1 /* $OpenBSD: uidswap.c,v 1.37 2015/01/16 06:40:12 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 #ifdef ANDROID
32 #include <private/android_filesystem_config.h>
33 #include <sys/capability.h>
34 #include <sys/prctl.h>
35 #endif
36 
37 /*
38  * Note: all these functions must work in all of the following cases:
39  *    1. euid=0, ruid=0
40  *    2. euid=0, ruid!=0
41  *    3. euid!=0, ruid!=0
42  * Additionally, they must work regardless of whether the system has
43  * POSIX saved uids or not.
44  */
45 
46 #if defined(_POSIX_SAVED_IDS) && !defined(BROKEN_SAVED_UIDS)
47 /* Lets assume that posix saved ids also work with seteuid, even though that
48    is not part of the posix specification. */
49 #define SAVED_IDS_WORK_WITH_SETEUID
50 /* Saved effective uid. */
51 static uid_t 	saved_euid = 0;
52 static gid_t	saved_egid = 0;
53 #endif
54 
55 /* Saved effective uid. */
56 static int	privileged = 0;
57 static int	temporarily_use_uid_effective = 0;
58 static gid_t	*saved_egroups = NULL, *user_groups = NULL;
59 static int	saved_egroupslen = -1, user_groupslen = -1;
60 
61 /*
62  * Temporarily changes to the given uid.  If the effective user
63  * id is not root, this does nothing.  This call cannot be nested.
64  */
65 void
temporarily_use_uid(struct passwd * pw)66 temporarily_use_uid(struct passwd *pw)
67 {
68 	/* Save the current euid, and egroups. */
69 #ifdef SAVED_IDS_WORK_WITH_SETEUID
70 	saved_euid = geteuid();
71 	saved_egid = getegid();
72 	debug("temporarily_use_uid: %u/%u (e=%u/%u)",
73 	    (u_int)pw->pw_uid, (u_int)pw->pw_gid,
74 	    (u_int)saved_euid, (u_int)saved_egid);
75 #ifndef HAVE_CYGWIN
76 	if (saved_euid != 0) {
77 		privileged = 0;
78 		return;
79 	}
80 #endif
81 #else
82 	if (geteuid() != 0) {
83 		privileged = 0;
84 		return;
85 	}
86 #endif /* SAVED_IDS_WORK_WITH_SETEUID */
87 
88 	privileged = 1;
89 	temporarily_use_uid_effective = 1;
90 
91 	saved_egroupslen = getgroups(0, NULL);
92 	if (saved_egroupslen < 0)
93 		fatal("getgroups: %.100s", strerror(errno));
94 	if (saved_egroupslen > 0) {
95 		saved_egroups = xrealloc(saved_egroups,
96 		    saved_egroupslen, sizeof(gid_t));
97 		if (getgroups(saved_egroupslen, saved_egroups) < 0)
98 			fatal("getgroups: %.100s", strerror(errno));
99 	} else { /* saved_egroupslen == 0 */
100 		free(saved_egroups);
101 	}
102 
103 	/* set and save the user's groups */
104 	if (user_groupslen == -1) {
105 		if (initgroups(pw->pw_name, pw->pw_gid) < 0)
106 			fatal("initgroups: %s: %.100s", pw->pw_name,
107 			    strerror(errno));
108 
109 		user_groupslen = getgroups(0, NULL);
110 		if (user_groupslen < 0)
111 			fatal("getgroups: %.100s", strerror(errno));
112 		if (user_groupslen > 0) {
113 			user_groups = xrealloc(user_groups,
114 			    user_groupslen, sizeof(gid_t));
115 			if (getgroups(user_groupslen, user_groups) < 0)
116 				fatal("getgroups: %.100s", strerror(errno));
117 		} else { /* user_groupslen == 0 */
118 			free(user_groups);
119 		}
120 	}
121 	/* Set the effective uid to the given (unprivileged) uid. */
122 	if (setgroups(user_groupslen, user_groups) < 0)
123 		fatal("setgroups: %.100s", strerror(errno));
124 #ifndef SAVED_IDS_WORK_WITH_SETEUID
125 	/* Propagate the privileged gid to all of our gids. */
126 	if (setgid(getegid()) < 0)
127 		debug("setgid %u: %.100s", (u_int) getegid(), strerror(errno));
128 	/* Propagate the privileged uid to all of our uids. */
129 	if (setuid(geteuid()) < 0)
130 		debug("setuid %u: %.100s", (u_int) geteuid(), strerror(errno));
131 #endif /* SAVED_IDS_WORK_WITH_SETEUID */
132 	if (setegid(pw->pw_gid) < 0)
133 		fatal("setegid %u: %.100s", (u_int)pw->pw_gid,
134 		    strerror(errno));
135 	if (seteuid(pw->pw_uid) == -1)
136 		fatal("seteuid %u: %.100s", (u_int)pw->pw_uid,
137 		    strerror(errno));
138 }
139 
140 void
permanently_drop_suid(uid_t uid)141 permanently_drop_suid(uid_t uid)
142 {
143 #ifndef HAVE_CYGWIN
144 	uid_t old_uid = getuid();
145 #endif
146 
147 	debug("permanently_drop_suid: %u", (u_int)uid);
148 	if (setresuid(uid, uid, uid) < 0)
149 		fatal("setresuid %u: %.100s", (u_int)uid, strerror(errno));
150 
151 #ifndef HAVE_CYGWIN
152 	/* Try restoration of UID if changed (test clearing of saved uid) */
153 	if (old_uid != uid &&
154 	    (setuid(old_uid) != -1 || seteuid(old_uid) != -1))
155 		fatal("%s: was able to restore old [e]uid", __func__);
156 #endif
157 
158 	/* Verify UID drop was successful */
159 	if (getuid() != uid || geteuid() != uid) {
160 		fatal("%s: euid incorrect uid:%u euid:%u (should be %u)",
161 		    __func__, (u_int)getuid(), (u_int)geteuid(), (u_int)uid);
162 	}
163 }
164 
165 /*
166  * Restores to the original (privileged) uid.
167  */
168 void
restore_uid(void)169 restore_uid(void)
170 {
171 	/* it's a no-op unless privileged */
172 	if (!privileged) {
173 		debug("restore_uid: (unprivileged)");
174 		return;
175 	}
176 	if (!temporarily_use_uid_effective)
177 		fatal("restore_uid: temporarily_use_uid not effective");
178 
179 #ifdef SAVED_IDS_WORK_WITH_SETEUID
180 	debug("restore_uid: %u/%u", (u_int)saved_euid, (u_int)saved_egid);
181 	/* Set the effective uid back to the saved privileged uid. */
182 	if (seteuid(saved_euid) < 0)
183 		fatal("seteuid %u: %.100s", (u_int)saved_euid, strerror(errno));
184 	if (setegid(saved_egid) < 0)
185 		fatal("setegid %u: %.100s", (u_int)saved_egid, strerror(errno));
186 #else /* SAVED_IDS_WORK_WITH_SETEUID */
187 	/*
188 	 * We are unable to restore the real uid to its unprivileged value.
189 	 * Propagate the real uid (usually more privileged) to effective uid
190 	 * as well.
191 	 */
192 	setuid(getuid());
193 	setgid(getgid());
194 #endif /* SAVED_IDS_WORK_WITH_SETEUID */
195 
196 	if (setgroups(saved_egroupslen, saved_egroups) < 0)
197 		fatal("setgroups: %.100s", strerror(errno));
198 	temporarily_use_uid_effective = 0;
199 }
200 
201 /*
202  * Permanently sets all uids to the given uid.  This cannot be
203  * called while temporarily_use_uid is effective.
204  */
205 void
permanently_set_uid(struct passwd * pw)206 permanently_set_uid(struct passwd *pw)
207 {
208 #ifndef HAVE_CYGWIN
209 	uid_t old_uid = getuid();
210 	gid_t old_gid = getgid();
211 #endif
212 #if defined(ANDROID)
213 	struct __user_cap_header_struct header;
214 	struct __user_cap_data_struct cap;
215 #endif
216 
217 	if (pw == NULL)
218 		fatal("permanently_set_uid: no user given");
219 	if (temporarily_use_uid_effective)
220 		fatal("permanently_set_uid: temporarily_use_uid effective");
221 	debug("permanently_set_uid: %u/%u", (u_int)pw->pw_uid,
222 	    (u_int)pw->pw_gid);
223 
224 #if defined(ANDROID)
225 	if (pw->pw_uid == AID_SHELL) {
226 		prctl(PR_SET_KEEPCAPS, 1, 0, 0, 0);
227 
228 		/* add extra groups needed for shell user:
229 		 * - AID_LOG to read system logs (adb logcat)
230 		 * - AID_INPUT to diagnose input issues (getevent)
231 		 * - AID_INET to diagnose network issues (netcfg, ping)
232 		 * - AID_GRAPHICS to access the frame buffer
233 		 * - AID_NET_BT and AID_NET_BT_ADMIN to diagnose bluetooth (hcidump)
234 		 * - AID_SDCARD_RW to allow writing to the SD card
235 		 * - AID_MOUNT to allow unmounting the SD card before rebooting
236 		 * - AID_NET_BW_STATS to read out qtaguid statistics. */
237 		gid_t groups[] = {AID_LOG,       AID_INPUT,  AID_INET,
238 				  AID_GRAPHICS,  AID_NET_BT, AID_NET_BT_ADMIN,
239 				  AID_SDCARD_RW, AID_MOUNT,  AID_NET_BW_STATS};
240 		setgroups(sizeof(groups)/sizeof(groups[0]), groups);
241 	}
242 #endif
243 
244 	if (setresgid(pw->pw_gid, pw->pw_gid, pw->pw_gid) < 0)
245 		fatal("setresgid %u: %.100s", (u_int)pw->pw_gid, strerror(errno));
246 
247 #ifdef __APPLE__
248 	/*
249 	 * OS X requires initgroups after setgid to opt back into
250 	 * memberd support for >16 supplemental groups.
251 	 */
252 	if (initgroups(pw->pw_name, pw->pw_gid) < 0)
253 		fatal("initgroups %.100s %u: %.100s",
254 		    pw->pw_name, (u_int)pw->pw_gid, strerror(errno));
255 #endif
256 
257 	if (setresuid(pw->pw_uid, pw->pw_uid, pw->pw_uid) < 0)
258 		fatal("setresuid %u: %.100s", (u_int)pw->pw_uid, strerror(errno));
259 
260 #ifndef HAVE_CYGWIN
261 	/* Try restoration of GID if changed (test clearing of saved gid) */
262 	if (old_gid != pw->pw_gid && pw->pw_uid != 0 &&
263 	    (setgid(old_gid) != -1 || setegid(old_gid) != -1))
264 		fatal("%s: was able to restore old [e]gid", __func__);
265 #endif
266 
267 	/* Verify GID drop was successful */
268 	if (getgid() != pw->pw_gid || getegid() != pw->pw_gid) {
269 		fatal("%s: egid incorrect gid:%u egid:%u (should be %u)",
270 		    __func__, (u_int)getgid(), (u_int)getegid(),
271 		    (u_int)pw->pw_gid);
272 	}
273 
274 #ifndef HAVE_CYGWIN
275 	/* Try restoration of UID if changed (test clearing of saved uid) */
276 	if (old_uid != pw->pw_uid &&
277 	    (setuid(old_uid) != -1 || seteuid(old_uid) != -1))
278 		fatal("%s: was able to restore old [e]uid", __func__);
279 #endif
280 
281 	/* Verify UID drop was successful */
282 	if (getuid() != pw->pw_uid || geteuid() != pw->pw_uid) {
283 		fatal("%s: euid incorrect uid:%u euid:%u (should be %u)",
284 		    __func__, (u_int)getuid(), (u_int)geteuid(),
285 		    (u_int)pw->pw_uid);
286 	}
287 
288 #ifdef ANDROID
289 	if (pw->pw_uid == AID_SHELL) {
290 		/* set CAP_SYS_BOOT capability, so "adb reboot" will succeed */
291 		header.version = _LINUX_CAPABILITY_VERSION;
292 		header.pid = 0;
293 		cap.effective = cap.permitted = (1 << CAP_SYS_BOOT);
294 		cap.inheritable = 0;
295 		capset(&header, &cap);
296 	}
297 #endif
298 
299 }
300