1 /*
2 * Copyright (C) 2010 The Android Open Source Project
3 *
4 * Licensed under the Apache License, Version 2.0 (the "License");
5 * you may not use this file except in compliance with the License.
6 * You may obtain a copy of the License at
7 *
8 * http://www.apache.org/licenses/LICENSE-2.0
9 *
10 * Unless required by applicable law or agreed to in writing, software
11 * distributed under the License is distributed on an "AS IS" BASIS,
12 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
13 * See the License for the specific language governing permissions and
14 * limitations under the License.
15 */
16
17 #include <errno.h>
18 #include <error.h>
19 #include <paths.h>
20 #include <pwd.h>
21 #include <stdlib.h>
22 #include <string.h>
23 #include <sys/capability.h>
24 #include <sys/stat.h>
25 #include <sys/types.h>
26 #include <unistd.h>
27
28 #include <string>
29 #include <vector>
30
31 #include <libminijail.h>
32 #include <scoped_minijail.h>
33
34 #include <android-base/properties.h>
35 #include <packagelistparser/packagelistparser.h>
36 #include <private/android_filesystem_config.h>
37 #include <selinux/android.h>
38
39 // The purpose of this program is to run a command as a specific
40 // application user-id. Typical usage is:
41 //
42 // run-as <package-name> <command> <args>
43 //
44 // The 'run-as' binary is installed with CAP_SETUID and CAP_SETGID file
45 // capabilities, but will check the following:
46 //
47 // - that the ro.boot.disable_runas property is not set
48 // - that it is invoked from the 'shell' or 'root' user (abort otherwise)
49 // - that '<package-name>' is the name of an installed and debuggable package
50 // - that the package's data directory is well-formed
51 //
52 // If so, it will drop to the application's user id / group id, cd to the
53 // package's data directory, then run the command there.
54 //
55 // This can be useful for a number of different things on production devices:
56 //
57 // - Allow application developers to look at their own application data
58 // during development.
59 //
60 // - Run the 'gdbserver' binary executable to allow native debugging
61 //
62
packagelist_parse_callback(pkg_info * this_package,void * userdata)63 static bool packagelist_parse_callback(pkg_info* this_package, void* userdata) {
64 pkg_info* p = reinterpret_cast<pkg_info*>(userdata);
65 if (strcmp(p->name, this_package->name) == 0) {
66 *p = *this_package;
67 return false; // Stop searching.
68 }
69 packagelist_free(this_package);
70 return true; // Keep searching.
71 }
72
check_directory(const char * path,uid_t uid)73 static void check_directory(const char* path, uid_t uid) {
74 struct stat st;
75 if (TEMP_FAILURE_RETRY(lstat(path, &st)) == -1) {
76 error(1, errno, "couldn't stat %s", path);
77 }
78
79 // Must be a real directory, not a symlink.
80 if (!S_ISDIR(st.st_mode)) {
81 error(1, 0, "%s not a directory: %o", path, st.st_mode);
82 }
83
84 // Must be owned by specific uid/gid.
85 if (st.st_uid != uid || st.st_gid != uid) {
86 error(1, 0, "%s has wrong owner: %d/%d, not %d", path, st.st_uid, st.st_gid, uid);
87 }
88
89 // Must not be readable or writable by others.
90 if ((st.st_mode & (S_IROTH | S_IWOTH)) != 0) {
91 error(1, 0, "%s readable or writable by others: %o", path, st.st_mode);
92 }
93 }
94
95 // This function is used to check the data directory path for safety.
96 // We check that every sub-directory is owned by the 'system' user
97 // and exists and is not a symlink. We also check that the full directory
98 // path is properly owned by the user ID.
check_data_path(const char * package_name,const char * data_path,uid_t uid)99 static void check_data_path(const char* package_name, const char* data_path, uid_t uid) {
100 // The path should be absolute.
101 if (data_path[0] != '/') {
102 error(1, 0, "%s data path not absolute: %s", package_name, data_path);
103 }
104
105 // Look for all sub-paths, we do that by finding
106 // directory separators in the input path and
107 // checking each sub-path independently.
108 for (int nn = 1; data_path[nn] != '\0'; nn++) {
109 char subpath[PATH_MAX];
110
111 /* skip non-separator characters */
112 if (data_path[nn] != '/') continue;
113
114 /* handle trailing separator case */
115 if (data_path[nn+1] == '\0') break;
116
117 /* found a separator, check that data_path is not too long. */
118 if (nn >= (int)(sizeof subpath)) {
119 error(1, 0, "%s data path too long: %s", package_name, data_path);
120 }
121
122 /* reject any '..' subpath */
123 if (nn >= 3 &&
124 data_path[nn-3] == '/' &&
125 data_path[nn-2] == '.' &&
126 data_path[nn-1] == '.') {
127 error(1, 0, "%s contains '..': %s", package_name, data_path);
128 }
129
130 /* copy to 'subpath', then check ownership */
131 memcpy(subpath, data_path, nn);
132 subpath[nn] = '\0';
133
134 check_directory(subpath, AID_SYSTEM);
135 }
136
137 // All sub-paths were checked, now verify that the full data
138 // directory is owned by the application uid.
139 check_directory(data_path, uid);
140 }
141
get_supplementary_gids(uid_t userAppId)142 std::vector<gid_t> get_supplementary_gids(uid_t userAppId) {
143 std::vector<gid_t> gids;
144 int size = getgroups(0, &gids[0]);
145 if (size < 0) {
146 error(1, errno, "getgroups failed");
147 }
148 gids.resize(size);
149 size = getgroups(size, &gids[0]);
150 if (size != static_cast<int>(gids.size())) {
151 error(1, errno, "getgroups failed");
152 }
153 // Profile guide compiled oat files (like /data/app/xxx/oat/arm64/base.odex) are not readable
154 // worldwide (DEXOPT_PUBLIC flag isn't set). To support reading them (needed by simpleperf for
155 // profiling), add shared app gid to supplementary groups.
156 gid_t shared_app_gid = userAppId % AID_USER_OFFSET - AID_APP_START + AID_SHARED_GID_START;
157 gids.push_back(shared_app_gid);
158 return gids;
159 }
160
main(int argc,char * argv[])161 int main(int argc, char* argv[]) {
162 // Check arguments.
163 if (argc < 2) {
164 error(1, 0, "usage: run-as <package-name> [--user <uid>] <command> [<args>]\n");
165 }
166
167 // This program runs with CAP_SETUID and CAP_SETGID capabilities on Android
168 // production devices. Check user id of caller --- must be 'shell' or 'root'.
169 if (getuid() != AID_SHELL && getuid() != AID_ROOT) {
170 error(1, 0, "only 'shell' or 'root' users can run this program");
171 }
172
173 // Some devices can disable running run-as, such as Chrome OS when running in
174 // non-developer mode.
175 if (android::base::GetBoolProperty("ro.boot.disable_runas", false)) {
176 error(1, 0, "run-as is disabled from the kernel commandline");
177 }
178
179 char* pkgname = argv[1];
180 int cmd_argv_offset = 2;
181
182 // Get user_id from command line if provided.
183 int userId = 0;
184 if ((argc >= 4) && !strcmp(argv[2], "--user")) {
185 userId = atoi(argv[3]);
186 if (userId < 0) error(1, 0, "negative user id: %d", userId);
187 cmd_argv_offset += 2;
188 }
189
190 // Retrieve package information from system, switching egid so we can read the file.
191 pkg_info info = {.name = pkgname};
192 gid_t old_egid = getegid();
193 if (setegid(AID_PACKAGE_INFO) == -1) error(1, errno, "setegid(AID_PACKAGE_INFO) failed");
194 if (!packagelist_parse(packagelist_parse_callback, &info)) {
195 error(1, errno, "packagelist_parse failed");
196 }
197 if (setegid(old_egid) == -1) error(1, errno, "couldn't restore egid");
198
199 if (info.uid == 0) {
200 error(1, 0, "unknown package: %s", pkgname);
201 }
202
203 // Verify that user id is not too big.
204 if ((UID_MAX - info.uid) / AID_USER_OFFSET < (uid_t)userId) {
205 error(1, 0, "user id too big: %d", userId);
206 }
207
208 // Calculate user app ID.
209 uid_t userAppId = (AID_USER_OFFSET * userId) + info.uid;
210
211 // Reject system packages.
212 if (userAppId < AID_APP) {
213 error(1, 0, "package not an application: %s", pkgname);
214 }
215
216 // Reject any non-debuggable package.
217 if (!info.debuggable) {
218 error(1, 0, "package not debuggable: %s", pkgname);
219 }
220
221 // Ensure we have the right data path for the specific user.
222 free(info.data_dir);
223 if (asprintf(&info.data_dir, "/data/user/%d/%s", userId, pkgname) == -1) {
224 error(1, errno, "asprintf failed");
225 }
226
227 // Check that the data directory path is valid.
228 check_data_path(pkgname, info.data_dir, userAppId);
229
230 // Ensure that we change all real/effective/saved IDs at the
231 // same time to avoid nasty surprises.
232 uid_t uid = userAppId;
233 uid_t gid = userAppId;
234 std::vector<gid_t> supplementary_gids = get_supplementary_gids(userAppId);
235 ScopedMinijail j(minijail_new());
236 minijail_change_uid(j.get(), uid);
237 minijail_change_gid(j.get(), gid);
238 minijail_set_supplementary_gids(j.get(), supplementary_gids.size(), supplementary_gids.data());
239 minijail_enter(j.get());
240
241 std::string seinfo = std::string(info.seinfo) + ":fromRunAs";
242 if (selinux_android_setcontext(uid, 0, seinfo.c_str(), pkgname) < 0) {
243 error(1, errno, "couldn't set SELinux security context '%s'", seinfo.c_str());
244 }
245
246 // cd into the data directory, and set $HOME correspondingly.
247 if (TEMP_FAILURE_RETRY(chdir(info.data_dir)) == -1) {
248 error(1, errno, "couldn't chdir to package's data directory '%s'", info.data_dir);
249 }
250 setenv("HOME", info.data_dir, 1);
251
252 // Reset parts of the environment, like su would.
253 setenv("PATH", _PATH_DEFPATH, 1);
254 unsetenv("IFS");
255
256 // Set the user-specific parts for this user.
257 passwd* pw = getpwuid(uid);
258 setenv("LOGNAME", pw->pw_name, 1);
259 setenv("SHELL", pw->pw_shell, 1);
260 setenv("USER", pw->pw_name, 1);
261
262 // User specified command for exec.
263 if ((argc >= cmd_argv_offset + 1) &&
264 (execvp(argv[cmd_argv_offset], argv+cmd_argv_offset) == -1)) {
265 error(1, errno, "exec failed for %s", argv[cmd_argv_offset]);
266 }
267
268 // Default exec shell.
269 execlp(_PATH_BSHELL, "sh", NULL);
270 error(1, errno, "exec failed");
271 }
272