uidswap.c revision 98675 
1/*
2 * Author: Tatu Ylonen <ylo@cs.hut.fi>
3 * Copyright (c) 1995 Tatu Ylonen <ylo@cs.hut.fi>, Espoo, Finland
4 *                    All rights reserved
5 * Code for uid-swapping.
6 *
7 * As far as I am concerned, the code I have written for this software
8 * can be used freely for any purpose.  Any derived versions of this
9 * software must be clearly marked as such, and if the derived work is
10 * incompatible with the protocol description in the RFC file, it must be
11 * called by a name other than "ssh" or "Secure Shell".
12 */
13
14#include "includes.h"
15RCSID("$OpenBSD: uidswap.c,v 1.22 2002/05/28 21:24:00 stevesk Exp $");
16
17#include "log.h"
18#include "uidswap.h"
19
20/*
21 * Note: all these functions must work in all of the following cases:
22 *    1. euid=0, ruid=0
23 *    2. euid=0, ruid!=0
24 *    3. euid!=0, ruid!=0
25 * Additionally, they must work regardless of whether the system has
26 * POSIX saved uids or not.
27 */
28
29/* Lets assume that posix saved ids also work with seteuid, even though that
30   is not part of the posix specification. */
31
32/* Saved effective uid. */
33static int	privileged = 0;
34static int	temporarily_use_uid_effective = 0;
35static uid_t	saved_euid = 0;
36static gid_t	saved_egid;
37static gid_t	saved_egroups[NGROUPS_MAX], user_groups[NGROUPS_MAX];
38static int	saved_egroupslen = -1, user_groupslen = -1;
39
40/*
41 * Temporarily changes to the given uid.  If the effective user
42 * id is not root, this does nothing.  This call cannot be nested.
43 */
44void
45temporarily_use_uid(struct passwd *pw)
46{
47	/* Save the current euid, and egroups. */
48	saved_euid = geteuid();
49	debug("temporarily_use_uid: %u/%u (e=%u)",
50	    (u_int)pw->pw_uid, (u_int)pw->pw_gid, (u_int)saved_euid);
51	if (saved_euid != 0) {
52		privileged = 0;
53		return;
54	}
55	privileged = 1;
56	temporarily_use_uid_effective = 1;
57	saved_egid = getegid();
58	saved_egroupslen = getgroups(NGROUPS_MAX, saved_egroups);
59	if (saved_egroupslen < 0)
60		fatal("getgroups: %.100s", strerror(errno));
61
62	/* set and save the user's groups */
63	if (user_groupslen == -1) {
64		if (initgroups(pw->pw_name, pw->pw_gid) < 0)
65			fatal("initgroups: %s: %.100s", pw->pw_name,
66			    strerror(errno));
67		user_groupslen = getgroups(NGROUPS_MAX, user_groups);
68		if (user_groupslen < 0)
69			fatal("getgroups: %.100s", strerror(errno));
70	}
71	/* Set the effective uid to the given (unprivileged) uid. */
72	if (setgroups(user_groupslen, user_groups) < 0)
73		fatal("setgroups: %.100s", strerror(errno));
74	if (setegid(pw->pw_gid) < 0)
75		fatal("setegid %u: %.100s", (u_int)pw->pw_gid,
76		    strerror(errno));
77	if (seteuid(pw->pw_uid) == -1)
78		fatal("seteuid %u: %.100s", (u_int)pw->pw_uid,
79		    strerror(errno));
80}
81
82/*
83 * Restores to the original (privileged) uid.
84 */
85void
86restore_uid(void)
87{
88	debug("restore_uid");
89	/* it's a no-op unless privileged */
90	if (!privileged)
91		return;
92	if (!temporarily_use_uid_effective)
93		fatal("restore_uid: temporarily_use_uid not effective");
94	/* Set the effective uid back to the saved privileged uid. */
95	if (seteuid(saved_euid) < 0)
96		fatal("seteuid %u: %.100s", (u_int)saved_euid, strerror(errno));
97	if (setgroups(saved_egroupslen, saved_egroups) < 0)
98		fatal("setgroups: %.100s", strerror(errno));
99	if (setegid(saved_egid) < 0)
100		fatal("setegid %u: %.100s", (u_int)saved_egid, strerror(errno));
101	temporarily_use_uid_effective = 0;
102}
103
104/*
105 * Permanently sets all uids to the given uid.  This cannot be
106 * called while temporarily_use_uid is effective.
107 */
108void
109permanently_set_uid(struct passwd *pw)
110{
111	if (temporarily_use_uid_effective)
112		fatal("permanently_set_uid: temporarily_use_uid effective");
113	if (setgid(pw->pw_gid) < 0)
114		fatal("setgid %u: %.100s", (u_int)pw->pw_gid, strerror(errno));
115	if (setuid(pw->pw_uid) < 0)
116		fatal("setuid %u: %.100s", (u_int)pw->pw_uid, strerror(errno));
117}
118