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/*
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us - User Switcher
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Copyright (C) 2021 Alessandro Mauri
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This program is free software: you can redistribute it and/or modify
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it under the terms of the GNU General Public License version 3.
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This program is distributed in the hope that it will be useful,
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but WITHOUT ANY WARRANTY; without even the implied warranty of
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MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
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GNU General Public License for more details.
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You should have received a copy of the GNU General Public License
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along with this program. If not, see <https://www.gnu.org/licenses/>.
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e-mail: alemauri001@tuta.io
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*/
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#define _POSIX_C_SOURCE 200809L
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#define _DEFAULT_SOURCE
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#include <sys/types.h>
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#include <stdio.h>
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#include <stdlib.h>
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#include <string.h>
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#include <errno.h>
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#include <pwd.h>
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#include <unistd.h>
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#include <limits.h>
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#include <grp.h>
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#include <security/pam_appl.h>
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#include <security/pam_misc.h>
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static void usage (void);
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static int perm_set (struct passwd *, struct group *);
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static int authenticate (const char *);
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static struct passwd* user_to_passwd (const char *);
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static struct group* group_to_grp (const char *);
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//static int execvpe(const char *, char *const *, char *const *);
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// FIXME: misc_conv is a separate library, should stick to plain PAM or make
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// our own pam module
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static struct pam_conv conv = {misc_conv, NULL};
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extern char **environ;
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int main (int argc, char *argv[])
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{
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char *t_usr = "root", *t_grp = NULL;
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struct passwd *t_pw;
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struct group *t_gr;
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int opt, err;
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int shellflag = 0, envflag = 0;
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while ((opt = getopt(argc, argv, "A:u:g:C:se")) != -1) {
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switch (opt) {
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case 'A':
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printf("-A is not yet implemented\n");
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exit(EXIT_FAILURE);
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break;
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case 'u':
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t_usr = optarg;
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break;
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case 'g':
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t_grp = optarg;
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break;
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case 'C':
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printf("-C is not yet implemented\n");
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exit(EXIT_FAILURE);
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break;
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case 's':
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shellflag = 1;
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break;
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case 'e':
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envflag = 1;
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break;
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case '?':
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usage();
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exit(EINVAL);
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break;
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}
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}
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/* Get user info */
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const char *uname;
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char *shell;
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uid_t ruid = getuid();
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struct passwd *my_pw = getpwuid(ruid);
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if (!my_pw) {
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fprintf(stderr, "getpwid: %s\n", strerror(errno));
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return errno;
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}
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uname = my_pw->pw_name;
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/* Authenticate */
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if (authenticate(uname) != PAM_SUCCESS)
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exit(EXIT_FAILURE);
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/* Get target user and group info */
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t_pw = user_to_passwd(t_usr);
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if (!t_pw) {
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fprintf(stderr, "user_to_passwd: %s\n", strerror(errno));
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return errno;
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}
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t_gr = group_to_grp(t_grp);
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/* Get target user's shell */
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if (!shellflag)
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shell = t_pw->pw_shell;
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else
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shell = getenv("SHELL");
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if (!shell)
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shell = "/bin/sh";
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/* Set argc and argv */
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int c_argc = argc - optind;
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char **c_argv;
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if (c_argc) {
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c_argv = malloc(sizeof(char *) * (c_argc + 1));
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if (!c_argv) {
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fprintf(stderr, "malloc: %s\n", strerror(errno));
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exit(errno);
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}
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for (int i = 0; optind < argc; optind++, i++) {
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c_argv[i] = strdup(argv[optind]);
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if (!c_argv[i]) {
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fprintf(stderr, "strdup: %s\n", strerror(errno));
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exit(errno);
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}
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}
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} else {
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c_argc = 1;
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c_argv = malloc(sizeof(char *) * (c_argc + 1));
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if (!c_argv) {
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fprintf(stderr, "malloc: %s\n", strerror(errno));
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exit(errno);
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}
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c_argv[0] = strdup(shell);
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if (!c_argv[0]) {
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fprintf(stderr, "strdup: %s\n", strerror(errno));
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exit(errno);
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}
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}
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c_argv[c_argc] = NULL;
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struct env_elem {
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char *name;
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char *value;
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};
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struct env_elem env_keep[] = {
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{"PATH", NULL},
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{"TERM", NULL},
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{"EDITOR", NULL},
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{"VISUAL", NULL},
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{"DISPLAY", NULL},
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{"XAUTHORITY", NULL},
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{NULL, NULL}
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};
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struct env_elem env_mod[] = {
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{"USER", t_pw->pw_name},
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{"LOGNAME", t_pw->pw_name},
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{"SHELL", t_pw->pw_shell},
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{"HOME", t_pw->pw_dir},
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{NULL, NULL}
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};
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if (envflag) { /* clear env */
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for (int i = 0; env_keep[i].name; i++)
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env_keep[i].value = strdup(getenv(env_keep[i].name));
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environ = NULL; // in place of clearenv
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}
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for (int i = 0; env_mod[i].name; i++) {
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err = setenv(env_mod[i].name, env_mod[i].value, 1);
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if (err == -1) {
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fprintf(stderr, "setenv: %s\n", strerror(errno));
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goto fail_end;
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}
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}
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if (envflag) {
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for (int i = 0; env_keep[i].name; i++) {
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if (env_keep[i].value) {
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err = setenv(env_keep[i].name, env_keep[i].value, 1);
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if (err == -1) {
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fprintf(stderr, "setenv: %s\n", strerror(errno));
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goto fail_end;
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}
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}
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}
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}
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// do not override, we might be under more levels of 'us'
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err = setenv("US_USER", my_pw->pw_name, 0);
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errno = 0;
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/* Set permissions */
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if (perm_set(t_pw, t_gr) == -1) {
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fprintf(stderr, "perm_set: %s\n", strerror(errno));
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goto fail_end;
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}
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/* Execute the command */
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err = execvp(c_argv[0], c_argv);
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if (err == -1)
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fprintf(stderr, "execl: %s\n", strerror(errno));
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/* Cleanup and return */
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fail_end:
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/* Free up the copied argv */
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for (int i=0; c_argv[i]; i++)
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free(c_argv[i]);
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free(c_argv);
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return errno;
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}
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static inline void usage (void)
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{
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// TODO: planned options
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// -a [program]: like sudo's askpass
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// -u [user]: change the default user from root to user
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// -g [group]: change the primary group to [gorup]
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// both -a and -g will accept numbers with #[num] like sudo
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// -c [file]: manually select config file
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// something about environment
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// something about non interactiveness
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printf("usage: us [-se] [-u user] [-g group] command [args]\n");
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}
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static int perm_set (struct passwd *pw, struct group *gr)
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{
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if (!pw) {
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errno = EINVAL;
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return -1;
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}
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uid_t uid;
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gid_t gid;
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uid = pw->pw_uid;
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gid = pw->pw_gid;
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if (gr)
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gid = gr->gr_gid;
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/* Set permissions, setting group perms first because in the case of
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* dropping from higher permissions setting the uid first results in
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* an error */
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int err;
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/* Non POSIX but implemented in most systems anyways */
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err = initgroups(pw->pw_name, pw->pw_gid);
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if (err == -1) {
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printf("initgroups failed\n");
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return -1;
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}
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if (setregid(gid, gid) == -1) {
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printf("setregid failed\n");
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return -1;
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}
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if (setreuid(uid, uid) == -1) {
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printf("setreuid failed\n");
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return -1;
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}
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return 0;
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}
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static int authenticate (const char *uname)
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{
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pam_handle_t *pamh;
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int pam_err, count = 0;
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pam_err = pam_start("User Switcher", uname, &conv, &pamh);
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if (pam_err != PAM_SUCCESS) {
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fprintf(stderr, "pam_start: %s\n", pam_strerror(pamh, pam_err));
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return pam_err;
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}
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do {
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pam_err = pam_authenticate(pamh, 0);
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if (pam_err != PAM_SUCCESS)
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printf("Auth failed: %s\n", pam_strerror(pamh, pam_err));
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// FIXME: count gets ignored because authentication service has
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// a set amount of retries giving an error:
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// Have exhausted maximum number of retries for service
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count++;
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} while (pam_err != PAM_SUCCESS && count < 4);
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if (pam_err != PAM_SUCCESS) {
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fprintf(stderr, "better luck next time\n");
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pam_end(pamh, pam_err);
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return pam_err;
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}
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// FIXME: check again for the validity of the login for more security
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// as in: https://docs.oracle.com/cd/E19120-01/open.solaris/819-2145/pam-20/index.html
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// FIXME: ^C [SIGINT] will interrupt this call possibly causing a
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// vulnerability
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return pam_end(pamh, pam_err);
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}
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static struct passwd* user_to_passwd (const char *user)
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{
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if (!user) {
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errno = EINVAL;
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return NULL;
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}
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struct passwd* pw;
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long uid_l;
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errno = 0;
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if (user[0] != '#') {
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pw = getpwnam(user);
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} else {
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uid_l = strtol(&user[1], NULL, 10);
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if (uid_l < 0 || errno) {
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errno = errno ? errno : EINVAL;
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return NULL;
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}
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pw = getpwuid((uid_t)uid_l);
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}
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if (!pw) {
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if (!errno)
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errno = EINVAL;
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return NULL;
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}
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return pw;
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}
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static struct group* group_to_grp (const char *group)
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{
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if (!group) {
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errno = EINVAL;
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return NULL;
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}
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struct group* gr;
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long gid_l;
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errno = 0;
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if (group[0] != '#') {
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gr = getgrnam(group);
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} else {
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gid_l = strtol(&group[1], NULL, 10);
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if (gid_l < 0 || errno) {
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errno = errno ? errno : EINVAL;
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return NULL;
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}
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gr = getgrgid((gid_t)gid_l);
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}
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if (!gr) {
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if (!errno)
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errno = EINVAL;
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return NULL;
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}
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return gr;
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}
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