summaryrefslogtreecommitdiff
path: root/src/setuid.c
blob: 57842302d6552b86471dadda3b007328eef4b34b (plain)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
/* -*- Mode: C; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 8 -*-
 *
 * setuid.c --- management of runtime privileges.
 *
 * xscreensaver, Copyright (c) 1993-1998 Jamie Zawinski <jwz@jwz.org>
 * Copyright (C) 2012-2021 MATE Developers
 *
 * Permission to use, copy, modify, distribute, and sell this software and its
 * documentation for any purpose is hereby granted without fee, provided that
 * the above copyright notice appear in all copies and that both that
 * copyright notice and this permission notice appear in supporting
 * documentation.  No representations are made about the suitability of this
 * software for any purpose.  It is provided "as is" without express or
 * implied warranty.
 */

#include "config.h"

#ifndef EPERM
#include <errno.h>
#endif

#include <stdio.h>
#include <string.h>
#include <sys/types.h>
#include <unistd.h>
#include <pwd.h>		/* for getpwnam() and struct passwd */
#include <grp.h>		/* for getgrgid() and struct group */
#include <glib.h>

#include "setuid.h"

static char *
uid_gid_string (uid_t uid,
                gid_t gid)
{
	static char   *buf;
	struct passwd *p = NULL;
	struct group  *g = NULL;

	p = getpwuid (uid);
	g = getgrgid (gid);

	buf = g_strdup_printf ("%s/%s (%ld/%ld)",
	                       (p && p->pw_name ? p->pw_name : "???"),
	                       (g && g->gr_name ? g->gr_name : "???"),
	                       (long) uid, (long) gid);

	return buf;
}

static gboolean
set_ids_by_number (uid_t  uid,
                   gid_t  gid,
                   char **message_ret)
{
	int uid_errno = 0;
	int gid_errno = 0;
	int sgs_errno = 0;
	struct passwd *p = getpwuid (uid);
	struct group  *g = getgrgid (gid);

	if (message_ret)
		*message_ret = NULL;

	/* Rumor has it that some implementations of of setuid() do nothing
	   when called with -1; therefore, if the "nobody" user has a uid of
	   -1, then that would be Really Bad.  Rumor further has it that such
	   systems really ought to be using -2 for "nobody", since that works.
	   So, if we get a uid (or gid, for good measure) of -1, switch to -2
	   instead.  Note that this must be done after we've looked up the
	   user/group names with getpwuid(-1) and/or getgrgid(-1).
	*/
	if (gid == (gid_t) -1) gid = (gid_t) -2;
	if (uid == (uid_t) -1) uid = (uid_t) -2;

	errno = 0;
	if (setgroups (1, &gid) < 0)
		sgs_errno = errno ? errno : -1;

	errno = 0;
	if (setgid (gid) != 0)
		gid_errno = errno ? errno : -1;

	errno = 0;
	if (setuid (uid) != 0)
		uid_errno = errno ? errno : -1;

	if (uid_errno == 0 && gid_errno == 0 && sgs_errno == 0)
	{
		static char *reason;
		reason = g_strdup_printf ("changed uid/gid to %s/%s (%ld/%ld).",
		                          (p && p->pw_name ? p->pw_name : "???"),
		                          (g && g->gr_name ? g->gr_name : "???"),
		                          (long) uid, (long) gid);
		if (message_ret)
			*message_ret = g_strdup (reason);

		g_free (reason);

		return TRUE;
	}
	else
	{
		char *reason = NULL;

		if (sgs_errno)
		{
			reason = g_strdup_printf ("couldn't setgroups to %s (%ld)",
			                          (g && g->gr_name ? g->gr_name : "???"),
			                          (long) gid);
			if (sgs_errno == -1)
				fprintf (stderr, "%s: unknown error\n", reason);
			else
			{
				errno = sgs_errno;
				perror (reason);
			}
			g_free (reason);
			reason = NULL;
		}

		if (gid_errno)
		{
			reason = g_strdup_printf ("couldn't set gid to %s (%ld)",
			                          (g && g->gr_name ? g->gr_name : "???"),
			                          (long) gid);
			if (gid_errno == -1)
				fprintf (stderr, "%s: unknown error\n", reason);
			else
			{
				errno = gid_errno;
				perror (reason);
			}
			g_free (reason);
			reason = NULL;
		}

		if (uid_errno)
		{
			reason = g_strdup_printf ("couldn't set uid to %s (%ld)",
			                          (p && p->pw_name ? p->pw_name : "???"),
			                          (long) uid);
			if (uid_errno == -1)
				fprintf (stderr, "%s: unknown error\n", reason);
			else
			{
				errno = uid_errno;
				perror (reason);
			}
			g_free (reason);
			reason = NULL;
		}
		return FALSE;
	}
	return FALSE;
}

/* If we've been run as setuid or setgid to someone else (most likely root)
   turn off the extra permissions so that random user-specified programs
   don't get special privileges.  (On some systems it is necessary to install
   this program as setuid root in order to read the passwd file to implement
   lock-mode.)

   *** WARNING: DO NOT DISABLE ANY OF THE FOLLOWING CODE!
   If you do so, you will open a security hole.  See the sections
   of the xscreensaver manual titled "LOCKING AND ROOT LOGINS",
   and "USING XDM".
*/

/* Returns TRUE if OK to lock, FALSE otherwise */
gboolean
hack_uid (char **nolock_reason,
          char **orig_uid,
          char **uid_message)
{
	char    *reason;
	gboolean ret;

	ret = TRUE;
	reason = NULL;

	if (nolock_reason != NULL)
	{
		*nolock_reason = NULL;
	}
	if (orig_uid != NULL)
	{
		*orig_uid = NULL;
	}
	if (uid_message != NULL)
	{
		*uid_message = NULL;
	}

	/* Discard privileges, and set the effective user/group ids to the
	   real user/group ids.  That is, give up our "chmod +s" rights.
	*/
	{
		uid_t euid = geteuid ();
		gid_t egid = getegid ();
		uid_t uid  = getuid ();
		gid_t gid  = getgid ();

		if (orig_uid != NULL)
		{
			*orig_uid = uid_gid_string (euid, egid);
		}

		if (uid != euid || gid != egid)
		{
#ifdef HAVE_BSDAUTH /* we need to setgid auth to run the bsd_auth(3) login_* helpers */
			struct group *authg = getgrnam("auth");
			if (!authg || !authg->gr_name || !*authg->gr_name) {
				reason = g_strdup ("no such group as \"auth\" for bsdauth.");
				ret = FALSE;
				goto out;
			}
			gid = authg->gr_gid;
#endif /* !HAVE_BSDAUTH */
			if (! set_ids_by_number (uid, gid, uid_message))
			{
				reason = g_strdup ("unable to discard privileges.");

				ret = FALSE;
				goto out;
			}
		}
	}

	/* Locking can't work when running as root, because we have no way of
	   knowing what the user id of the logged in user is (so we don't know
	   whose password to prompt for.)

	   *** WARNING: DO NOT DISABLE THIS CODE!
	   If you do so, you will open a security hole.  See the sections
	   of the xscreensaver manual titled "LOCKING AND ROOT LOGINS",
	   and "USING XDM".
	*/
	if (getuid () == (uid_t) 0)
	{
		reason = g_strdup ("running as root");
		ret = FALSE;
		goto out;
	}

out:
	if (nolock_reason != NULL)
	{
		*nolock_reason = g_strdup (reason);
	}
	g_free (reason);

	return ret;
}