summaryrefslogtreecommitdiffstats
path: root/source/xap/xscreensaver/setuid.c
blob: 343dcf097dba0bb5b0b8a234492420a6ddeb6145 (about) (plain) (blame)
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
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
/* setuid.c --- management of runtime privileges.
 * xscreensaver, Copyright (c) 1993-1998 Jamie Zawinski <jwz@jwz.org>
 *
 * 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.
 */

#ifdef HAVE_CONFIG_H
# include "config.h"
#endif

#include <X11/Xlib.h>		/* not used for much... */

/* This file doesn't need the Xt headers, so stub these types out... */
#undef XtPointer
#define XtAppContext void*
#define XrmDatabase  void*
#define XtIntervalId void*
#define XtPointer    void*
#define Widget       void*

#include "xscreensaver.h"

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

#include <pwd.h>		/* for getpwnam() and struct passwd */
#include <grp.h>		/* for getgrgid() and struct group */

static const char *
uid_gid_string (uid_t uid, gid_t gid)
{
  static char buf[255];
  struct passwd *p = 0;
  struct group *g = 0;
  p = getpwuid (uid);
  g = getgrgid (gid);
  sprintf (buf, "%s/%s (%ld/%ld)",
	   (p && p->pw_name ? p->pw_name : "???"),
	   (g && g->gr_name ? g->gr_name : "???"),
	   (long) uid, (long) gid);
  return buf;
}


void
describe_uids (saver_info *si, FILE *out)
{
  uid_t uid = getuid();
  gid_t gid = getgid();
  uid_t euid = geteuid();
  gid_t egid = getegid();
  char *s1 = strdup (uid_gid_string (uid, gid));
  char *s2 = strdup (uid_gid_string (euid, egid));

  if (si->orig_uid && *si->orig_uid &&
      (!!strcmp (si->orig_uid, s1) ||
       !!strcmp (si->orig_uid, s2)))
    fprintf (out, "%s: initial effective uid/gid was %s\n", blurb(),
	     si->orig_uid);

  fprintf (out, "%s: running as %s", blurb(), s1);
  if (uid != euid || gid != egid)
    fprintf (out, "; effectively %s", s2);
  fprintf(out, "\n");
  free(s1);
  free(s2);
}


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

  if (message_ret)
    *message_ret = 0;

  /* 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 (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)
    {
      static char buf [1024];
      sprintf (buf, "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 = buf;
      return 0;
    }
  else
    {
      char buf [1024];
      if (gid_errno)
	{
	  sprintf (buf, "%s: couldn't set gid to %s (%ld)",
		   blurb(),
		   (g && g->gr_name ? g->gr_name : "???"),
		   (long) gid);
	  if (gid_errno == -1)
	    fprintf(stderr, "%s: unknown error\n", buf);
	  else
	    perror(buf);
	}

      if (uid_errno)
	{
	  sprintf (buf, "%s: couldn't set uid to %s (%ld)",
		   blurb(),
		   (p && p->pw_name ? p->pw_name : "???"),
		   (long) uid);
	  if (uid_errno == -1)
	    fprintf(stderr, "%s: unknown error\n", buf);
	  else
	    perror(buf);
	}

      return -1;
    }
}


/* 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".
 */
void
hack_uid (saver_info *si)
{

  /* 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();

    si->orig_uid = strdup (uid_gid_string (euid, egid));

    if (uid != euid || gid != egid)
      if (set_ids_by_number (uid, gid, &si->uid_message) != 0)
	saver_exit (si, 1, 0);
  }


  /* 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)
    {
      si->locking_disabled_p = True;
      si->nolock_reason = "running as root";
    }


  /* If we're running as root, switch to a safer user.  This is above and
     beyond the fact that we've disabling locking, above -- the theory is
     that running graphics demos as root is just always a stupid thing
     to do, since they have probably never been security reviewed and are
     more likely to be buggy than just about any other kind of program.
     (And that assumes non-malicious code.  There are also attacks here.)

     *** 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)
    {
      struct passwd *p;

      p = getpwnam ("nobody");
      if (! p) p = getpwnam ("noaccess");
      if (! p) p = getpwnam ("daemon");
      if (! p)
	{
	  fprintf (stderr,
		   "%s: running as root, and couldn't find a safer uid.\n",
		   blurb());
	  saver_exit(si, 1, 0);
	}

      if (set_ids_by_number (p->pw_uid, p->pw_gid, &si->uid_message) != 0)
	saver_exit (si, -1, 0);
    }


  /* If there's anything even remotely funny looking about the passwd struct,
     or if we're running as some other user from the list below (a
     non-comprehensive selection of users known to be privileged in some way,
     and not normal end-users) then disable locking.  If it was possible,
     switching to "nobody" would be the thing to do, but only root itself has
     the privs to do that.

     *** 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".
   */
  {
    uid_t uid = getuid ();		/* get it again */
    struct passwd *p = getpwuid (uid);	/* get it again */

    if (!p ||
	uid == (uid_t)  0 ||
	uid == (uid_t) -1 ||
	uid == (uid_t) -2 ||
	p->pw_uid == (uid_t)  0 ||
	p->pw_uid == (uid_t) -1 ||
	p->pw_uid == (uid_t) -2 ||
	!p->pw_name ||
	!*p->pw_name ||
	!strcmp (p->pw_name, "root") ||
	!strcmp (p->pw_name, "nobody") ||
	!strcmp (p->pw_name, "noaccess") ||
	!strcmp (p->pw_name, "operator") ||
	!strcmp (p->pw_name, "daemon") ||
	!strcmp (p->pw_name, "bin") ||
	!strcmp (p->pw_name, "adm") ||
	!strcmp (p->pw_name, "sys") ||
	!strcmp (p->pw_name, "games"))
      {
	static char buf [1024];
	sprintf (buf, "running as %s",
		 (p && p->pw_name && *p->pw_name
		  ? p->pw_name : "<unknown>"));
	si->nolock_reason = buf;
	si->locking_disabled_p = True;
	si->dangerous_uid_p = True;
      }
  }
}