Pluggable Authentication Modules
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# A. Sample PAM Application

The following is a minimal implementation of su(1) using PAM. Note that it uses the OpenPAM-specific openpam_ttyconv(3) conversation function, which is prototyped in `security/openpam.h`. If you wish build this application on a system with a different PAM library, you will have to provide your own conversation function. A robust conversation function is surprisingly difficult to implement; the one presented in the Sample PAM Conversation Function appendix is a good starting point, but should not be used in real-world applications.

    #include <sys/param.h>
    #include <sys/wait.h>

    #include <err.h>
    #include <pwd.h>
    #include <stdio.h>
    #include <stdlib.h>
    #include <string.h>
    #include <syslog.h>
    #include <unistd.h>

    #include <security/pam_appl.h>
    #include <security/openpam.h>   /* for openpam_ttyconv() */

    extern char **environ;

    static pam_handle_t *pamh;
    static struct pam_conv pamc;

    static void
    usage(void)
    {

        fprintf(stderr, "Usage: su [login [args]]\n");
        exit(1);
    }

    int
    main(int argc, char *argv[])
    {
        char hostname[MAXHOSTNAMELEN];
        const char *user, *tty;
        char **args, **pam_envlist, **pam_env;
        struct passwd *pwd;
        int o, pam_err, status;
        pid_t pid;

        while ((o = getopt(argc, argv, "h")) != -1)
            switch (o) {
            case 'h':
            default:
                usage();
            }

        argc -= optind;
        argv += optind;

        if (argc > 0) {
            user = *argv;
            --argc;
            ++argv;
        } else {
            user = "root";
        }

        /* initialize PAM */
        pamc.conv = &openpam_ttyconv;
        pam_start("su", user, &pamc, &pamh);

        /* set some items */
        gethostname(hostname, sizeof(hostname));
        if ((pam_err = pam_set_item(pamh, PAM_RHOST, hostname)) != PAM_SUCCESS)
            goto pamerr;
        user = getlogin();
        if ((pam_err = pam_set_item(pamh, PAM_RUSER, user)) != PAM_SUCCESS)
            goto pamerr;
        tty = ttyname(STDERR_FILENO);
        if ((pam_err = pam_set_item(pamh, PAM_TTY, tty)) != PAM_SUCCESS)
            goto pamerr;

        /* authenticate the applicant */
        if ((pam_err = pam_authenticate(pamh, 0)) != PAM_SUCCESS)
            goto pamerr;
        if ((pam_err = pam_acct_mgmt(pamh, 0)) == PAM_NEW_AUTHTOK_REQD)
            pam_err = pam_chauthtok(pamh, PAM_CHANGE_EXPIRED_AUTHTOK);
        if (pam_err != PAM_SUCCESS)
            goto pamerr;

        /* establish the requested credentials */
        if ((pam_err = pam_setcred(pamh, PAM_ESTABLISH_CRED)) != PAM_SUCCESS)
            goto pamerr;

        /* authentication succeeded; open a session */
        if ((pam_err = pam_open_session(pamh, 0)) != PAM_SUCCESS)
            goto pamerr;

        /* get mapped user name; PAM may have changed it */
        pam_err = pam_get_item(pamh, PAM_USER, (const void **)&user);
        if (pam_err != PAM_SUCCESS || (pwd = getpwnam(user)) == NULL)
            goto pamerr;

        /* export PAM environment */
        if ((pam_envlist = pam_getenvlist(pamh)) != NULL) {
            for (pam_env = pam_envlist; *pam_env != NULL; ++pam_env) {
                putenv(*pam_env);
                free(*pam_env);
            }
            free(pam_envlist);
        }

        /* build argument list */
        if ((args = calloc(argc + 2, sizeof *args)) == NULL) {
            warn("calloc()");
            goto err;
        }
        *args = pwd->pw_shell;
        memcpy(args + 1, argv, argc * sizeof *args);

        /* fork and exec */
        switch ((pid = fork())) {
        case -1:
            warn("fork()");
            goto err;
        case 0:
            /* child: give up privs and start a shell */

            /* set uid and groups */
            if (initgroups(pwd->pw_name, pwd->pw_gid) == -1) {
                warn("initgroups()");
                _exit(1);
            }
            if (setgid(pwd->pw_gid) == -1) {
                warn("setgid()");
                _exit(1);
            }
            if (setuid(pwd->pw_uid) == -1) {
                warn("setuid()");
                _exit(1);
            }
            execve(*args, args, environ);
            warn("execve()");
            _exit(1);
        default:
            /* parent: wait for child to exit */
            waitpid(pid, &status, 0);

            /* close the session and release PAM resources */
            pam_err = pam_close_session(pamh, 0);
            pam_end(pamh, pam_err);

            exit(WEXITSTATUS(status));
        }

    pamerr:
        fprintf(stderr, "Sorry\n");
    err:
        pam_end(pamh, pam_err);
        exit(1);
    }

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