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add unveil(2):

chpass(1) without parameters enters in edit mode by default, in here it will
need to execute _PATH_BSHELL to spawn a new EDITOR, _PATH_SHELLS to check
(read) if we are changing from/to a non-standard shell (in case we are not
root) and read access to `tempname' to verify if the file has valid entries and
create to unlink it.

If -s is used to change a user's shell then it will need read access to
_PATH_SHELLS by the same reason already mentioned above.

Unconditionally we need to unveil _PATH_MASTERPASSWD_LOCK with write/create
permissions, _PATH_MASTERPASSWD with read and _PATH_PWD_MKDB to execute
pwd_mkdb(8).

In the -a case I'm not unveiling /etc/spwd.db since we can get it through
pledge "getpw", which can be added later for completeness of all code paths.
Note also that the first pledges need "unveil" since we will call unveil(2)
afterwards.

"looks good" deraadt@

/*	$OpenBSD: chpass.c,v 1.45 2019/04/30 18:28:45 mestre Exp $	*/
/*	$NetBSD: chpass.c,v 1.8 1996/05/15 21:50:43 jtc Exp $	*/

/*-
 * Copyright (c) 1988, 1993, 1994
 *	The Regents of the University of California.  All rights reserved.
 *
 * 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. Neither the name of the University 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 REGENTS 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 REGENTS 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/resource.h>
#include <sys/stat.h>
#include <sys/time.h>
#include <sys/uio.h>

#include <err.h>
#include <errno.h>
#include <fcntl.h>
#include <paths.h>
#include <pwd.h>
#include <signal.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <util.h>

#include "chpass.h"

extern char *__progname;

enum { NEWSH, LOADENTRY, EDITENTRY } op;
uid_t uid;

void	baduser(void);
void	kbintr(int);
void	usage(void);

int
main(int argc, char *argv[])
{
	struct passwd *pw = NULL, *opw = NULL, lpw;
	int i, ch, pfd, tfd, dfd;
	char *tz, *arg = NULL;
	sigset_t fullset;

	/* We need to use the system timezone for date conversions. */
	if ((tz = getenv("TZ")) != NULL) {
	    unsetenv("TZ");
	    tzset();
	    setenv("TZ", tz, 1);
	}

	op = EDITENTRY;
	while ((ch = getopt(argc, argv, "a:s:")) != -1)
		switch(ch) {
		case 'a':
			op = LOADENTRY;
			arg = optarg;
			break;
		case 's':
			op = NEWSH;
			arg = optarg;
			break;
		case '?':
		default:
			usage();
		}
	argc -= optind;
	argv += optind;

	uid = getuid();

	if (op == EDITENTRY || op == NEWSH)
		switch(argc) {
		case 0:
			pw = getpwuid_shadow(uid);
			if (!pw)
				errx(1, "unknown user: uid %u", uid);
			break;
		case 1:
			pw = getpwnam_shadow(*argv);
			if (!pw)
				errx(1, "unknown user: %s", *argv);
			if (uid && uid != pw->pw_uid)
				baduser();
			break;
		default:
			usage();
		}

	if (op == LOADENTRY) {
		if (argc != 0)
			errx(1, "option -a does not accept user argument");
		if (uid)
			baduser();
		pw = &lpw;
		if (!pw_scan(arg, pw, NULL))
			exit(1);
		opw = getpwnam_shadow(pw->pw_name);
	}
	if (opw == NULL && (opw = pw_dup(pw)) == NULL)
		err(1, NULL);

	/* Edit the user passwd information if requested. */
	if (op == EDITENTRY) {
		char tempname[] = _PATH_VARTMP "pw.XXXXXXXXXX";
		int edit_status;

		if ((pw = pw_dup(pw)) == NULL)
			pw_error(NULL, 1, 1);
		dfd = mkostemp(tempname, O_CLOEXEC);
		if (dfd == -1)
			pw_error(tempname, 1, 1);
		display(tempname, dfd, pw);

		if (unveil(_PATH_BSHELL, "x") == -1)
			err(1, "unveil");
		if (unveil(_PATH_SHELLS, "r") == -1)
			err(1, "unveil");
		if (unveil(tempname, "rc") == -1)
			err(1, "unveil");
		if (pledge("stdio rpath wpath cpath id proc exec unveil",
		    NULL) == -1)
			err(1, "pledge");

		edit_status = edit(tempname, pw);
		close(dfd);
		unlink(tempname);

		switch (edit_status) {
		case EDIT_OK:
			break;
		case EDIT_NOCHANGE:
			pw_error(NULL, 0, 0);
			break;
		case EDIT_ERROR:
		default:
			pw_error(tempname, 1, 1);
			break;
		}
	}

	if (op == NEWSH) {
		if (unveil(_PATH_SHELLS, "r") == -1)
			err(1, "unveil");
		if (pledge("stdio rpath wpath cpath id proc exec unveil",
		    NULL) == -1)
			err(1, "pledge");

		/* protect p_shell -- it thinks NULL is /bin/sh */
		if (!arg[0])
			usage();
		if (p_shell(arg, pw, NULL))
			pw_error(NULL, 0, 1);
	}

	/* Drop user's real uid and block all signals to avoid a DoS. */
	setuid(0);
	sigfillset(&fullset);
	sigdelset(&fullset, SIGINT);
	sigprocmask(SIG_BLOCK, &fullset, NULL);

	if (unveil(_PATH_MASTERPASSWD_LOCK, "wc") == -1)
		err(1, "unveil");
	if (unveil(_PATH_MASTERPASSWD, "r") == -1)
		err(1, "unveil");
	if (unveil(_PATH_PWD_MKDB, "x") == -1)
		err(1, "unveil");
	if (pledge("stdio rpath wpath cpath proc exec", NULL) == -1)
		err(1, "pledge");

	/* Get the passwd lock file and open the passwd file for reading. */
	pw_init();
	for (i = 1; (tfd = pw_lock(0)) == -1; i++) {
		if (i == 4)
			(void)fputs("Attempting to lock password file, "
			    "please wait or press ^C to abort", stderr);
		(void)signal(SIGINT, kbintr);
		if (i % 16 == 0)
			fputc('.', stderr);
		usleep(250000);
		(void)signal(SIGINT, SIG_IGN);
	}
	if (i >= 4)
		fputc('\n', stderr);
	pfd = open(_PATH_MASTERPASSWD, O_RDONLY|O_CLOEXEC, 0);
	if (pfd == -1)
		pw_error(_PATH_MASTERPASSWD, 1, 1);

	/* Copy the passwd file to the lock file, updating pw. */
	pw_copy(pfd, tfd, pw, opw);

	/* If username changed we need to rebuild the entire db. */
	arg = !strcmp(opw->pw_name, pw->pw_name) ? pw->pw_name : NULL;

	/* Now finish the passwd file update. */
	if (pw_mkdb(arg, 0) == -1)
		pw_error(NULL, 0, 1);
	exit(0);
}

void
baduser(void)
{

	errx(1, "%s", strerror(EACCES));
}

/* ARGSUSED */
void
kbintr(int signo)
{
	dprintf(STDERR_FILENO, "\n%s: %s unchanged\n",
	    __progname, _PATH_MASTERPASSWD);
	_exit(1);
}

void
usage(void)
{

	(void)fprintf(stderr, "usage: %s [-s newshell] [user]\n", __progname);
	(void)fprintf(stderr, "       %s -a list\n", __progname);
	exit(1);
}