us/us.c

/*
    us - User Switcher
    Copyright (C) 2021  Alessandro Mauri

    This program is free software: you can redistribute it and/or modify
    it under the terms of the GNU General Public License version 3.

    This program is distributed in the hope that it will be useful,
    but WITHOUT ANY WARRANTY; without even the implied warranty of
    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
    GNU General Public License for more details.

    You should have received a copy of the GNU General Public License
    along with this program.  If not, see <https://www.gnu.org/licenses/>.

    e-mail: alemauri001@tuta.io
*/

#define _POSIX_C_SOURCE 200809L
#define _DEFAULT_SOURCE

#include <sys/types.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <errno.h>
#include <pwd.h>
#include <unistd.h>
#include <limits.h>
#include <grp.h>
#include <security/pam_appl.h>
#include <security/pam_misc.h>

static void usage (void);
static int perm_set (struct passwd *, struct group *);
static int authenticate (const char *);
static struct passwd* user_to_passwd (const char *);
static struct group* group_to_grp (const char *);
//static int execvpe(const char *, char *const *, char *const *);

// FIXME: misc_conv is a separate library, should stick to plain PAM or make
// our own pam module
static struct pam_conv conv = {misc_conv, NULL};

extern char **environ;

int main (int argc, char *argv[])
{
	char *t_usr = "root", *t_grp = NULL;
	struct passwd *t_pw;
	struct group *t_gr;
	int opt, err;
	int shellflag = 0, envflag = 0;
	while ((opt = getopt(argc, argv, "A:u:g:C:se")) != -1) {
		switch (opt) {
		case 'A':
			printf("-A is not yet implemented\n");
			exit(EXIT_FAILURE);
			break;
		case 'u':
			t_usr = optarg;
			break;
		case 'g':
			t_grp = optarg;
			break;
		case 'C':
			printf("-C is not yet implemented\n");
			exit(EXIT_FAILURE);
			break;
		case 's':
			shellflag = 1;
			break;
		case 'e':
			envflag = 1;
			break;
		case '?':
			usage();
			exit(EINVAL);
			break;
		}
	}

	/* Get user info */
	const char *uname;
	char *shell;
	uid_t ruid = getuid();
	struct passwd *my_pw = getpwuid(ruid);
	if (!my_pw) {
		fprintf(stderr, "getpwid: %s\n", strerror(errno));
		return errno;
	}
	uname = my_pw->pw_name;

	/* Authenticate */
	if (authenticate(uname) != PAM_SUCCESS)
		exit(EXIT_FAILURE);


	/* Get target user and group info */
	t_pw = user_to_passwd(t_usr);
	if (!t_pw) {
		fprintf(stderr, "user_to_passwd: %s\n", strerror(errno));
		return errno;
	}
	t_gr = group_to_grp(t_grp);

	/* Get target user's shell */
	if (!shellflag)
		shell = t_pw->pw_shell;
	else
		shell = getenv("SHELL");
	if (!shell)
		shell = "/bin/sh";

	/* Set argc and argv */
	int c_argc = argc - optind;
	char **c_argv;
	if (c_argc) {
		c_argv = malloc(sizeof(char *) * (c_argc + 1));
		if (!c_argv) {
			fprintf(stderr, "malloc: %s\n", strerror(errno));
			exit(errno);
		}
		for (int i = 0; optind < argc; optind++, i++) {
			c_argv[i] = strdup(argv[optind]);
			if (!c_argv[i]) {
				fprintf(stderr, "strdup: %s\n", strerror(errno));
				exit(errno);
			}
		}
	} else {
		c_argc = 1;
		c_argv = malloc(sizeof(char *) * (c_argc + 1));
		if (!c_argv) {
			fprintf(stderr, "malloc: %s\n", strerror(errno));
			exit(errno);
		}
		c_argv[0] = strdup(shell);
		if (!c_argv[0]) {
			fprintf(stderr, "strdup: %s\n", strerror(errno));
			exit(errno);
		}
	}
	c_argv[c_argc] = NULL;

	struct env_elem {
		char *name;
		char *value;
	};

	struct env_elem env_keep[] = {
		{"PATH", NULL},
		{"TERM", NULL},
		{"EDITOR", NULL},
		{"VISUAL", NULL},
		{"DISPLAY", NULL},
		{"XAUTHORITY", NULL},
		{NULL, NULL}
	};

	struct env_elem env_mod[] = {
		{"USER", t_pw->pw_name},
		{"LOGNAME", t_pw->pw_name},
		{"SHELL", t_pw->pw_shell},
		{"HOME", t_pw->pw_dir},
		{NULL, NULL}
	};

	if (envflag) {	/* clear env */
		for (int i = 0; env_keep[i].name; i++)
			env_keep[i].value = strdup(getenv(env_keep[i].name));
		environ = NULL; // in place of clearenv
	}

	for (int i = 0; env_mod[i].name; i++) {
		err = setenv(env_mod[i].name, env_mod[i].value, 1);
		if (err == -1) {
			fprintf(stderr, "setenv: %s\n", strerror(errno));
			goto fail_end;
		}	
	}

	if (envflag) {
		for (int i = 0; env_keep[i].name; i++) {
			if (env_keep[i].value) {
				err = setenv(env_keep[i].name, env_keep[i].value, 1);
				if (err == -1) {
					fprintf(stderr, "setenv: %s\n", strerror(errno));
					goto fail_end;
				}
			}	
		}
	}
	// do not override, we might be under more levels of 'us'
	err = setenv("US_USER", my_pw->pw_name, 0);

	errno = 0;
	/* Set permissions */
	if (perm_set(t_pw, t_gr) == -1) {
		fprintf(stderr, "perm_set: %s\n", strerror(errno));
		goto fail_end;
	}

	/* Execute the command */
	err = execvp(c_argv[0], c_argv);
	if (err == -1)
		fprintf(stderr, "execl: %s\n", strerror(errno));

	/* Cleanup and return */
	fail_end:
	/* Free up the copied argv */
	for (int i=0; c_argv[i]; i++)
		free(c_argv[i]);
	free(c_argv);
	return errno;
}

static inline void usage (void)
{
	// TODO: planned options
	// -a [program]: like sudo's askpass
	// -u [user]: change the default user from root to user
	// -g [group]: change the primary group to [gorup]
	// 	both -a and -g will accept numbers with #[num] like sudo
	// -c [file]: manually select config file
	// something about environment
	// something about non interactiveness
	printf("usage: us [-se] [-u user] [-g group] command [args]\n");
}

static int perm_set (struct passwd *pw, struct group *gr)
{
	if (!pw) {
		errno = EINVAL;
		return -1;
	}

	uid_t uid;
	gid_t gid;

	uid = pw->pw_uid;
	gid = pw->pw_gid;

	if (gr)
		gid = gr->gr_gid;

	/* Set permissions, setting group perms first because in the case of
	 * dropping from higher permissions setting the uid first results in
	 * an error */
	int err;
	/* Non POSIX but implemented in most systems anyways */
	err = initgroups(pw->pw_name, pw->pw_gid);
	if (err == -1) {
		printf("initgroups failed\n");
		return -1;
	}

	if (setregid(gid, gid) == -1) {
		printf("setregid failed\n");
		return -1;
	}
	
	if (setreuid(uid, uid) == -1) {
		printf("setreuid failed\n");
		return -1;
	}

	return 0;
}

static int authenticate (const char *uname)
{
	pam_handle_t *pamh;
	int pam_err, count = 0;

	do {
		pam_err = pam_start("User Switcher", uname, &conv, &pamh);
		if (pam_err != PAM_SUCCESS) {
			fprintf(stderr, "pam_start: %s\n", pam_strerror(pamh, pam_err));
			return pam_err;
		}

		pam_err = pam_authenticate(pamh, 0);
		if (pam_err == PAM_SUCCESS) {
			pam_err = pam_acct_mgmt(pamh, 0);
		}

		if (pam_err != PAM_SUCCESS) {
			printf("Auth failed: %s\n", pam_strerror(pamh, pam_err));
			pam_end(pamh, pam_err);
		}

		count++;
	} while (pam_err != PAM_SUCCESS && count < 3);

	if (pam_err != PAM_SUCCESS) {
		fprintf(stderr, "better luck next time\n");
		return pam_err;
	}
	// FIXME: check again for the validity of the login for more security
	// as in: https://docs.oracle.com/cd/E19120-01/open.solaris/819-2145/pam-20/index.html
	// FIXME: ^C [SIGINT] will interrupt this call possibly causing a
	// vulnerability

	return pam_end(pamh, pam_err);
}

static struct passwd* user_to_passwd (const char *user)
{
	if (!user) {
		errno = EINVAL;
		return NULL;
	}

	struct passwd* pw;
	long uid_l;
	errno = 0;
	if (user[0] != '#') {
		pw = getpwnam(user);
	} else {
		uid_l = strtol(&user[1], NULL, 10);
		if (uid_l < 0 || errno) {
			errno = errno ? errno : EINVAL;
			return NULL;
		}
		pw = getpwuid((uid_t)uid_l);
	}

	if (!pw) {
		if (!errno)
			errno = EINVAL;
		return NULL;
	}

	return pw;
}

static struct group* group_to_grp (const char *group)
{
	if (!group) {
		errno = EINVAL;
		return NULL;
	}

	struct group* gr;
	long gid_l;
	errno = 0;
	if (group[0] != '#') {
		gr = getgrnam(group);
	} else {
		gid_l = strtol(&group[1], NULL, 10);
		if (gid_l < 0 || errno) {
			errno = errno ? errno : EINVAL;
			return NULL;
		}
		gr = getgrgid((gid_t)gid_l);
	}

	if (!gr) {
		if (!errno)
			errno = EINVAL;
		return NULL;
	}
	return gr;
}