized applications.
• Implement access controls for hardware features: If your hardware has sensitive features, ensure they are protected by appropriate access control mechanisms.