sec-design-deep-analysis.md

Okay, let's dive deep into the security analysis of KernelSU, based on the provided design review and the GitHub repository.

1. Objective, Scope, and Methodology

Objective:

The primary objective of this deep analysis is to perform a thorough security assessment of KernelSU's key components, identifying potential vulnerabilities, attack vectors, and weaknesses in its design and implementation. This includes analyzing the interaction between the KernelSU Manager App, the SU Daemon, the KernelSU Module, and the Android operating system itself. The goal is to provide actionable recommendations to improve KernelSU's security posture and mitigate identified risks.

Scope:

This analysis will focus on the following key components of KernelSU:

• KernelSU Module (Kernel Space): This is the core of the system, and its security is paramount. We'll examine its system call interception, permission enforcement, and overall interaction with the Android kernel.
• SU Daemon (User Space): This daemon handles root requests from applications. We'll analyze its communication with the kernel module, its access control mechanisms, and its vulnerability to user-space attacks.
• KernelSU Manager App (User Space): This app provides the user interface for managing KernelSU. We'll examine its authentication to the SU Daemon, its configuration handling, and its potential for privilege escalation.
• Installation/Update Mechanism (Recovery Mode Flashing): We'll analyze the security of the installation process, focusing on the integrity and authenticity of the distributed binaries.
• Build Process (GitHub Actions): We'll examine the build pipeline for potential vulnerabilities and ensure secure build practices.
• Interaction with SELinux: We'll assess how KernelSU interacts with SELinux and whether it maintains or compromises the security provided by SELinux.

Methodology:

1. Architecture and Data Flow Inference: Based on the provided C4 diagrams, documentation, and (if necessary) code snippets from the GitHub repository, we will infer the detailed architecture, data flow, and component interactions within KernelSU.
2. Threat Modeling: We will identify potential threats and attack vectors against each component, considering the business priorities, accepted risks, and existing security controls. We'll use a combination of STRIDE (Spoofing, Tampering, Repudiation, Information Disclosure, Denial of Service, Elevation of Privilege) and other relevant threat modeling techniques.
3. Vulnerability Analysis: We will analyze each component for potential vulnerabilities based on common kernel and user-space exploitation techniques. This includes, but is not limited to:
   • Kernel Module: Race conditions, use-after-free, null pointer dereferences, integer overflows, out-of-bounds reads/writes, improper access control, information leaks.
   • SU Daemon: Command injection, buffer overflows, privilege escalation, insecure communication, logic flaws.
   • Manager App: Insecure storage of sensitive data, improper permission handling, intent spoofing, UI manipulation.
   • Installation Process: Man-in-the-middle attacks, malicious image substitution, downgrade attacks.
4. Mitigation Strategy Recommendation: For each identified vulnerability or weakness, we will provide specific, actionable, and tailored mitigation strategies that can be implemented by the KernelSU developers.

2. Security Implications of Key Components

Let's break down the security implications of each component:

2.1 KernelSU Module (Kernel Space)

• System Call Interception: This is a critical area. KernelSU likely intercepts system calls related to process execution (e.g., execve), file access, and potentially others to enforce its root access policies.
  • Threats: Incorrectly handling system calls can lead to race conditions, allowing malicious apps to bypass security checks. Bugs in the interception logic can lead to kernel panics (DoS) or arbitrary code execution (privilege escalation). Improper validation of system call arguments can lead to various kernel exploits.
  • Mitigation: Thorough input validation of all system call arguments. Use of appropriate locking mechanisms to prevent race conditions. Extensive testing, including fuzzing, of the system call interception logic. Employ kernel hardening techniques like KASLR, KPTI, and CFI (if supported by the target kernel).
• Permission Enforcement: The module must enforce the root access policies defined by the SU Daemon.
  • Threats: Logic errors in the permission enforcement mechanism could allow unauthorized root access. Bypassing the checks could lead to complete system compromise.
  • Mitigation: Implement a clear and well-defined permission model. Use a whitelist approach (deny by default, explicitly allow). Regularly audit the permission enforcement code. Consider using formal verification techniques if feasible.
• Interaction with Android Kernel: The module interacts directly with the kernel's internal structures and functions.
  • Threats: Incorrectly interacting with kernel structures can lead to memory corruption, instability, and vulnerabilities. Exploiting existing kernel vulnerabilities through the module is a significant risk.
  • Mitigation: Minimize the module's footprint and complexity. Adhere strictly to kernel coding best practices. Keep the module up-to-date with the latest kernel security patches. Use memory protection techniques (e.g., CONFIG_HARDENED_USERCOPY).
• Module Loading and Unloading:
  • Threats: Improper handling during module loading or unloading can lead to use-after-free vulnerabilities or leave the system in an inconsistent state.
  • Mitigation: Ensure proper resource cleanup during unloading. Use reference counting to prevent premature freeing of resources.

2.2 SU Daemon (User Space)

• Communication with Kernel Module: This communication channel is a prime target for attackers.
  • Threats: Man-in-the-middle attacks, injection of malicious data, denial-of-service attacks.
  • Mitigation: Use a secure communication channel, such as a custom ioctl interface with strict input validation. Implement a robust protocol with authentication and integrity checks. Consider using Binder with appropriate permissions and SELinux context.
• Access Control Mechanisms: The daemon enforces the whitelist/blacklist of applications allowed root access.
  • Threats: Logic flaws in the access control logic could allow unauthorized applications to gain root. Bypassing the daemon's checks would grant root access.
  • Mitigation: Use a simple and well-defined access control model (whitelist preferred). Store the configuration securely (e.g., encrypted, with appropriate file permissions). Regularly audit the access control code.
• Vulnerability to User-Space Attacks: The daemon runs in user space and is susceptible to common user-space vulnerabilities.
  • Threats: Buffer overflows, command injection, format string vulnerabilities, etc.
  • Mitigation: Use secure coding practices. Validate all inputs from applications and the Manager App. Use memory-safe languages (e.g., Rust) if feasible. Employ ASLR and DEP/NX.

2.3 KernelSU Manager App (User Space)

• Authentication to SU Daemon: The app needs to authenticate with the daemon to make configuration changes.
  • Threats: Weak authentication could allow malicious apps to modify the root access configuration.
  • Mitigation: Use a strong authentication mechanism, such as a challenge-response protocol or Binder with appropriate permissions and SELinux context. Avoid storing credentials insecurely.
• Configuration Handling: The app manages the root access configuration (whitelist/blacklist).
  • Threats: Improper handling of the configuration could lead to unauthorized root access or denial of service.
  • Mitigation: Store the configuration securely (e.g., encrypted, with appropriate file permissions). Validate all user inputs to prevent malicious configurations.
• Privilege Escalation: The app itself might have vulnerabilities that could be exploited to gain higher privileges.
  • Threats: Intent spoofing, UI manipulation, insecure storage of data.
  • Mitigation: Follow Android's security best practices for app development. Use appropriate permissions and protect sensitive data. Implement robust input validation.

2.4 Installation/Update Mechanism (Recovery Mode Flashing)

• Integrity and Authenticity of Binaries: This is crucial to prevent attackers from distributing malicious versions of KernelSU.
  • Threats: Man-in-the-middle attacks during download. Substitution of the official image with a malicious one.
  • Mitigation: Use HTTPS for downloads. Provide checksums (e.g., SHA-256) for users to verify the integrity of the downloaded image. Digitally sign the image and verify the signature in recovery mode (if supported by the recovery). Consider using a trusted distribution channel (e.g., a dedicated website with HTTPS).
• Downgrade Attacks:
  • Threats: An attacker could trick the user into installing an older, vulnerable version of KernelSU.
  • Mitigation: Include version information in the image and check it during installation. Reject installation of older versions.

2.5 Build Process (GitHub Actions)

• Controlled and Automated Environment: GitHub Actions provides a good foundation for secure builds.
  • Threats: Compromise of the build environment. Injection of malicious code during the build process. Use of vulnerable dependencies.
  • Mitigation: Regularly review and update the GitHub Actions workflow. Use dependency scanning tools to identify and mitigate vulnerable dependencies. Implement code signing as part of the build process. Use static analysis tools to detect potential vulnerabilities. Restrict access to the GitHub repository and require multi-factor authentication.

2.6 Interaction with SELinux

• SELinux Compatibility: KernelSU aims to be compatible with SELinux.
  • Threats: Incorrectly interacting with SELinux could weaken its security guarantees or lead to system instability. Bypassing SELinux policies would significantly increase the attack surface.
  • Mitigation: Define specific SELinux policies for KernelSU components (daemon, module, manager app). Ensure that KernelSU does not disable or bypass SELinux. Thoroughly test KernelSU with SELinux in enforcing mode. Use audit2allow to generate policies based on observed behavior, but carefully review the generated policies.

3. Actionable Mitigation Strategies (Summary and Prioritization)

Here's a prioritized summary of the most critical mitigation strategies:

High Priority (Must Implement):

1. Kernel Module Hardening:
   • Thorough input validation for all system call arguments.
   • Use of appropriate locking mechanisms to prevent race conditions.
   • Extensive testing (including fuzzing) of system call interception.
   • Employ kernel hardening techniques (KASLR, KPTI, CFI, CONFIG_HARDENED_USERCOPY).
   • Strict adherence to kernel coding best practices.
   • Regular security audits and code reviews.
2. Secure Communication (Kernel Module <-> SU Daemon):
   • Use a custom ioctl interface with strict input validation.
   • Implement a robust protocol with authentication and integrity checks.
   • Consider using Binder with appropriate permissions and SELinux context.
3. Secure Installation:
   • Use HTTPS for downloads.
   • Provide and verify checksums (SHA-256).
   • Digitally sign the image and verify the signature in recovery (if supported).
4. SU Daemon Access Control:
   • Whitelist approach (deny by default).
   • Secure storage of configuration (encrypted, proper file permissions).
5. SELinux Integration:
   • Define specific SELinux policies for all KernelSU components.
   • Ensure KernelSU does not disable or bypass SELinux.
   • Thorough testing with SELinux in enforcing mode.

Medium Priority (Strongly Recommended):

1. Manager App Security:
   • Strong authentication to the SU Daemon.
   • Secure storage of configuration.
   • Robust input validation.
   • Follow Android security best practices.
2. Build Process Security:
   • Dependency scanning and mitigation.
   • Code signing as part of the build.
   • Static analysis tools.
   • Restrict repository access and require MFA.
3. Kernel Module - Minimize Footprint:
   • Reduce complexity and code size.
   • Keep up-to-date with kernel security patches.
4. Downgrade attack prevention:
   • Version information included and checked.
   • Rejection of older versions installation.

Low Priority (Consider for Enhanced Security):

1. Formal Verification: Consider using formal verification techniques for critical parts of the kernel module.
2. Memory-Safe Languages: Explore using Rust for the SU Daemon or parts of the kernel module.
3. User-Confirmation Dialogs: Implement user-confirmation dialogs for granting root access (can be optional).
4. Integration with SafetyNet: Investigate ways to minimize the impact on SafetyNet (challenging, but beneficial).

This deep analysis provides a comprehensive overview of the security considerations for KernelSU. By implementing these mitigation strategies, the developers can significantly improve the security and robustness of their project, protecting users from potential threats and ensuring the long-term viability of KernelSU as a secure rooting solution. Continuous security review and updates are essential to maintain a strong security posture in the face of evolving threats.