int API minijail_change_user(struct minijail *j, const char *user)
{
char *buf = NULL;
struct passwd pw;
struct passwd *ppw = NULL;
ssize_t sz = sysconf(_SC_GETPW_R_SIZE_MAX);
if (sz == -1)
sz = 65536; /* your guess is as good as mine... */
/*
* sysconf(_SC_GETPW_R_SIZE_MAX), under glibc, is documented to return
* the maximum needed size of the buffer, so we don't have to search.
*/
buf = malloc(sz);
if (!buf)
return -ENOMEM;
getpwnam_r(user, &pw, buf, sz, &ppw);
/*
* We're safe to free the buffer here. The strings inside pw point
* inside buf, but we don't use any of them; this leaves the pointers
* dangling but it's safe. ppw points at pw if getpwnam_r succeeded.
*/
free(buf);
/* getpwnam_r(3) does *not* set errno when |ppw| is NULL. */
if (!ppw)
return -1;
minijail_change_uid(j, ppw->pw_uid);
j->user = strdup(user);
if (!j->user)
return -ENOMEM;
j->usergid = ppw->pw_gid;
return 0;
}
static void drop_privs(uid_t uid, gid_t gid) {
ScopedMinijail j(minijail_new());
minijail_set_supplementary_gids(j.get(), arraysize(kGroups), kGroups);
minijail_change_gid(j.get(), gid);
minijail_change_uid(j.get(), uid);
/* minijail_enter() will abort if priv-dropping fails. */
minijail_enter(j.get());
}
static void drop_privileges(int server_port) {
std::unique_ptr<minijail, void (*)(minijail*)> jail(minijail_new(),
&minijail_destroy);
// Add extra groups:
// AID_ADB to access the USB driver
// AID_LOG to read system logs (adb logcat)
// AID_INPUT to diagnose input issues (getevent)
// AID_INET to diagnose network issues (ping)
// AID_NET_BT and AID_NET_BT_ADMIN to diagnose bluetooth (hcidump)
// AID_SDCARD_R to allow reading from the SD card
// AID_SDCARD_RW to allow writing to the SD card
// AID_NET_BW_STATS to read out qtaguid statistics
// AID_READPROC for reading /proc entries across UID boundaries
gid_t groups[] = {AID_ADB, AID_LOG, AID_INPUT,
AID_INET, AID_NET_BT, AID_NET_BT_ADMIN,
AID_SDCARD_R, AID_SDCARD_RW, AID_NET_BW_STATS,
AID_READPROC};
if (minijail_set_supplementary_gids(
jail.get(),
sizeof(groups) / sizeof(groups[0]),
groups) != 0) {
LOG(FATAL) << "Could not configure supplementary groups";
}
// Don't listen on a port (default 5037) if running in secure mode.
// Don't run as root if running in secure mode.
if (should_drop_privileges()) {
drop_capabilities_bounding_set_if_needed();
minijail_change_gid(jail.get(), AID_SHELL);
minijail_change_uid(jail.get(), AID_SHELL);
// minijail_enter() will abort if any priv-dropping step fails.
minijail_enter(jail.get());
D("Local port disabled");
} else {
// minijail_enter() will abort if any priv-dropping step fails.
minijail_enter(jail.get());
if (root_seclabel != nullptr) {
if (selinux_android_setcon(root_seclabel) < 0) {
LOG(FATAL) << "Could not set SELinux context";
}
}
std::string error;
std::string local_name =
android::base::StringPrintf("tcp:%d", server_port);
if (install_listener(local_name, "*smartsocket*", nullptr, 0,
&error)) {
LOG(FATAL) << "Could not install *smartsocket* listener: "
<< error;
}
}
}
int main(int argc, char* argv[]) {
// Check arguments.
if (argc < 2) {
error(1, 0, "usage: run-as <package-name> [--user <uid>] <command> [<args>]\n");
}
// This program runs with CAP_SETUID and CAP_SETGID capabilities on Android
// production devices. Check user id of caller --- must be 'shell' or 'root'.
if (getuid() != AID_SHELL && getuid() != AID_ROOT) {
error(1, 0, "only 'shell' or 'root' users can run this program");
}
// Some devices can disable running run-as, such as Chrome OS when running in
// non-developer mode.
if (android::base::GetBoolProperty("ro.boot.disable_runas", false)) {
error(1, 0, "run-as is disabled from the kernel commandline");
}
char* pkgname = argv[1];
int cmd_argv_offset = 2;
// Get user_id from command line if provided.
int userId = 0;
if ((argc >= 4) && !strcmp(argv[2], "--user")) {
userId = atoi(argv[3]);
if (userId < 0) error(1, 0, "negative user id: %d", userId);
cmd_argv_offset += 2;
}
// Retrieve package information from system, switching egid so we can read the file.
gid_t old_egid = getegid();
if (setegid(AID_PACKAGE_INFO) == -1) error(1, errno, "setegid(AID_PACKAGE_INFO) failed");
pkg_info info;
memset(&info, 0, sizeof(info));
info.name = pkgname;
if (!packagelist_parse(packagelist_parse_callback, &info)) {
error(1, errno, "packagelist_parse failed");
}
// Handle a multi-user data path
if (userId > 0) {
free(info.data_dir);
if (asprintf(&info.data_dir, "/data/user/%d/%s", userId, pkgname) == -1) {
error(1, errno, "asprintf failed");
}
}
if (info.uid == 0) {
error(1, 0, "unknown package: %s", pkgname);
}
if (setegid(old_egid) == -1) error(1, errno, "couldn't restore egid");
// Verify that user id is not too big.
if ((UID_MAX - info.uid) / AID_USER_OFFSET < (uid_t)userId) {
error(1, 0, "user id too big: %d", userId);
}
// Calculate user app ID.
uid_t userAppId = (AID_USER_OFFSET * userId) + info.uid;
// Reject system packages.
if (userAppId < AID_APP) {
error(1, 0, "package not an application: %s", pkgname);
}
// Reject any non-debuggable package.
if (!info.debuggable) {
error(1, 0, "package not debuggable: %s", pkgname);
}
// Check that the data directory path is valid.
if (!check_data_path(info.data_dir, userAppId)) {
error(1, 0, "package has corrupt installation: %s", pkgname);
}
// Ensure that we change all real/effective/saved IDs at the
// same time to avoid nasty surprises.
uid_t uid = userAppId;
uid_t gid = userAppId;
ScopedMinijail j(minijail_new());
minijail_change_uid(j.get(), uid);
minijail_change_gid(j.get(), gid);
minijail_keep_supplementary_gids(j.get());
minijail_enter(j.get());
std::string seinfo = std::string(info.seinfo) + ":fromRunAs";
if (selinux_android_setcontext(uid, 0, seinfo.c_str(), pkgname) < 0) {
error(1, errno, "couldn't set SELinux security context");
}
// cd into the data directory, and set $HOME correspondingly.
if (TEMP_FAILURE_RETRY(chdir(info.data_dir)) == -1) {
error(1, errno, "couldn't chdir to package's data directory");
}
setenv("HOME", info.data_dir, 1);
// Reset parts of the environment, like su would.
setenv("PATH", _PATH_DEFPATH, 1);
unsetenv("IFS");
// Set the user-specific parts for this user.
passwd* pw = getpwuid(uid);
setenv("LOGNAME", pw->pw_name, 1);
setenv("SHELL", pw->pw_shell, 1);
setenv("USER", pw->pw_name, 1);
// User specified command for exec.
if ((argc >= cmd_argv_offset + 1) &&
(execvp(argv[cmd_argv_offset], argv+cmd_argv_offset) == -1)) {
error(1, errno, "exec failed for %s", argv[cmd_argv_offset]);
}
// Default exec shell.
execlp(_PATH_BSHELL, "sh", NULL);
error(1, errno, "exec failed");
}