Security Policy Unit Testing

In this lesson, you will learn how to verify your security policies by writing automated unit tests that programmatically assert specific access grants and denials.

Why Unit Test Security Policies?

As SELinux policies grow in complexity, manually checking for vulnerabilities becomes error-prone. "Policy Unit Testing" allows you to treat your security rules like code, ensuring that every time you recompile your module, your intended constraints remain intact. By utilizing tools like sepolgen or custom scripts that interface with the libselinux library, we can simulate policy decisions without needing to deploy the module to a production kernel.

Asserting Access with sesearch

The most common way to test a policy is to query the compiled binary policy for specific rules. If you have defined a custom domain, you should write a test script that ensures your domain is denied access to sensitive objects.

For example, if you want to ensure your web_t domain can never execute files labeled as shadow_t, you can write a simple test script:

# Test Script: test_web_policy.sh
# Assert: web_t should NOT have read access to shadow_t
if sesearch -s web_t -t shadow_t -c file -p read /sys/fs/selinux/policy | grep -q "allow"; then
    echo "FAIL: web_t has forbidden read access to shadow_t"
    exit 1
else
    echo "PASS: Access correctly denied."
fi

Testing Explicit Grants

Unit tests are equally vital for ensuring functional requirements. If you have defined an interface allowing your app_t domain to write to log_t files, your test suite should assert that this permission exists:

# Assert: app_t must have write access to log_t
sesearch -s app_t -t log_t -c file -p write /sys/fs/selinux/policy | grep -q "allow"
if [ $? -eq 0 ]; then
    echo "PASS: Required write access exists."
else
    echo "FAIL: app_t missing required write access to log_t"
    exit 1
fi

By integrating these checks into your deployment pipeline, you move from "hoping" your policy is secure to "proving" it through automated verification. These tests catch regressions where a developer might accidentally introduce an allow rule that violates the principle of least privilege.

Key Takeaway: Security Policy Unit Testing turns abstract access requirements into deterministic, automated assertions, ensuring your policy remains compliant throughout its lifecycle.