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);
}