How to use the command 'audit2allow' (with examples)

SELinux, or Security-Enhanced Linux, is a mandatory access control (MAC) security mechanism integrated into the Linux kernel. The audit2allow command is an essential tool for managing and customizing SELinux policies. It is used to generate policy allow rules from audit logs that contain records of denied access attempts. This command simplifies the process of creating exceptions in SELinux by translating security audit logs into policy modules that can be loaded to modify the behavior of SELinux without altering the core security policies. However, caution is required as it may inadvertently permit more access than intended, so reviewing the generated policies is crucial.

Use case 1: Generate a local policy to allow access for all denied services

Code:

sudo audit2allow --all -M local_policy_name

Motivation:
System administrators often face situations where legitimate services or applications are being blocked by SELinux due to overly restrictive policies. By generating a local policy with audit2allow using the --all flag, administrators can quickly create a comprehensive policy module to address these denials, ensuring smoother operation of affected services.

Explanation:

• sudo: This command is run with superuser privileges because modifying SELinux policies requires administrative rights.
• audit2allow: This is the main command used to generate SELinux policy modules from audit logs.
• --all: This flag indicates that the tool should consider all denials found in the audit logs when generating the policy. It targets comprehensive policy creation rather than a specific process or application.
• -M local_policy_name: This option specifies that the output should be in the form of a module named local_policy_name, where policies can be easily managed and installed.

Example Output:
Upon execution, a .te (Type Enforcement) and a .pp (Policy Package) file named local_policy_name.te and local_policy_name.pp would be generated, containing all the necessary rules to allow access blocked before.

Use case 2: Generate a local policy module to grant access to a specific process/service/command from the audit logs

Code:

sudo grep apache2 /var/log/audit/audit.log | sudo audit2allow -M local_policy_name

Motivation:
When a specific service, like an Apache server, is being denied access by SELinux, pinpointing and resolving such issues without affecting the entire system’s security posture is essential. By targeting log entries related to apache2, administrators can refine their policies to minimize security risks while facilitating necessary service functions.

Explanation:

• sudo grep apache2 /var/log/audit/audit.log: This part of the command filters the audit logs to only display entries related to apache2, ensuring that only relevant denials are addressed.
• |: This pipe operator feeds the filtered log entries as input to the subsequent audit2allow command.
• sudo audit2allow -M local_policy_name: The audit2allow tool then processes these inputs to create a specific policy module. Here, -M local_policy_name defines the name for the generated module.

Example Output:
This process outputs the local_policy_name.te and local_policy_name.pp files customized to address the specific denials affecting the apache2 service.

Use case 3: Inspect and review the Type Enforcement (.te) file for a local policy

Code:

vim local_policy_name.te

Motivation:
Before applying any local policy changes, it’s crucial to review the generated Type Enforcement file. This ensures that the changes will only address the intended denials without introducing vulnerabilities by granting excessive permissions. By using a text editor like vim, administrators can thoroughly examine and understand the implications of the policy rules.

Explanation:

• vim: Launches the Vim text editor, a tool used for reviewing and editing files directly from the command line.
• local_policy_name.te: The file being opened, which contains the uncompiled policy rules that audit2allow generated, allowing for a detailed review.

Example Output:
You will see a set of policy rules defining the permissions that will be granted. Each entry corresponds to a specific denial and details how that access is to be allowed.

Use case 4: Install a local policy module

Code:

sudo semodule -i local_policy_name.pp

Motivation:
After confirming that the generated policy is appropriate and won’t compromise system security, administrators must install the policy to apply the changes system-wide. This is the final step to ensure that SELinux accommodates the necessary permissions for intended operations.

Explanation:

• sudo: Superuser privileges are required to install SELinux policies.
• semodule: This command manages SELinux modules, allowing for installation, removal, and listing of available policy modules.
• -i local_policy_name.pp: Specifies the installation (import) of the policy package with the given name, effectively updating SELinux.

Example Output:
The output may be minimal, but successful execution means the .pp policy module is now active, and services or applications affected by previous denials should function without interruption.

Conclusion:

The audit2allow command serves as a powerful utility in the SELinux toolkit, aiding administrators in customizing security policies according to the unique needs of their configurations. When used responsibly, it can significantly enhance system operability while maintaining robust security safeguards. Always remember to review generated policies thoroughly to avoid inadvertently opening security vulnerabilities.