1/*
2**
3** Copyright 2010, The Android Open Source Project
4**
5** Licensed under the Apache License, Version 2.0 (the "License");
6** you may not use this file except in compliance with the License.
7** You may obtain a copy of the License at
8**
9**     http://www.apache.org/licenses/LICENSE-2.0
10**
11** Unless required by applicable law or agreed to in writing, software
12** distributed under the License is distributed on an "AS IS" BASIS,
13** WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
14** See the License for the specific language governing permissions and
15** limitations under the License.
16*/
17
18#define PROGNAME  "run-as"
19#define LOG_TAG   PROGNAME
20
21#include <stdio.h>
22#include <stdlib.h>
23#include <string.h>
24#include <sys/types.h>
25#include <sys/stat.h>
26#include <dirent.h>
27#include <errno.h>
28#include <unistd.h>
29#include <time.h>
30#include <stdarg.h>
31
32#include <private/android_filesystem_config.h>
33#include "package.h"
34
35/*
36 *  WARNING WARNING WARNING WARNING
37 *
38 *  This program runs as set-uid root on Android production devices.
39 *  Be very conservative when modifying it to avoid any serious
40 *  security issue. Keep in mind the following:
41 *
42 *  - This program should only run for the 'root' or 'shell' users
43 *
44 *  - Avoid anything that is more complex than simple system calls
45 *    until the uid/gid has been dropped to that of a normal user
46 *    or you are sure to exit.
47 *
48 *    This avoids depending on environment variables, system properties
49 *    and other external factors that may affect the C library in
50 *    unpredictable ways.
51 *
52 *  - Do not trust user input and/or the filesystem whenever possible.
53 *
54 *  Read README.TXT for more details.
55 *
56 *
57 *
58 * The purpose of this program is to run a command as a specific
59 * application user-id. Typical usage is:
60 *
61 *   run-as <package-name> <command> <args>
62 *
63 *  The 'run-as' binary is setuid, but will check the following:
64 *
65 *  - that it is invoked from the 'shell' or 'root' user (abort otherwise)
66 *  - that '<package-name>' is the name of an installed and debuggable package
67 *  - that the package's data directory is well-formed (see package.c)
68 *
69 *  If so, it will cd to the package's data directory, drop to the application's
70 *  user id / group id then run the command there.
71 *
72 *  This can be useful for a number of different things on production devices:
73 *
74 *  - Allow application developers to look at their own applicative data
75 *    during development.
76 *
77 *  - Run the 'gdbserver' binary executable to allow native debugging
78 */
79
80static void
81usage(void)
82{
83    const char*  str = "Usage: " PROGNAME " <package-name> <command> [<args>]\n\n";
84    write(1, str, strlen(str));
85    exit(1);
86}
87
88
89static void
90panic(const char* format, ...)
91{
92    va_list args;
93
94    fprintf(stderr, "%s: ", PROGNAME);
95    va_start(args, format);
96    vfprintf(stderr, format, args);
97    va_end(args);
98    exit(1);
99}
100
101
102int main(int argc, char **argv)
103{
104    const char* pkgname;
105    int myuid, uid, gid;
106    PackageInfo info;
107
108    /* check arguments */
109    if (argc < 2)
110        usage();
111
112    /* check userid of caller - must be 'shell' or 'root' */
113    myuid = getuid();
114    if (myuid != AID_SHELL && myuid != AID_ROOT) {
115        panic("only 'shell' or 'root' users can run this program\n");
116    }
117
118    /* retrieve package information from system */
119    pkgname = argv[1];
120    if (get_package_info(pkgname, &info) < 0) {
121        panic("Package '%s' is unknown\n", pkgname);
122        return 1;
123    }
124
125    /* reject system packages */
126    if (info.uid < AID_APP) {
127        panic("Package '%s' is not an application\n", pkgname);
128        return 1;
129    }
130
131    /* reject any non-debuggable package */
132    if (!info.isDebuggable) {
133        panic("Package '%s' is not debuggable\n", pkgname);
134        return 1;
135    }
136
137    /* check that the data directory path is valid */
138    if (check_data_path(info.dataDir, info.uid) < 0) {
139        panic("Package '%s' has corrupt installation\n", pkgname);
140        return 1;
141    }
142
143    /* then move to it */
144    {
145        int ret;
146        do {
147            ret = chdir(info.dataDir);
148        } while (ret < 0 && errno == EINTR);
149
150        if (ret < 0) {
151            panic("Could not cd to package's data directory: %s\n", strerror(errno));
152            return 1;
153        }
154    }
155
156    /* Ensure that we change all real/effective/saved IDs at the
157     * same time to avoid nasty surprises.
158     */
159    uid = gid = info.uid;
160    if(setresgid(gid,gid,gid) || setresuid(uid,uid,uid)) {
161        panic("Permission denied\n");
162        return 1;
163    }
164
165    /* User specified command for exec. */
166    if (argc >= 3 ) {
167        if (execvp(argv[2], argv+2) < 0) {
168            panic("exec failed for %s Error:%s\n", argv[2], strerror(errno));
169            return -errno;
170        }
171    }
172
173    /* Default exec shell. */
174    execlp("/system/bin/sh", "sh", NULL);
175
176    panic("exec failed\n");
177    return 1;
178}
179