us/us.c

#define _POSIX_C_SOURCE 200809L

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

static void usage (void);
static int perm_set (uid_t, gid_t);

// FIXME: misc_conv is a separate library, should stick to plain PAM
static struct pam_conv conv = {misc_conv, NULL};

int main (int argc, char *argv[])
{
	// FIXME: change the default program to execute SHELL
	if (argc < 2) {
		usage();
		exit(1);
	}

	uid_t ruid = getuid();
	gid_t rgid = getgid();
	struct passwd *pw = getpwuid(ruid);
	if (!pw) {
		fprintf(stderr, "getpwid: %s\n", strerror(errno));
		exit(errno);
	}
	const char *uname = pw->pw_name;

	pam_handle_t *pamh;
	int pam_err, count = 0;

	// TODO: Add arguments

	// TODO: add PAM authentication
	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));
		exit(pam_err);
	}

	do {
		pam_err = pam_authenticate(pamh, 0);
		if (pam_err != PAM_SUCCESS)
			printf("Auth failed: %s\n", pam_strerror(pamh, pam_err));
		count++;
	} while (pam_err != PAM_SUCCESS && count < 4);
	if (pam_err != PAM_SUCCESS) {
		fprintf(stderr, "better luck next time\n");
		pam_end(pamh, pam_err);
		exit(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

	pam_end(pamh, pam_err);
	// TODO: clean up env

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

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

	/* Execute the command */
	if (execvp(*c_argv, c_argv) == -1) // execvp searches in path
		fprintf(stderr, "execv: %s\n", strerror(errno));

	/* If exec fails reset the permissions */
	if (perm_set(ruid, rgid) == -1) { // 0 = root
		fprintf(stderr, "perm_set: %s\n", strerror(errno));
		goto fail_end;
	}

	/* Cleanup and return */
	fail_end:

	/* Free up the copied argv */
	for (int i = 0; i < c_argc; 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 [command]\n");
}

static int perm_set (uid_t uid, gid_t gid)
{
	// FIXME: This doesn't change the uid and gid, it only changes the
	// e(ffective)uid and egid keeping the original gid, uid and groups list
	// to make it work like sudo, the uid gid and groups list must be all
	// set to 0 (or the corresponding value for other users).
	// An option could be to add an argument to keep the groups list,
	// another to change only the euid/egid.
	// For more information run:
	// $ sudo id
	// and check the output
	int err = 0;
	// FIXME: using setresuid() and setresgid() is preferred
	if ((err = seteuid(uid)) == -1)
		return err;
	else if ((err = setegid(gid)) == -1)
		return err;
	// FIXME: to set the group list use initgroups()
	return err;
}