Why does util-linux runuser (su) fork?

 The `runuser` or `su` command in the util-linux package is used for switching to another user account on a Unix-like system. When a user runs `su`, it often appears that the command forks or spawns a new process. This behavior has a fundamental purpose and is linked to the way Unix-like systems handle user authentication and privilege separation.


The process of forking in the context of `runuser` or `su` can be explained in detail:


1. **User Authentication:** When a user runs the `runuser` or `su` command, they typically want to execute commands or access resources with the privileges of a different user account. To do this, the system must perform user authentication to ensure that the user requesting the switch is allowed to become the target user.


2. **Password Verification:** Traditionally, the authentication process involves verifying the user's password. The user running `su` is asked to provide the password for the target user account. The system then needs to check if the provided password matches the stored password for the target account. This password verification is a critical security step.


3. **Forking a New Process:** To handle the password verification securely, `runuser` or `su` forks or creates a new process. This new process runs with the effective user ID of the target user account. The original process, which was running as the initiating user, continues to exist but with its privileges unchanged.


4. **Password Entry Mode:** The new process created after forking is in a mode where it can safely prompt the user for the password, and, if necessary, it can securely perform password checks without revealing sensitive information or affecting the original process.


5. **Privilege Separation:** This forking mechanism ensures privilege separation, meaning that the process responsible for password verification and switching to the target user runs with the minimal required privileges, while the original process remains with the privileges of the initiating user.


6. **Communication Between Processes:** The two processes communicate to complete the user switching operation. If the password is correctly verified, the new process continues execution with the privileges of the target user account, and it can execute the requested commands or access the desired resources.


The purpose of this forking behavior is to minimize potential security risks and vulnerabilities. It isolates the password verification process from the user's shell or the commands they want to execute. If password verification were handled within the same process, there would be a greater risk of an attacker exploiting vulnerabilities to gain unauthorized access or compromise the security of the system.


By forking a new process for authentication and user switching, `runuser` or `su` follows the principle of least privilege, which is a fundamental concept in computer security. It ensures that the process responsible for verifying the password and switching users has the minimum privileges required to perform these tasks and nothing more.


In conclusion, the forking behavior of `runuser` or `su` when switching to another user account is designed to enhance security by separating the process of password verification and user switching from the user's current session. It helps prevent security risks and vulnerabilities associated with privilege escalation and unauthorized access. This practice aligns with the core principles of Unix-like system security and access control.

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