1 /* $OpenBSD: uidswap.c,v 1.42 2019/06/28 13:35:04 deraadt Exp $ */
2 /*
3 * Author: Tatu Ylonen <ylo@cs.hut.fi>
4 * Copyright (c) 1995 Tatu Ylonen <ylo@cs.hut.fi>, Espoo, Finland
5 * All rights reserved
6 * Code for uid-swapping.
7 *
8 * As far as I am concerned, the code I have written for this software
9 * can be used freely for any purpose. Any derived versions of this
10 * software must be clearly marked as such, and if the derived work is
11 * incompatible with the protocol description in the RFC file, it must be
12 * called by a name other than "ssh" or "Secure Shell".
13 */
14
15 #include "includes.h"
16
17 #include <errno.h>
18 #include <pwd.h>
19 #include <string.h>
20 #include <unistd.h>
21 #include <limits.h>
22 #include <stdarg.h>
23 #include <stdlib.h>
24
25 #include <grp.h>
26
27 #include "log.h"
28 #include "uidswap.h"
29 #include "xmalloc.h"
30
31 /*
32 * Note: all these functions must work in all of the following cases:
33 * 1. euid=0, ruid=0
34 * 2. euid=0, ruid!=0
35 * 3. euid!=0, ruid!=0
36 * Additionally, they must work regardless of whether the system has
37 * POSIX saved uids or not.
38 */
39
40 #if defined(_POSIX_SAVED_IDS) && !defined(BROKEN_SAVED_UIDS)
41 /* Lets assume that posix saved ids also work with seteuid, even though that
42 is not part of the posix specification. */
43 #define SAVED_IDS_WORK_WITH_SETEUID
44 /* Saved effective uid. */
45 static uid_t saved_euid = 0;
46 static gid_t saved_egid = 0;
47 #endif
48
49 /* Saved effective uid. */
50 static int privileged = 0;
51 static int temporarily_use_uid_effective = 0;
52 static uid_t user_groups_uid;
53 static gid_t *saved_egroups = NULL, *user_groups = NULL;
54 static int saved_egroupslen = -1, user_groupslen = -1;
55
56 /*
57 * Temporarily changes to the given uid. If the effective user
58 * id is not root, this does nothing. This call cannot be nested.
59 */
60 void
temporarily_use_uid(struct passwd * pw)61 temporarily_use_uid(struct passwd *pw)
62 {
63 /* Save the current euid, and egroups. */
64 #ifdef SAVED_IDS_WORK_WITH_SETEUID
65 saved_euid = geteuid();
66 saved_egid = getegid();
67 debug("temporarily_use_uid: %u/%u (e=%u/%u)",
68 (u_int)pw->pw_uid, (u_int)pw->pw_gid,
69 (u_int)saved_euid, (u_int)saved_egid);
70 #ifndef HAVE_CYGWIN
71 if (saved_euid != 0) {
72 privileged = 0;
73 return;
74 }
75 #endif
76 #else
77 if (geteuid() != 0) {
78 privileged = 0;
79 return;
80 }
81 #endif /* SAVED_IDS_WORK_WITH_SETEUID */
82
83 privileged = 1;
84 temporarily_use_uid_effective = 1;
85
86 saved_egroupslen = getgroups(0, NULL);
87 if (saved_egroupslen == -1)
88 fatal("getgroups: %.100s", strerror(errno));
89 if (saved_egroupslen > 0) {
90 saved_egroups = xreallocarray(saved_egroups,
91 saved_egroupslen, sizeof(gid_t));
92 if (getgroups(saved_egroupslen, saved_egroups) == -1)
93 fatal("getgroups: %.100s", strerror(errno));
94 } else { /* saved_egroupslen == 0 */
95 free(saved_egroups);
96 saved_egroups = NULL;
97 }
98
99 /* set and save the user's groups */
100 if (user_groupslen == -1 || user_groups_uid != pw->pw_uid) {
101 if (initgroups(pw->pw_name, pw->pw_gid) == -1)
102 fatal("initgroups: %s: %.100s", pw->pw_name,
103 strerror(errno));
104
105 user_groupslen = getgroups(0, NULL);
106 if (user_groupslen == -1)
107 fatal("getgroups: %.100s", strerror(errno));
108 if (user_groupslen > 0) {
109 user_groups = xreallocarray(user_groups,
110 user_groupslen, sizeof(gid_t));
111 if (getgroups(user_groupslen, user_groups) == -1)
112 fatal("getgroups: %.100s", strerror(errno));
113 } else { /* user_groupslen == 0 */
114 free(user_groups);
115 user_groups = NULL;
116 }
117 user_groups_uid = pw->pw_uid;
118 }
119 /* Set the effective uid to the given (unprivileged) uid. */
120 if (setgroups(user_groupslen, user_groups) == -1)
121 fatal("setgroups: %.100s", strerror(errno));
122 #ifndef SAVED_IDS_WORK_WITH_SETEUID
123 /* Propagate the privileged gid to all of our gids. */
124 if (setgid(getegid()) == -1)
125 debug("setgid %u: %.100s", (u_int) getegid(), strerror(errno));
126 /* Propagate the privileged uid to all of our uids. */
127 if (setuid(geteuid()) == -1)
128 debug("setuid %u: %.100s", (u_int) geteuid(), strerror(errno));
129 #endif /* SAVED_IDS_WORK_WITH_SETEUID */
130 if (setegid(pw->pw_gid) == -1)
131 fatal("setegid %u: %.100s", (u_int)pw->pw_gid,
132 strerror(errno));
133 if (seteuid(pw->pw_uid) == -1)
134 fatal("seteuid %u: %.100s", (u_int)pw->pw_uid,
135 strerror(errno));
136 }
137
138 /*
139 * Restores to the original (privileged) uid.
140 */
141 void
restore_uid(void)142 restore_uid(void)
143 {
144 /* it's a no-op unless privileged */
145 if (!privileged) {
146 debug("restore_uid: (unprivileged)");
147 return;
148 }
149 if (!temporarily_use_uid_effective)
150 fatal("restore_uid: temporarily_use_uid not effective");
151
152 #ifdef SAVED_IDS_WORK_WITH_SETEUID
153 debug("restore_uid: %u/%u", (u_int)saved_euid, (u_int)saved_egid);
154 /* Set the effective uid back to the saved privileged uid. */
155 if (seteuid(saved_euid) == -1)
156 fatal("seteuid %u: %.100s", (u_int)saved_euid, strerror(errno));
157 if (setegid(saved_egid) == -1)
158 fatal("setegid %u: %.100s", (u_int)saved_egid, strerror(errno));
159 #else /* SAVED_IDS_WORK_WITH_SETEUID */
160 /*
161 * We are unable to restore the real uid to its unprivileged value.
162 * Propagate the real uid (usually more privileged) to effective uid
163 * as well.
164 */
165 if (setuid(getuid()) == -1)
166 fatal("%s: setuid failed: %s", __func__, strerror(errno));
167 if (setgid(getgid()) == -1)
168 fatal("%s: setgid failed: %s", __func__, strerror(errno));
169 #endif /* SAVED_IDS_WORK_WITH_SETEUID */
170
171 if (setgroups(saved_egroupslen, saved_egroups) == -1)
172 fatal("setgroups: %.100s", strerror(errno));
173 temporarily_use_uid_effective = 0;
174 }
175
176 /*
177 * Permanently sets all uids to the given uid. This cannot be
178 * called while temporarily_use_uid is effective.
179 */
180 void
permanently_set_uid(struct passwd * pw)181 permanently_set_uid(struct passwd *pw)
182 {
183 #ifndef NO_UID_RESTORATION_TEST
184 uid_t old_uid = getuid();
185 gid_t old_gid = getgid();
186 #endif
187
188 if (pw == NULL)
189 fatal("permanently_set_uid: no user given");
190 if (temporarily_use_uid_effective)
191 fatal("permanently_set_uid: temporarily_use_uid effective");
192 debug("permanently_set_uid: %u/%u", (u_int)pw->pw_uid,
193 (u_int)pw->pw_gid);
194
195 if (setresgid(pw->pw_gid, pw->pw_gid, pw->pw_gid) == -1)
196 fatal("setresgid %u: %.100s", (u_int)pw->pw_gid, strerror(errno));
197
198 #ifdef __APPLE__
199 /*
200 * OS X requires initgroups after setgid to opt back into
201 * memberd support for >16 supplemental groups.
202 */
203 if (initgroups(pw->pw_name, pw->pw_gid) == -1)
204 fatal("initgroups %.100s %u: %.100s",
205 pw->pw_name, (u_int)pw->pw_gid, strerror(errno));
206 #endif
207
208 if (setresuid(pw->pw_uid, pw->pw_uid, pw->pw_uid) == -1)
209 fatal("setresuid %u: %.100s", (u_int)pw->pw_uid, strerror(errno));
210
211 #ifndef NO_UID_RESTORATION_TEST
212 /* Try restoration of GID if changed (test clearing of saved gid) */
213 if (old_gid != pw->pw_gid && pw->pw_uid != 0 &&
214 (setgid(old_gid) != -1 || setegid(old_gid) != -1))
215 fatal("%s: was able to restore old [e]gid", __func__);
216 #endif
217
218 /* Verify GID drop was successful */
219 if (getgid() != pw->pw_gid || getegid() != pw->pw_gid) {
220 fatal("%s: egid incorrect gid:%u egid:%u (should be %u)",
221 __func__, (u_int)getgid(), (u_int)getegid(),
222 (u_int)pw->pw_gid);
223 }
224
225 #ifndef NO_UID_RESTORATION_TEST
226 /* Try restoration of UID if changed (test clearing of saved uid) */
227 if (old_uid != pw->pw_uid &&
228 (setuid(old_uid) != -1 || seteuid(old_uid) != -1))
229 fatal("%s: was able to restore old [e]uid", __func__);
230 #endif
231
232 /* Verify UID drop was successful */
233 if (getuid() != pw->pw_uid || geteuid() != pw->pw_uid) {
234 fatal("%s: euid incorrect uid:%u euid:%u (should be %u)",
235 __func__, (u_int)getuid(), (u_int)geteuid(),
236 (u_int)pw->pw_uid);
237 }
238 }
239