uidswap.c

/*
 * Author: Tatu Ylonen <ylo@cs.hut.fi>
 * Copyright (c) 1995 Tatu Ylonen <ylo@cs.hut.fi>, Espoo, Finland
 *                    All rights reserved
 * Code for uid-swapping.
 *
 * As far as I am concerned, the code I have written for this software
 * can be used freely for any purpose.  Any derived versions of this
 * software must be clearly marked as such, and if the derived work is
 * incompatible with the protocol description in the RFC file, it must be
 * called by a name other than "ssh" or "Secure Shell".
 */

#include "includes.h"
RCSID("$OpenBSD: uidswap.c,v 1.15 2001/04/08 11:24:33 markus Exp $");

#include "log.h"
#include "uidswap.h"

/*
 * Note: all these functions must work in all of the following cases:
 *    1. euid=0, ruid=0
 *    2. euid=0, ruid!=0
 *    3. euid!=0, ruid!=0
 * Additionally, they must work regardless of whether the system has
 * POSIX saved uids or not.
 */

/* Lets assume that posix saved ids also work with seteuid, even though that
   is not part of the posix specification. */
 
/* Saved effective uid. */
static int	privileged = 0;
static int	temporarily_use_uid_effective = 0;
static uid_t	saved_euid = 0;
static gid_t	saved_egid;
static gid_t	saved_egroups[NGROUPS_MAX], user_groups[NGROUPS_MAX];
static int	saved_egroupslen = -1, user_groupslen = -1;
  
/*
 * Temporarily changes to the given uid.  If the effective user
 * id is not root, this does nothing.  This call cannot be nested.
 */
void
temporarily_use_uid(struct passwd *pw)
{
 	/* Save the current euid, and egroups. */
  	saved_euid = geteuid();
 	debug("temporarily_use_uid: %d/%d (e=%d)",
 	    pw->pw_uid, pw->pw_gid, saved_euid);
 	if (saved_euid != 0) {
 		privileged = 0;
 		return;
 	}
 	privileged = 1;
 	temporarily_use_uid_effective = 1;
	saved_egid = getegid();
	saved_egroupslen = getgroups(NGROUPS_MAX, saved_egroups);
 	if (saved_egroupslen < 0)
 		fatal("getgroups: %.100s", strerror(errno));
 
 	/* set and save the user's groups */
 	if (user_groupslen == -1) {
 		if (initgroups(pw->pw_name, pw->pw_gid) < 0)
 			fatal("initgroups: %s: %.100s", pw->pw_name,
 			    strerror(errno));
 		user_groupslen = getgroups(NGROUPS_MAX, user_groups);                           
 		if (user_groupslen < 0)
 			fatal("getgroups: %.100s", strerror(errno));
 	}
  	/* Set the effective uid to the given (unprivileged) uid. */
	if (setgroups(user_groupslen, user_groups) < 0)
		fatal("setgroups: %.100s", strerror(errno));
	pw->pw_gid = pw->pw_gid;
	if (setegid(pw->pw_gid) < 0)
 		fatal("setegid %u: %.100s", (u_int) pw->pw_gid,
 		    strerror(errno));
 	if (seteuid(pw->pw_uid) == -1)
 		fatal("seteuid %u: %.100s", (u_int) pw->pw_uid,
 		    strerror(errno));
}
  
/*
 * Restores to the original uid.
 */
void
restore_uid(void)
{
	debug("restore_uid");
	/* it's a no-op unless privileged */
	if (!privileged)
		return;
	if (!temporarily_use_uid_effective)
		fatal("restore_uid: temporarily_use_uid not effective");
	/* Set the effective uid back to the saved uid. */
	if (seteuid(saved_euid) < 0)
		fatal("seteuid %u: %.100s", (u_int) saved_euid, strerror(errno));
	if (setgroups(saved_egroupslen, saved_egroups) < 0)
		fatal("setgroups: %.100s", strerror(errno));
	if (setegid(saved_egid) < 0)
		fatal("setegid %u: %.100s", (u_int) saved_egid, strerror(errno));
	temporarily_use_uid_effective = 0;
}

/*
 * Permanently sets all uids to the given uid.  This cannot be
 * called while temporarily_use_uid is effective.
 */
void
permanently_set_uid(struct passwd *pw)
{
	if (temporarily_use_uid_effective)
		fatal("restore_uid: temporarily_use_uid effective");
	if (setuid(pw->pw_uid) < 0)
		fatal("setuid %u: %.100s", (u_int) pw->pw_uid, strerror(errno));
	if (setgid(pw->pw_gid) < 0)
		fatal("setgid %u: %.100s", (u_int) pw->pw_gid, strerror(errno));
}