Google Boosts Linux Security with Array Checks

Buffer overflows have been the cause of many security breaches by exploiting C's limitations in type safety and memory management. Realizing the impact of this persistent issue, Google has taken proactive steps to improve Linux security by introducing the counted_by attribute for flexible array members. This is a significant advancement in the ongoing fight against security vulnerabilities. To help you understand this initiative, I'll discuss this attribute in more detail, explore its significance in the realm of Linux security, and explain how you can use this attribute to improve the security of your Linux administration.

Introduction to the counted_by Attribute

The C language has a powerful feature that allows structures to include an array with variable size at the end of their structure. A compromise in security accompanies the flexibility, as their size can only be determined at runtime. This makes it difficult for the compilers to perform bounds checks. In the past, the alloc_size property of malloc() was used to perform local bounds checks within the same function. The allocated object did not carry the critical information needed for bounds checks, leaving a security hole when the array was accessed outside its allocation scope. 

The introduction of the counted_by attribute significantly reduces this drawback. This attribute explicitly links the flexible array to the field that stores the size of the array. This linkage empowers the array bounds sanitizer (enabled by -fsanitize=array-bounds) to accurately verify operations on flexible array members by utilizing the count field to ensure that array bounds are not exceeded. 

What Is the Significance of This Attribute for Linux Admins Looking to Improve Security?

This development has profound implications for Linux administrators. The counted_by feature is a powerful tool that can protect applications from various security flaws, including heap buffer overflows. By ensuring flexible array operations adhere to predefined bounds, the risk of exploits that rely on buffer overflows - such as unauthorized code execution and unauthorized access - is substantially reduced.

This innovation enhances the security of applications and the overall resilience and robustness of the Linux kernel. The kernel is the core of the Linux OS, so its security significantly impacts the security of countless systems and devices that run Linux.

How Can Admins Use This Feature to Improve Security?

Linux administrators can use the counted_by feature to improve security in several ways. Here's how:

• Code Review and Updates: Admins may initiate code reviews for existing codebases that contain flexible array members. If applicable, they can include the counted_by attribute in the code, which specifies the field that stores the array size. 
• Secure Coding Practices: Secure coding practices should include the counted_by attribute. When designing structures that have flexible array members, explicitly tying the array to its counter size can prevent potential vulnerabilities. 
• Fortification Efforts: The counted_by attribute is part of a larger effort to fortify Linux to prevent buffer overflows. By using this feature alongside other compiler-sanitization flags and fortification macros like _FORTIFY_SOURCE, administrators can harden systems against an array of attack vectors.
• Security Auditing: This attribute allows for a more comprehensive audit of security. Tools enabled by -fsanitize=array-bounds can now detect and report potentially unsafe operations involving flexible array members, facilitating early intervention.

Developers must follow specific rules to use the counted_by attribute effectively:

• The flexible array member must also be contained within the same nonanonymous struct.
• Before accessing an array, the count field must first be set.
• The array field should always have the same number of elements.
• The number of elements in the count field can be changed but must not exceed that assigned initially.

Our Final Thoughts on Google's Efforts to Bolster Linux Security

Google's counted_by feature is a significant step in securing Linux environments. This attribute provides a robust method for checking the bounds of flexible array members and addresses a vulnerability in the C programming language. This feature is a must-have for Linux administrators who want to protect their systems from the constant threat of buffer overflows. In the future, collaboration between developers and administrators to implement features like counted_bywill be essential to maintaining the integrity and resilience of our digital infrastructure.