Secure Programming Cookbook for C and C++

1.6. Creating a Child Process Securely

Problem

Your program needs to create a child process either to perform work within the same program or, more frequently, to execute another program.

Solution

On Unix, creating a child process is done by calling fork( ) . When fork( ) completes successfully, a nearly identical copy of the calling process is created as a new process. Most frequently, a new program is immediately executed using one of the exec*( ) family of functions (see Recipe 1.7). However, especially in the days before threading, it was common to use fork( ) to create separate “threads” of execution within a program.^[4]

If the newly created process is going to continue running the same program, any pseudo-random number generators (PRNGs) must be reseeded so that the two processes will each yield different random data as they continue to execute. In addition, any inherited file descriptors that are not needed should be closed; they remain open in the other process because the new process only has a copy of them.

Finally, if the original process had extra privileges from being executed as setuid or setgid, those privileges will be inherited by the new process, and they should be dropped immediately if they are not needed. In particular, if the new process is going to be used to execute a new program, privileges should always be dropped so that the new program does not inherit privileges that it should not have.

Discussion

When fork( ) is used to create a new process, the new ...