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defend against malicious line in ut_line, which could cause unwanted
writes to anything under /dev.  revoke setuid/gid privs earlier.
From: xs@kittenz.org

/*	$NetBSD: utmp_update.c,v 1.4 2002/08/16 20:21:48 itojun Exp $	 */

/*-
 * Copyright (c) 2002 The NetBSD Foundation, Inc.
 * All rights reserved.
 *
 * This code is derived from software contributed to The NetBSD Foundation
 * by Christos Zoulas.
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions
 * are met:
 * 1. Redistributions of source code must retain the above copyright
 *    notice, this list of conditions and the following disclaimer.
 * 2. Redistributions in binary form must reproduce the above copyright
 *    notice, this list of conditions and the following disclaimer in the
 *    documentation and/or other materials provided with the distribution.
 * 3. All advertising materials mentioning features or use of this software
 *    must display the following acknowledgement:
 *        This product includes software developed by the NetBSD
 *        Foundation, Inc. and its contributors.
 * 4. Neither the name of The NetBSD Foundation nor the names of its
 *    contributors may be used to endorse or promote products derived
 *    from this software without specific prior written permission.
 *
 * THIS SOFTWARE IS PROVIDED BY THE NETBSD FOUNDATION, INC. AND CONTRIBUTORS
 * ``AS IS'' AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED
 * TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
 * PURPOSE ARE DISCLAIMED.  IN NO EVENT SHALL THE FOUNDATION OR CONTRIBUTORS
 * BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
 * CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
 * SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
 * INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
 * CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
 * ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
 * POSSIBILITY OF SUCH DAMAGE.
 */

#include <sys/stat.h>

#include <stdio.h>
#include <vis.h>
#include <err.h>
#include <fcntl.h>
#include <pwd.h>
#include <utmpx.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>

int main(int, char *[]);

int
main(int argc, char *argv[])
{
	struct utmpx *utx;
	size_t len;
	struct passwd *pwd;
	struct stat st;
	int fd;
	uid_t euid;

	euid = geteuid();
	if (seteuid(getuid()) == -1)
		err(1, "seteuid");

	if (argc != 2) {
		(void)fprintf(stderr, "Usage: %s <vis-utmpx-entry>\n",
			getprogname());
		exit(1);
	}

	len = strlen(argv[1]);

	if (len > sizeof(*utx) * 4 + 1 || len < sizeof(*utx))
		errx(1, "Bad argument");

	if ((utx = malloc(len)) == NULL)
		err(1, NULL);

	if (strunvis((char *)utx, argv[1]) != sizeof(*utx))
		errx(1, "Decoding error");

	switch (utx->ut_type) {
	case USER_PROCESS:
	case DEAD_PROCESS:
		break;
	default:
		errx(1, "Invalid utmpx type %d", (int)utx->ut_type);
	}

	if ((pwd = getpwuid(getuid())) == NULL)
		errx(1, "User %lu does not exist in password database",
		    (long)getuid());

	if (strcmp(pwd->pw_name, utx->ut_name) != 0)
		errx(1, "Current user `%s' does not match `%s' in utmpx entry",
		    pwd->pw_name, utx->ut_name);

	fd = open(utx->ut_line, O_RDONLY, 0);
	if (fd == -1)
		err(1, "Cannot open `%s'", utx->ut_line);
	if (fstat(fd, &st) == -1)
		err(1, "Cannot stat `%s'", utx->ut_line);
	if (st.st_uid != getuid())
		errx(1, "%s: Is not owned by you", utx->ut_line);
	if (!isatty(fd))
		errx(1, "%s: Not a tty device", utx->ut_line);
	close(fd);
	if (access(utx->ut_line, W_OK|R_OK) == -1)
		err(1, "%s", utx->ut_line);

	(void)seteuid(euid);
	pututxline(utx);

	return 0;
}