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Check for symlinks before overwriting kerberos ticket files.
While i'm at it, make sure not to use Kerberos at all if there is no
local srvtab.
Based on patch from Todd Miller.  Reported by <jouko@solutions.fi>.

/*	$OpenBSD: su.c,v 1.35 2000/12/02 22:44:49 hin Exp $	*/

/*
 * Copyright (c) 1988 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. All advertising materials mentioning features or use of this software
 *    must display the following acknowledgement:
 *	This product includes software developed by the University of
 *	California, Berkeley and its contributors.
 * 4. 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.
 */

#ifndef lint
char copyright[] =
"@(#) Copyright (c) 1988 The Regents of the University of California.\n\
 All rights reserved.\n";
#endif /* not lint */

#ifndef lint
/*static char sccsid[] = "from: @(#)su.c	5.26 (Berkeley) 7/6/91";*/
static char rcsid[] = "$OpenBSD: su.c,v 1.35 2000/12/02 22:44:49 hin Exp $";
#endif /* not lint */

#include <sys/param.h>
#include <sys/time.h>
#include <sys/resource.h>

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

#ifdef  SKEY
#include <skey.h>                                                       
#endif                                                                       

#ifdef KERBEROS
#include <des.h>
#include <kerberosIV/krb.h>
#include <netdb.h>

int kerberos __P((char *username, char *user, int uid));

#define	ARGSTR	"-Kc:flm"

int use_kerberos = 1;
char krbtkfile[MAXPATHLEN];
char lrealm[REALM_SZ];
int ksettkfile(char *);
#else
#define	ARGSTR	"-c:flm"
#endif

char   *ontty __P((void));
int	chshell __P((char *));

int
main(argc, argv)
	int argc;
	char **argv;
{
	extern char **environ;
	register struct passwd *pwd;
	register char *p, **g;
	struct group *gr;
	uid_t ruid;
	login_cap_t *lc;
	int asme, ch, asthem, fastlogin, prio;
	enum { UNSET, YES, NO } iscsh;
	char *user, *shell, *avshell, *username, *class, **np;
	char shellbuf[MAXPATHLEN], avshellbuf[MAXPATHLEN];

	iscsh = UNSET;
	shell = class = NULL;
	asme = asthem = fastlogin = 0;
	while ((ch = getopt(argc, argv, ARGSTR)) != -1)
		switch((char)ch) {
#ifdef KERBEROS
		case 'K':
			use_kerberos = 0;
			break;
#endif
		case 'c':
			class = optarg;
			break;
		case 'f':
			fastlogin = 1;
			break;
		case '-':
		case 'l':
			asme = 0;
			asthem = 1;
			break;
		case 'm':
			asme = 1;
			asthem = 0;
			break;
		case '?':
		default:
			(void)fprintf(stderr,
			    "usage: su [%s] [login [shell arguments]]\n",
			    ARGSTR);
			exit(1);
		}
	argv += optind;

	errno = 0;
	prio = getpriority(PRIO_PROCESS, 0);
	if (errno)
		prio = 0;
	(void)setpriority(PRIO_PROCESS, 0, -2);
	openlog("su", LOG_CONS, 0);

	/* get current login name and shell */
	ruid = getuid();
	username = getlogin();
	if (username == NULL || (pwd = getpwnam(username)) == NULL ||
	    pwd->pw_uid != ruid)
		pwd = getpwuid(ruid);
	if (pwd == NULL)
		errx(1, "who are you?");
	if ((username = strdup(pwd->pw_name)) == NULL)
		err(1, "can't allocate memory");
	if (asme) {
		if (pwd->pw_shell && *pwd->pw_shell) {
			shell = strncpy(shellbuf, pwd->pw_shell, sizeof(shellbuf) - 1);
			shellbuf[sizeof(shellbuf) - 1] = '\0';
		} else {
			shell = _PATH_BSHELL;
			iscsh = NO;
		}
	}

	/* get target login information, default to root */
	user = *argv ? *argv : "root";
	np = *argv ? argv : argv-1;

	if ((pwd = getpwnam(user)) == NULL)
		errx(1, "unknown login %s", user);
	if ((user = strdup(pwd->pw_name)) == NULL)
		err(1, "can't allocate memory");

	/* If the user specified a login class and we are root, use it */
	if (ruid && class)
		errx(1, "only the superuser may specify a login class");
	if (class)
		pwd->pw_class = class;
	if ((lc = login_getclass(pwd->pw_class)) == NULL)
		errx(1, "no such login class: %s",
		    class ? class : LOGIN_DEFCLASS);

#if KERBEROS
	if (ksettkfile(user))
		use_kerberos = 0;
#endif

	if (ruid) {
#ifdef KERBEROS
	    if (!use_kerberos || kerberos(username, user, pwd->pw_uid))
#endif
	    {
		/* only allow those in group zero to su to root. */
		if (pwd->pw_uid == 0 && (gr = getgrgid((gid_t)0))
		    && gr->gr_mem && *(gr->gr_mem))
			for (g = gr->gr_mem;; ++g) {
				if (!*g)
					errx(1, "you are not in the correct group to su %s.", user);
				if (strcmp(username, *g) == 0)
					break;
		}
		/* if target requires a password, verify it */
		if (*pwd->pw_passwd) {
			p = getpass("Password:");
#ifdef SKEY
			if (strcasecmp(p, "s/key") == 0) {
				if (skey_authenticate(user))
					goto badlogin;
			} else
#endif
			if (strcmp(pwd->pw_passwd, crypt(p, pwd->pw_passwd))) {
badlogin:
				fprintf(stderr, "Sorry\n");
				syslog(LOG_AUTH|LOG_WARNING,
					"BAD SU %s to %s%s", username,
					user, ontty());
				exit(1);
			}
		}
	    }
	    if (pwd->pw_expire && time(NULL) >= pwd->pw_expire) {
		    fprintf(stderr, "Sorry - account expired\n");
		    syslog(LOG_AUTH|LOG_WARNING, "BAD SU %s to %s%s", username,
			    user, ontty());
		    exit(1);
	    }
	}

	if (asme) {
		/* if asme and non-standard target shell, must be root */
		if (!chshell(pwd->pw_shell) && ruid)
			errx(1, "permission denied (shell).");
	} else if (pwd->pw_shell && *pwd->pw_shell) {
		shell = pwd->pw_shell;
		iscsh = UNSET;
	} else {
		shell = _PATH_BSHELL;
		iscsh = NO;
	}

	if ((p = strrchr(shell, '/')))
		avshell = p+1;
	else
		avshell = shell;

	/* if we're forking a csh, we want to slightly muck the args */
	if (iscsh == UNSET)
		iscsh = strcmp(avshell, "csh") ? NO : YES;

	if (!asme) {
		if (asthem) {
			p = getenv("TERM");
			if ((environ = calloc(1, sizeof (char *))) == NULL)
				errx(1, "calloc");
			if (setusercontext(lc, pwd, pwd->pw_uid, LOGIN_SETPATH))
				err(1, "unable to set user context");
			if (p) {
				if (setenv("TERM", p, 1) == -1)
					err(1, "unable to set environment");
			}

			seteuid(pwd->pw_uid);
			setegid(pwd->pw_gid);
			if (chdir(pwd->pw_dir) < 0)
				err(1, "%s", pwd->pw_dir);
			seteuid(0);
			setegid(0);	/* XXX use a saved gid instead? */
		} else if (pwd->pw_uid == 0) {
			/* XXX - this seems questionable to me */
			if (setusercontext(lc,
			    pwd, pwd->pw_uid, LOGIN_SETPATH|LOGIN_SETUMASK))
				err(1, "unable to set user context");
		}
		if (asthem || pwd->pw_uid) {
			if (setenv("LOGNAME", pwd->pw_name, 1) == -1 ||
			    setenv("USER", pwd->pw_name, 1) == -1)
				err(1, "unable to set environment");
		}
		if (setenv("HOME", pwd->pw_dir, 1) == -1 ||
		    setenv("SHELL", shell, 1) == -1)
			err(1, "unable to set environment");
	}

#ifdef KERBEROS
	if (*krbtkfile) {
		if (setenv("KRBTKFILE", krbtkfile, 1) == -1)
			err(1, "unable to set environment");
	}
#endif

	if (iscsh == YES) {
		if (fastlogin)
			*np-- = "-f";
		if (asme)
			*np-- = "-m";
	}

	if (asthem) {
		avshellbuf[0] = '-';
		strncpy(avshellbuf+1, avshell, sizeof(avshellbuf) - 2);
		avshellbuf[sizeof(avshellbuf) - 1] = '\0';
		avshell = avshellbuf;
	} else if (iscsh == YES) {
		/* csh strips the first character... */
		avshellbuf[0] = '_';
		strncpy(avshellbuf+1, avshell, sizeof(avshellbuf) - 2);
		avshellbuf[sizeof(avshellbuf) - 1] = '\0';
		avshell = avshellbuf;
	}
			
	*np = avshell;

	if (ruid != 0)
		syslog(LOG_NOTICE|LOG_AUTH, "%s to %s%s",
		    username, user, ontty());

	(void)setpriority(PRIO_PROCESS, 0, prio);
	if (setusercontext(lc, pwd, pwd->pw_uid,
	    (asthem ? (LOGIN_SETPRIORITY | LOGIN_SETUMASK) : 0) |
	    LOGIN_SETRESOURCES | LOGIN_SETGROUP | LOGIN_SETUSER))
		err(1, "unable to set user context");

	execv(shell, np);
	err(1, "%s", shell);
}

int
chshell(sh)
	char *sh;
{
	register char *cp;

	while ((cp = getusershell()) != NULL)
		if (strcmp(cp, sh) == 0)
			return (1);
	return (0);
}

char *
ontty()
{
	char *p, *ttyname();
	static char buf[MAXPATHLEN + 4];

	buf[0] = 0;
	if ((p = ttyname(STDERR_FILENO)))
		snprintf(buf, sizeof(buf), " on %s", p);
	return (buf);
}

#ifdef KERBEROS
int koktologin __P((char *, char *, char *));

int
kerberos(username, user, uid)
	char *username, *user;
	int uid;
{
	KTEXT_ST ticket;
	AUTH_DAT authdata;
	struct hostent *hp;
	int kerno, fd;
	in_addr_t faddr;
	char hostname[MAXHOSTNAMELEN], savehost[MAXHOSTNAMELEN];
	char *ontty(), *krb_get_phost();

	/* Don't bother with Kerberos if there is no srvtab file */
	if ((fd = open(KEYFILE, O_RDONLY, 0)) < 0)
		return (1);
	close(fd);

	if (koktologin(username, lrealm, user) && !uid) {
		(void)fprintf(stderr, "kerberos su: not in %s's ACL.\n", user);
		return (1);
	}
	(void)krb_set_tkt_string(krbtkfile);

	/*
	 * Set real as well as effective ID to 0 for the moment,
	 * to make the kerberos library do the right thing.
	 */
	if (setuid(0) < 0) {
		warn("setuid");
		return (1);
	}

	/*
	 * Little trick here -- if we are su'ing to root,
	 * we need to get a ticket for "xxx.root", where xxx represents
	 * the name of the person su'ing.  Otherwise (non-root case),
	 * we need to get a ticket for "yyy.", where yyy represents
	 * the name of the person being su'd to, and the instance is null
	 */

	printf("%s%s@%s's ", (uid == 0 ? username : user), 
	       (uid == 0 ? ".root" : ""), lrealm);
	fflush(stdout);
	kerno = krb_get_pw_in_tkt((uid == 0 ? username : user),
		(uid == 0 ? "root" : ""), lrealm,
	    	"krbtgt", lrealm, DEFAULT_TKT_LIFE, 0);

	if (kerno != KSUCCESS) {
		if (kerno == KDC_PR_UNKNOWN) {
			warnx("kerberos principal unknown: %s.%s@%s",
				(uid == 0 ? username : user),
				(uid == 0 ? "root" : ""), lrealm);
			return (1);
		}
		warnx("unable to su: %s", krb_err_txt[kerno]);
		syslog(LOG_NOTICE|LOG_AUTH,
		    "BAD Kerberos SU: %s to %s%s: %s",
		    username, user, ontty(), krb_err_txt[kerno]);
		return (1);
	}

	/*
	 * Set the owner of the ticket file to root but bail if someone
	 * has nefariously swapped a link in place of the file.
	 */
	fd = open(krbtkfile, O_RDWR|O_NOFOLLOW, 0);
	if (fd == -1) {
		warn("unable to open ticket file");
		(void)unlink(krbtkfile);
		return (1);
	}
	if (fchown(fd, uid, -1) < 0) {
		warn("fchown");
		(void)unlink(krbtkfile);
		return (1);
	}
	close(fd);

	(void)setpriority(PRIO_PROCESS, 0, -2);

	if (gethostname(hostname, sizeof(hostname)) == -1) {
		warn("gethostname");
		dest_tkt();
		return (1);
	}

	(void)strncpy(savehost, krb_get_phost(hostname), sizeof(savehost) - 1);
	savehost[sizeof(savehost) - 1] = '\0';

	kerno = krb_mk_req(&ticket, "rcmd", savehost, lrealm, 33);

	if (kerno == KDC_PR_UNKNOWN) {
		warnx("Warning: TGT not verified.");
		syslog(LOG_NOTICE|LOG_AUTH,
		    "%s to %s%s, TGT not verified (%s); %s.%s not registered?",
		    username, user, ontty(), krb_err_txt[kerno],
		    "rcmd", savehost);
	} else if (kerno != KSUCCESS) {
		warnx("Unable to use TGT: %s", krb_err_txt[kerno]);
		syslog(LOG_NOTICE|LOG_AUTH, "failed su: %s to %s%s: %s",
		    username, user, ontty(), krb_err_txt[kerno]);
		dest_tkt();
		return (1);
	} else {
		if (!(hp = gethostbyname(hostname))) {
			warnx("can't get addr of %s", hostname);
			dest_tkt();
			return (1);
		}
		(void)memcpy((void *)&faddr, (void *)hp->h_addr, sizeof(faddr));

		if ((kerno = krb_rd_req(&ticket, "rcmd", savehost, faddr,
		    &authdata, "")) != KSUCCESS) {
			warnx("unable to verify rcmd ticket: %s",
			      krb_err_txt[kerno]);
			syslog(LOG_NOTICE|LOG_AUTH,
			    "failed su: %s to %s%s: %s", username,
			     user, ontty(), krb_err_txt[kerno]);
			dest_tkt();
			return (1);
		}
	}
	return (0);
}

int
koktologin(name, realm, toname)
	char *name, *realm, *toname;
{
	register AUTH_DAT *kdata;
	AUTH_DAT kdata_st;

	memset((void *)&kdata_st, 0, sizeof(kdata_st));
	kdata = &kdata_st;

	(void)strncpy(kdata->pname, name, sizeof(kdata->pname) - 1);
	kdata->pname[sizeof(kdata->pname) - 1] = '\0';

	(void)strncpy(kdata->pinst,
	    ((strcmp(toname, "root") == 0) ? "root" : ""), sizeof(kdata->pinst) - 1);
	kdata->pinst[sizeof(kdata->pinst) -1] = '\0';

	(void)strncpy(kdata->prealm, realm, sizeof(kdata->prealm) - 1);
	kdata->prealm[sizeof(kdata->prealm) -1] = '\0';

	return (kuserok(kdata, toname));
}

int
ksettkfile(user)
	char *user;
{
	if (krb_get_lrealm(lrealm, 1) != KSUCCESS)
		return (1);
	(void)snprintf(krbtkfile, sizeof(krbtkfile), "%s_%s_%u", TKT_ROOT,
		user, getuid());
	return (0);
}
#endif