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The string with path to shell could be taken directly from struct passwd.
At some point later the data it points to is overridden by getpwuid() call,
resulting in garbage. The problem could be easily demonstreated by double
doas call:

  $ doas doas -su _sndio
  doas: mpty: command not found

The fix is easy: just strdup() the pw_shell field value.

okay tedu@, tweaks from & okay natano@

/* $OpenBSD: doas.c,v 1.60 2016/07/18 16:46:30 zhuk 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 <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 "doas.h"

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

size_t
arraylen(const char **arr)
{
	size_t cnt = 0;

	while (*arr) {
		cnt++;
		arr++;
	}
	return cnt;
}

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, 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)
{
	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);
	}
}

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 myname[_PW_NAME_LEN + 1];
	struct passwd *pw;
	struct rule *rule;
	uid_t uid;
	uid_t target = 0;
	gid_t groups[NGROUPS_MAX + 1];
	int ngroups;
	int i, ch;
	int sflag = 0;
	int nflag = 0;
	char cwdpath[PATH_MAX];
	const char *cwd;
	char *login_style = NULL;
	char **envp;

	setprogname("doas");

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

	closefrom(STDERR_FILENO + 1);

	uid = getuid();

	while ((ch = getopt(argc, argv, "a:C:nsu:")) != -1) {
		switch (ch) {
		case 'a':
			login_style = optarg;
			break;
		case 'C':
			confpath = optarg;
			break;
		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();

	pw = getpwuid(uid);
	if (!pw)
		err(1, "getpwuid failed");
	if (strlcpy(myname, pw->pw_name, sizeof(myname)) >= sizeof(myname))
		errx(1, "pw_name too long");
	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] = strdup(pw->pw_shell);
			if (shargv[0] == NULL)
				err(1, NULL);
		} else
			shargv[0] = sh;
		argv = shargv;
		argc = 1;
	}

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

	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", myname, cmdline);
		errc(1, EPERM, NULL);
	}

	if (!(rule->options & NOPASS)) {
		char *challenge = NULL, *response, rbuf[1024], cbuf[128];
		auth_session_t *as;

		if (nflag)
			errx(1, "Authorization required");

		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)) {
			syslog(LOG_AUTHPRIV | LOG_NOTICE,
			    "failed auth for %s", myname);
			errc(1, EPERM, NULL);
		}
		explicit_bzero(rbuf, sizeof(rbuf));
	}

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

	pw = getpwuid(target);
	if (!pw)
		errx(1, "no passwd entry for target");

	if (setusercontext(NULL, pw, target, LOGIN_SETGROUP |
	    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",
	    myname, cmdline, pw->pw_name, cwd);

	envp = prepenv(rule);

	if (rule->cmd) {
		if (setenv("PATH", safepath, 1) == -1)
			err(1, "failed to set PATH '%s'", safepath);
	}
	execvpe(cmd, argv, envp);
	if (errno == ENOENT)
		errx(1, "%s: command not found", cmd);
	err(1, "%s", cmd);
}