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Revert previous commit.  We still want to call setgroups in the case where
there are zero groups to remove any that we might otherwise inherit (as
pointed out by grawity at gmail.com) and since the 2nd argument to
setgroups is always a static global it's always valid to dereference in
this case.  ok deraadt@ djm@

/* $OpenBSD: uidswap.c,v 1.39 2015/06/24 01:49:19 dtucker Exp $ */
/*
 * 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 <errno.h>
#include <pwd.h>
#include <string.h>
#include <unistd.h>
#include <limits.h>
#include <stdarg.h>
#include <stdlib.h>

#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();
	saved_egid = getegid();
	debug("temporarily_use_uid: %u/%u (e=%u/%u)",
	    (u_int)pw->pw_uid, (u_int)pw->pw_gid,
	    (u_int)saved_euid, (u_int)saved_egid);
	if (saved_euid != 0) {
		privileged = 0;
		return;
	}
	privileged = 1;
	temporarily_use_uid_effective = 1;
	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));
	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 (privileged) uid.
 */
void
restore_uid(void)
{
	/* it's a no-op unless privileged */
	if (!privileged) {
		debug("restore_uid: (unprivileged)");
		return;
	}
	if (!temporarily_use_uid_effective)
		fatal("restore_uid: temporarily_use_uid not effective");
	debug("restore_uid: %u/%u", (u_int)saved_euid, (u_int)saved_egid);
	/* Set the effective uid back to the saved privileged 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("permanently_set_uid: temporarily_use_uid effective");
	debug("permanently_set_uid: %u/%u", (u_int)pw->pw_uid,
	    (u_int)pw->pw_gid);
	if (setresgid(pw->pw_gid, pw->pw_gid, pw->pw_gid) != 0)
		fatal("setresgid %u: %s", (u_int)pw->pw_gid, strerror(errno));
	if (setresuid(pw->pw_uid, pw->pw_uid, pw->pw_uid) != 0)
		fatal("setresuid %u: %s", (u_int)pw->pw_uid, strerror(errno));
}

void
permanently_drop_suid(uid_t uid)
{
	debug("permanently_drop_suid: %u", (u_int)uid);
	if (setresuid(uid, uid, uid) != 0)
		fatal("setresuid %u: %s", (u_int)uid, strerror(errno));
}