Debugging Policy Transitions

In this lesson, you will learn how to identify and resolve failures that occur when a process attempts to change its security domain during execution.

Understanding Domain Transitions

In Mandatory Access Control (MAC) systems like SELinux, a domain transition occurs when a process execution call (e.g., execve) triggers a switch from its current domain to a new, more specific domain. This is how a web server, initially running in a generic init_t domain, "transitions" into a restricted httpd_t domain.

A failure here means the security policy does not contain a valid type_transition rule for the combination of the current domain and the file being executed. When this happens, the process fails to gain its intended security context, often resulting in "Permission Denied" errors or the process staying trapped in an overly permissive or incorrect domain.

Investigating the Failure

When a process fails to transition, your first step is to consult the audit logs. If the policy lacks the necessary transition rule, the kernel will log an avc: denied message. You need to look for scontext (source context) and tcontext (target context of the executable file).

Consider this example of an audit log entry:

type=AVC msg=audit(1625091234.567:101): avc: denied { transition } 
for scontext=system_u:system_r:init_t:s0 tcontext=system_u:object_r:web_exec_t:s0 
tclass=process permissive=0

This log tells you that the init_t domain is being denied the right to transition into the domain associated with web_exec_t. To debug this, verify three things:

1. Rule Existence: Does a type_transition rule exist in your policy source linking the source type (init_t) and the executable type (web_exec_t)?
2. Permission Bits: Does the source domain have the process { transition } permission allowed for the target domain?
3. Entrypoint: Does the target domain have an entrypoint permission rule defined for the file type web_exec_t?

If any of these links are missing, the process will fail to shift domains. Use seinfo -x or search your policy modules to ensure that the transition logic is explicitly defined for both the source domain and the executable file.

Key Takeaway: Debugging a transition requires confirming that both the type_transition rule exists in the policy and that the source domain has explicit permission to transition into the target domain's entry point.