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redo the environment inheritance to not inherit. it was intended to make
life easier, but it can be surprising or even unsafe. instead, reset just
about everything to the target user's values.
ok deraadt martijn
Thanks to Sander Bos in particular for pointing out some nasty edge cases.

/* $OpenBSD: doas.c,v 1.77 2019/06/16 18:16:34 tedu Exp $ */
/*
 * Copyright (c) 2015 Ted Unangst <tedu@openbsd.org>
 *
 * Permission to use, copy, modify, and distribute this software for any
 * purpose with or without fee is hereby granted, provided that the above
 * copyright notice and this permission notice appear in all copies.
 *
 * THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
 * WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
 * MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
 * ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
 * WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN
 * ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
 * OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
 */

#include <sys/types.h>
#include <sys/stat.h>
#include <sys/ioctl.h>

#include <limits.h>
#include <login_cap.h>
#include <bsd_auth.h>
#include <readpassphrase.h>
#include <string.h>
#include <stdio.h>
#include <stdlib.h>
#include <err.h>
#include <unistd.h>
#include <pwd.h>
#include <grp.h>
#include <syslog.h>
#include <errno.h>
#include <fcntl.h>

#include "doas.h"

static void __dead
usage(void)
{
	fprintf(stderr, "usage: doas [-Lns] [-a style] [-C config] [-u user]"
	    " command [args]\n");
	exit(1);
}

static int
parseuid(const char *s, uid_t *uid)
{
	struct passwd *pw;
	const char *errstr;

	if ((pw = getpwnam(s)) != NULL) {
		*uid = pw->pw_uid;
		return 0;
	}
	*uid = strtonum(s, 0, UID_MAX, &errstr);
	if (errstr)
		return -1;
	return 0;
}

static int
uidcheck(const char *s, uid_t desired)
{
	uid_t uid;

	if (parseuid(s, &uid) != 0)
		return -1;
	if (uid != desired)
		return -1;
	return 0;
}

static int
parsegid(const char *s, gid_t *gid)
{
	struct group *gr;
	const char *errstr;

	if ((gr = getgrnam(s)) != NULL) {
		*gid = gr->gr_gid;
		return 0;
	}
	*gid = strtonum(s, 0, GID_MAX, &errstr);
	if (errstr)
		return -1;
	return 0;
}

static int
match(uid_t uid, gid_t *groups, int ngroups, uid_t target, const char *cmd,
    const char **cmdargs, struct rule *r)
{
	int i;

	if (r->ident[0] == ':') {
		gid_t rgid;
		if (parsegid(r->ident + 1, &rgid) == -1)
			return 0;
		for (i = 0; i < ngroups; i++) {
			if (rgid == groups[i])
				break;
		}
		if (i == ngroups)
			return 0;
	} else {
		if (uidcheck(r->ident, uid) != 0)
			return 0;
	}
	if (r->target && uidcheck(r->target, target) != 0)
		return 0;
	if (r->cmd) {
		if (strcmp(r->cmd, cmd))
			return 0;
		if (r->cmdargs) {
			/* if arguments were given, they should match explicitly */
			for (i = 0; r->cmdargs[i]; i++) {
				if (!cmdargs[i])
					return 0;
				if (strcmp(r->cmdargs[i], cmdargs[i]))
					return 0;
			}
			if (cmdargs[i])
				return 0;
		}
	}
	return 1;
}

static int
permit(uid_t uid, gid_t *groups, int ngroups, const struct rule **lastr,
    uid_t target, const char *cmd, const char **cmdargs)
{
	int i;

	*lastr = NULL;
	for (i = 0; i < nrules; i++) {
		if (match(uid, groups, ngroups, target, cmd,
		    cmdargs, rules[i]))
			*lastr = rules[i];
	}
	if (!*lastr)
		return 0;
	return (*lastr)->action == PERMIT;
}

static void
parseconfig(const char *filename, int checkperms)
{
	extern FILE *yyfp;
	extern int yyparse(void);
	struct stat sb;

	yyfp = fopen(filename, "r");
	if (!yyfp)
		err(1, checkperms ? "doas is not enabled, %s" :
		    "could not open config file %s", filename);

	if (checkperms) {
		if (fstat(fileno(yyfp), &sb) != 0)
			err(1, "fstat(\"%s\")", filename);
		if ((sb.st_mode & (S_IWGRP|S_IWOTH)) != 0)
			errx(1, "%s is writable by group or other", filename);
		if (sb.st_uid != 0)
			errx(1, "%s is not owned by root", filename);
	}

	yyparse();
	fclose(yyfp);
	if (parse_errors)
		exit(1);
}

static void __dead
checkconfig(const char *confpath, int argc, char **argv,
    uid_t uid, gid_t *groups, int ngroups, uid_t target)
{
	const struct rule *rule;

	setresuid(uid, uid, uid);
	parseconfig(confpath, 0);
	if (!argc)
		exit(0);

	if (permit(uid, groups, ngroups, &rule, target, argv[0],
	    (const char **)argv + 1)) {
		printf("permit%s\n", (rule->options & NOPASS) ? " nopass" : "");
		exit(0);
	} else {
		printf("deny\n");
		exit(1);
	}
}

static void
authuser(char *myname, char *login_style, int persist)
{
	char *challenge = NULL, *response, rbuf[1024], cbuf[128];
	auth_session_t *as;
	int fd = -1;

	if (persist)
		fd = open("/dev/tty", O_RDWR);
	if (fd != -1) {
		if (ioctl(fd, TIOCCHKVERAUTH) == 0)
			goto good;
	}

	if (!(as = auth_userchallenge(myname, login_style, "auth-doas",
	    &challenge)))
		errx(1, "Authorization failed");
	if (!challenge) {
		char host[HOST_NAME_MAX + 1];
		if (gethostname(host, sizeof(host)))
			snprintf(host, sizeof(host), "?");
		snprintf(cbuf, sizeof(cbuf),
		    "\rdoas (%.32s@%.32s) password: ", myname, host);
		challenge = cbuf;
	}
	response = readpassphrase(challenge, rbuf, sizeof(rbuf),
	    RPP_REQUIRE_TTY);
	if (response == NULL && errno == ENOTTY) {
		syslog(LOG_AUTHPRIV | LOG_NOTICE,
		    "tty required for %s", myname);
		errx(1, "a tty is required");
	}
	if (!auth_userresponse(as, response, 0)) {
		explicit_bzero(rbuf, sizeof(rbuf));
		syslog(LOG_AUTHPRIV | LOG_NOTICE,
		    "failed auth for %s", myname);
		errx(1, "Authorization failed");
	}
	explicit_bzero(rbuf, sizeof(rbuf));
good:
	if (fd != -1) {
		int secs = 5 * 60;
		ioctl(fd, TIOCSETVERAUTH, &secs);
		close(fd);
	}
}

int
unveilcommands(const char *ipath, const char *cmd)
{
	char *path = NULL, *p;
	int unveils = 0;

	if (strchr(cmd, '/') != NULL) {
		if (unveil(cmd, "x") != -1)
			unveils++;
		goto done;
	}

	if (!ipath) {
		errno = ENOENT;
		goto done;
	}
	path = strdup(ipath);
	if (!path) {
		errno = ENOENT;
		goto done;
	}
	for (p = path; p && *p; ) {
		char buf[PATH_MAX];
		char *cp = strsep(&p, ":");

		if (cp) {
			int r = snprintf(buf, sizeof buf, "%s/%s", cp, cmd);
			if (r != -1 && r < sizeof buf) {
				if (unveil(buf, "x") != -1)
					unveils++;
			}
		}
	}
done:
	free(path);
	return (unveils);
}

int
main(int argc, char **argv)
{
	const char *safepath = "/bin:/sbin:/usr/bin:/usr/sbin:"
	    "/usr/local/bin:/usr/local/sbin";
	const char *confpath = NULL;
	char *shargv[] = { NULL, NULL };
	char *sh;
	const char *cmd;
	char cmdline[LINE_MAX];
	char mypwbuf[_PW_BUF_LEN], targpwbuf[_PW_BUF_LEN];
	struct passwd mypwstore, targpwstore;
	struct passwd *mypw, *targpw;
	const struct rule *rule;
	uid_t uid;
	uid_t target = 0;
	gid_t groups[NGROUPS_MAX + 1];
	int ngroups;
	int i, ch, rv;
	int sflag = 0;
	int nflag = 0;
	char cwdpath[PATH_MAX];
	const char *cwd;
	char *login_style = NULL;
	char **envp;

	setprogname("doas");

	closefrom(STDERR_FILENO + 1);

	uid = getuid();

	while ((ch = getopt(argc, argv, "a:C:Lnsu:")) != -1) {
		switch (ch) {
		case 'a':
			login_style = optarg;
			break;
		case 'C':
			confpath = optarg;
			break;
		case 'L':
			i = open("/dev/tty", O_RDWR);
			if (i != -1)
				ioctl(i, TIOCCLRVERAUTH);
			exit(i == -1);
		case 'u':
			if (parseuid(optarg, &target) != 0)
				errx(1, "unknown user");
			break;
		case 'n':
			nflag = 1;
			break;
		case 's':
			sflag = 1;
			break;
		default:
			usage();
			break;
		}
	}
	argv += optind;
	argc -= optind;

	if (confpath) {
		if (sflag)
			usage();
	} else if ((!sflag && !argc) || (sflag && argc))
		usage();

	rv = getpwuid_r(uid, &mypwstore, mypwbuf, sizeof(mypwbuf), &mypw);
	if (rv != 0)
		err(1, "getpwuid_r failed");
	if (mypw == NULL)
		errx(1, "no passwd entry for self");
	ngroups = getgroups(NGROUPS_MAX, groups);
	if (ngroups == -1)
		err(1, "can't get groups");
	groups[ngroups++] = getgid();

	if (sflag) {
		sh = getenv("SHELL");
		if (sh == NULL || *sh == '\0') {
			shargv[0] = mypw->pw_shell;
		} else
			shargv[0] = sh;
		argv = shargv;
		argc = 1;
	}

	if (confpath) {
		checkconfig(confpath, argc, argv, uid, groups, ngroups,
		    target);
		exit(1);	/* fail safe */
	}

	if (geteuid())
		errx(1, "not installed setuid");

	parseconfig("/etc/doas.conf", 1);

	/* cmdline is used only for logging, no need to abort on truncate */
	(void)strlcpy(cmdline, argv[0], sizeof(cmdline));
	for (i = 1; i < argc; i++) {
		if (strlcat(cmdline, " ", sizeof(cmdline)) >= sizeof(cmdline))
			break;
		if (strlcat(cmdline, argv[i], sizeof(cmdline)) >= sizeof(cmdline))
			break;
	}

	cmd = argv[0];
	if (!permit(uid, groups, ngroups, &rule, target, cmd,
	    (const char **)argv + 1)) {
		syslog(LOG_AUTHPRIV | LOG_NOTICE,
		    "failed command for %s: %s", mypw->pw_name, cmdline);
		errc(1, EPERM, NULL);
	}

	if (!(rule->options & NOPASS)) {
		if (nflag)
			errx(1, "Authorization required");

		authuser(mypw->pw_name, login_style, rule->options & PERSIST);
	}

	if (unveil(_PATH_LOGIN_CONF, "r") == -1)
		err(1, "unveil");
	if (rule->cmd) {
		if (setenv("PATH", safepath, 1) == -1)
			err(1, "failed to set PATH '%s'", safepath);
	}
	if (unveilcommands(getenv("PATH"), cmd) == 0)
		goto fail;

	if (pledge("stdio rpath getpw exec id", NULL) == -1)
		err(1, "pledge");

	rv = getpwuid_r(target, &targpwstore, targpwbuf, sizeof(targpwbuf), &targpw);
	if (rv != 0)
		err(1, "getpwuid_r failed");
	if (targpw == NULL)
		errx(1, "no passwd entry for target");

	if (setusercontext(NULL, targpw, target, LOGIN_SETGROUP |
	    LOGIN_SETPATH |
	    LOGIN_SETPRIORITY | LOGIN_SETRESOURCES | LOGIN_SETUMASK |
	    LOGIN_SETUSER) != 0)
		errx(1, "failed to set user context for target");

	if (pledge("stdio rpath exec", NULL) == -1)
		err(1, "pledge");

	if (getcwd(cwdpath, sizeof(cwdpath)) == NULL)
		cwd = "(failed)";
	else
		cwd = cwdpath;

	if (pledge("stdio exec", NULL) == -1)
		err(1, "pledge");

	syslog(LOG_AUTHPRIV | LOG_INFO, "%s ran command %s as %s from %s",
	    mypw->pw_name, cmdline, targpw->pw_name, cwd);

	envp = prepenv(rule, mypw, targpw);

	if (rule->cmd) {
		/* do this again after setusercontext reset it */
		if (setenv("PATH", safepath, 1) == -1)
			err(1, "failed to set PATH '%s'", safepath);
	}
	execvpe(cmd, argv, envp);
fail:
	if (errno == ENOENT)
		errx(1, "%s: command not found", cmd);
	err(1, "%s", cmd);
}