summaryrefslogtreecommitdiffstats
path: root/run-as/run-as.c
blob: 368b8f1e6a708c94b1c291ad16f5a9afe38ca16a (plain)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
/*
**
** Copyright 2010, The Android Open Source Project
**
** Licensed under the Apache License, Version 2.0 (the "License");
** you may not use this file except in compliance with the License.
** You may obtain a copy of the License at
**
**     http://www.apache.org/licenses/LICENSE-2.0
**
** Unless required by applicable law or agreed to in writing, software
** distributed under the License is distributed on an "AS IS" BASIS,
** WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
** See the License for the specific language governing permissions and
** limitations under the License.
*/

#define PROGNAME "run-as"
#define LOG_TAG  PROGNAME

#include <dirent.h>
#include <errno.h>
#include <stdarg.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/capability.h>
#include <sys/cdefs.h>
#include <sys/stat.h>
#include <sys/types.h>
#include <time.h>
#include <unistd.h>

#include <private/android_filesystem_config.h>
#include <selinux/android.h>

#include "package.h"

/*
 *  WARNING WARNING WARNING WARNING
 *
 *  This program runs with CAP_SETUID and CAP_SETGID capabilities on Android
 *  production devices. Be very conservative when modifying it to avoid any
 *  serious security issue. Keep in mind the following:
 *
 *  - This program should only run for the 'root' or 'shell' users
 *
 *  - Avoid anything that is more complex than simple system calls
 *    until the uid/gid has been dropped to that of a normal user
 *    or you are sure to exit.
 *
 *    This avoids depending on environment variables, system properties
 *    and other external factors that may affect the C library in
 *    unpredictable ways.
 *
 *  - Do not trust user input and/or the filesystem whenever possible.
 *
 *  Read README.TXT for more details.
 *
 *
 *
 * The purpose of this program is to run a command as a specific
 * application user-id. Typical usage is:
 *
 *   run-as <package-name> <command> <args>
 *
 *  The 'run-as' binary is installed with CAP_SETUID and CAP_SETGID file
 *  capabilities, but will check the following:
 *
 *  - that it is invoked from the 'shell' or 'root' user (abort otherwise)
 *  - that '<package-name>' is the name of an installed and debuggable package
 *  - that the package's data directory is well-formed (see package.c)
 *
 *  If so, it will drop to the application's user id / group id, cd to the
 *  package's data directory, then run the command there.
 *
 *  NOTE: In the future it might not be possible to cd to the package's data
 *  directory under that package's user id / group id, in which case this
 *  utility will need to be changed accordingly.
 *
 *  This can be useful for a number of different things on production devices:
 *
 *  - Allow application developers to look at their own applicative data
 *    during development.
 *
 *  - Run the 'gdbserver' binary executable to allow native debugging
 */

__noreturn static void
panic(const char* format, ...)
{
    va_list args;
    int e = errno;

    fprintf(stderr, "%s: ", PROGNAME);
    va_start(args, format);
    vfprintf(stderr, format, args);
    va_end(args);
    exit(e ? -e : 1);
}

static void
usage(void)
{
    panic("Usage:\n    " PROGNAME " <package-name> <command> [<args>]\n");
}

int main(int argc, char **argv)
{
    const char* pkgname;
    int myuid, uid, gid;
    PackageInfo info;
    struct __user_cap_header_struct capheader;
    struct __user_cap_data_struct capdata[2];

    /* check arguments */
    if (argc < 2) {
        usage();
    }

    /* check userid of caller - must be 'shell' or 'root' */
    myuid = getuid();
    if (myuid != AID_SHELL && myuid != AID_ROOT) {
        panic("only 'shell' or 'root' users can run this program\n");
    }

    memset(&capheader, 0, sizeof(capheader));
    memset(&capdata, 0, sizeof(capdata));
    capheader.version = _LINUX_CAPABILITY_VERSION_3;
    capdata[CAP_TO_INDEX(CAP_SETUID)].effective |= CAP_TO_MASK(CAP_SETUID);
    capdata[CAP_TO_INDEX(CAP_SETGID)].effective |= CAP_TO_MASK(CAP_SETGID);
    capdata[CAP_TO_INDEX(CAP_SETUID)].permitted |= CAP_TO_MASK(CAP_SETUID);
    capdata[CAP_TO_INDEX(CAP_SETGID)].permitted |= CAP_TO_MASK(CAP_SETGID);

    if (capset(&capheader, &capdata[0]) < 0) {
        panic("Could not set capabilities: %s\n", strerror(errno));
    }

    /* retrieve package information from system (does setegid) */
    pkgname = argv[1];
    if (get_package_info(pkgname, &info) < 0) {
        panic("Package '%s' is unknown\n", pkgname);
    }

    /* reject system packages */
    if (info.uid < AID_APP) {
        panic("Package '%s' is not an application\n", pkgname);
    }

    /* reject any non-debuggable package */
    if (!info.isDebuggable) {
        panic("Package '%s' is not debuggable\n", pkgname);
    }

    /* check that the data directory path is valid */
    if (check_data_path(info.dataDir, info.uid) < 0) {
        panic("Package '%s' has corrupt installation\n", pkgname);
    }

    /* Ensure that we change all real/effective/saved IDs at the
     * same time to avoid nasty surprises.
     */
    uid = gid = info.uid;
    if(setresgid(gid,gid,gid) || setresuid(uid,uid,uid)) {
        panic("Permission denied\n");
    }

    /* Required if caller has uid and gid all non-zero */
    memset(&capdata, 0, sizeof(capdata));
    if (capset(&capheader, &capdata[0]) < 0) {
        panic("Could not clear all capabilities: %s\n", strerror(errno));
    }

    if (selinux_android_setcontext(uid, 0, info.seinfo, pkgname) < 0) {
        panic("Could not set SELinux security context: %s\n", strerror(errno));
    }

    /* cd into the data directory */
    if (TEMP_FAILURE_RETRY(chdir(info.dataDir)) < 0) {
        panic("Could not cd to package's data directory: %s\n", strerror(errno));
    }

    /* User specified command for exec. */
    if ((argc >= 3) && (execvp(argv[2], argv+2) < 0)) {
        panic("exec failed for %s: %s\n", argv[2], strerror(errno));
    }

    /* Default exec shell. */
    execlp("/system/bin/sh", "sh", NULL);

    panic("exec failed: %s\n", strerror(errno));
}