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The program must follow the principle of least privilege while carefully separating the binding and bookkeeping tasks. To minimize the chance of a flaw in the program from compromising the superuser level account, it must drop superuser privileges as soon as the privileged operations are completed. In the code shown below, privileges are dropped permanently as soon as the bind() operation is carried out. The code also ensures privileges may not be regained after being permanently dropped, as per POS37-C. Ensure that privilege relinquishment is successful.
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/* Code with elevated privileges */
int establish(void) {
/* This will store the listening socket's address */
struct sockaddr_in sa;
/* This will hold the listening socket */
int s;
/* Fill up the structure with address and port number */
sa.sin_port = htons(portnum);
/* Other system calls like socket() */
if (bind(
s,
(struct sockaddr *) &sa,
sizeof(struct sockaddr_in)) < 0)
{
/* Perform cleanup */
}
/* Return */
}
int main(void) {
int s = establish();
/* Drop privileges permanently */
if (setuid(getuid()) == -1) {
/* Handle the error */
}
if (setuid(0) != -1) {
/* Privileges can be restored, handle error */
}
/* Block with accept() until a client connects */
switch (fork()) {
case -1: /* Error, clean up and quit */
case 0: /* Close all open file descriptors
* This is the child, handle the client
*/
default: /* This is the parent, continue blocking */
}
}
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