xref: /openssh-portable/uidswap.c (revision 4d28fa78)
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 	setuid(getuid());
166 	setgid(getgid());
167 #endif /* SAVED_IDS_WORK_WITH_SETEUID */
168 
169 	if (setgroups(saved_egroupslen, saved_egroups) == -1)
170 		fatal("setgroups: %.100s", strerror(errno));
171 	temporarily_use_uid_effective = 0;
172 }
173 
174 /*
175  * Permanently sets all uids to the given uid.  This cannot be
176  * called while temporarily_use_uid is effective.
177  */
178 void
permanently_set_uid(struct passwd * pw)179 permanently_set_uid(struct passwd *pw)
180 {
181 #ifndef NO_UID_RESTORATION_TEST
182 	uid_t old_uid = getuid();
183 	gid_t old_gid = getgid();
184 #endif
185 
186 	if (pw == NULL)
187 		fatal("permanently_set_uid: no user given");
188 	if (temporarily_use_uid_effective)
189 		fatal("permanently_set_uid: temporarily_use_uid effective");
190 	debug("permanently_set_uid: %u/%u", (u_int)pw->pw_uid,
191 	    (u_int)pw->pw_gid);
192 
193 	if (setresgid(pw->pw_gid, pw->pw_gid, pw->pw_gid) == -1)
194 		fatal("setresgid %u: %.100s", (u_int)pw->pw_gid, strerror(errno));
195 
196 #ifdef __APPLE__
197 	/*
198 	 * OS X requires initgroups after setgid to opt back into
199 	 * memberd support for >16 supplemental groups.
200 	 */
201 	if (initgroups(pw->pw_name, pw->pw_gid) == -1)
202 		fatal("initgroups %.100s %u: %.100s",
203 		    pw->pw_name, (u_int)pw->pw_gid, strerror(errno));
204 #endif
205 
206 	if (setresuid(pw->pw_uid, pw->pw_uid, pw->pw_uid) == -1)
207 		fatal("setresuid %u: %.100s", (u_int)pw->pw_uid, strerror(errno));
208 
209 #ifndef NO_UID_RESTORATION_TEST
210 	/* Try restoration of GID if changed (test clearing of saved gid) */
211 	if (old_gid != pw->pw_gid && pw->pw_uid != 0 &&
212 	    (setgid(old_gid) != -1 || setegid(old_gid) != -1))
213 		fatal("%s: was able to restore old [e]gid", __func__);
214 #endif
215 
216 	/* Verify GID drop was successful */
217 	if (getgid() != pw->pw_gid || getegid() != pw->pw_gid) {
218 		fatal("%s: egid incorrect gid:%u egid:%u (should be %u)",
219 		    __func__, (u_int)getgid(), (u_int)getegid(),
220 		    (u_int)pw->pw_gid);
221 	}
222 
223 #ifndef NO_UID_RESTORATION_TEST
224 	/* Try restoration of UID if changed (test clearing of saved uid) */
225 	if (old_uid != pw->pw_uid &&
226 	    (setuid(old_uid) != -1 || seteuid(old_uid) != -1))
227 		fatal("%s: was able to restore old [e]uid", __func__);
228 #endif
229 
230 	/* Verify UID drop was successful */
231 	if (getuid() != pw->pw_uid || geteuid() != pw->pw_uid) {
232 		fatal("%s: euid incorrect uid:%u euid:%u (should be %u)",
233 		    __func__, (u_int)getuid(), (u_int)geteuid(),
234 		    (u_int)pw->pw_uid);
235 	}
236 }
237