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Jun 28, 2020
Jun 28, 2020

README.md

A5/A5X checkm8

checkm8 port for S5L8940X/S5L8942X/S5L8945X based on Arduino and MAX3421E-based USB Host Shield

Building

Follow the instructions here. Note that the patch for USB Host Library Rev. 2.0 is different.

USB Host Library Rev. 2.0 installation and patching

cd path/to/Arduino/libraries
git clone https://github.com/felis/USB_Host_Shield_2.0
cd USB_Host_Shield_2.0
git checkout cd87628af4a693eeafe1bf04486cf86ba01d29b8
git apply path/to/usb_host_library.patch

SoC selection

Before using the exploit change this line in the beginning of checkm8-a5.ino with target SoC CPID:

#define A5_8942

S5L8940X/S5L8942X/S5L8945X-specific exploitation notes

This article may be helpful for better understanding.

1. HOST2DEVICE control request without data phase processing

In newer SoCs this request is processed as follows. Note that there are different cases for request_handler_ret == 0 and request_handler_ret > 0:

void __fastcall usb_core_handle_usb_control_receive(void *ep0_rx_buffer, __int64 is_setup, __int64 data_rcvd, bool *data_phase)
{
  ...
  // get interface control request handler
  request_handler = ep.registered_interfaces[(unsigned __int16)setup_request.wIndex]->request_handler;
  if ( !request_handler )
    goto error_handling;
  // call to interface control request handler
  // set global buffer pointer
  request_handler_ret = request_handler(&g_setup_request, &ep0_data_phase_buffer);
  if ( !(g_setup_request.bmRequestType & 0x80000000) )
  {
    // HOST2DEVICE
    if ( request_handler_ret >= 1 )
    {
      // set global data phase length and ifnum
      ep0_data_phase_length = request_handler_ret; 
      ep0_data_phase_if_num = intf_num;
      goto continue;
    }
    if ( !request_handler_ret )
    {
      // acknowledge transaction with zlp, do not touch global state
      usb_core_send_zlp();
      goto continue;
    }
  ...
}

But in our target SoCs this request is processed slightly different:

unsigned int __fastcall usb_core_handle_usb_control_receive(unsigned int rx_buffer, int is_setup, unsigned int data_received, int *data_phase)
{
  ...
  if ( is_setup )
  {
    v11 = memmove((unsigned int)&g_setup_packet, rx_buffer, 8);
    if ( g_setup_packet.bmRequestType & 0x60 )
    {
      if ( (g_setup_packet.bmRequestType & 0x60) != 0x20
        || (g_setup_packet.bmRequestType & 0x1F) != 1
        || (v29 = LOBYTE(g_setup_packet.wIndex) | (HIBYTE(g_setup_packet.wIndex) << 8), v29 >= ep_size)
        || (request_handler = *(int (__fastcall **)(setup_req *, _DWORD *))(ep_array[LOBYTE(g_setup_packet.wIndex) | (HIBYTE(g_setup_packet.wIndex) << 8)]
                                                                          + 32)) == 0
        || (request_handler_ret = request_handler(&g_setup_packet, &ep0_data_phase_buffer), request_handler_ret < 0) )
      {
        // error handling
        rx_buffer = sub_2E7C(0x80, 1);
        v10 = 0;
        goto LABEL_103;
      }
      // if request_handler_ret >= 0, then data phase length will be touched anyway
      ep0_data_phase_length = request_handler_ret;
      ep0_data_phase_if_num = v29;
      *data_phase = 1;
      goto continue;
    }
  ...
}

So, if we send any HOST2DEVICE control request without data phase, then ep0_data_phase_length will be reset to zero. Because of this we can't use ctrlReq(0x21,4,0) to reenter DFU. But there is another way to reenter DFU:

  1. ctrlReq(bmRequestType = 0x21,bRequest = 1,wLength = 0x40) with any data
  2. ctrlReq(0x21,1,0)
  3. ctrlReq(0xa1,3,1)
  4. ctrlReq(0xa1,3,1)
  5. USB bus reset

So, to be able to write to the freed io_buffer, we do steps 1-4, then send an incomplete HOST2DEVICE control transaction to set global state, then trigger bus reset. This algorithm is fully described here by littlelailo.

But, if we use any normal OS with default USB stack for exploitation, then we can't avoid standard device requests (e.g. SET_ADDRESS, see this for more info) sent by OS before we can work with device. Because of it in our PoC we use Arduino and MAX3421E to control early initialization of USB.

2. Zero length packet processing

In newer SoCs data packets are processed as follows. Note that in case of zero length packet processing is not performed.

void __fastcall usb_core_handle_usb_control_receive(void *ep0_rx_buffer, __int64 is_setup, __int64 data_rcvd, bool *data_phase)
{
  ...
  if ( !(is_setup & 1) )
  {
    if ( !(_DWORD)data_rcvd ) // check for zero length packet
      return;
    if ( ep0_data_phase_rcvd + (unsigned int)data_rcvd <= ep0_data_phase_length )
    {
      if ( ep0_data_phase_length - ep0_data_phase_rcvd >= (unsigned int)data_rcvd )
        to_copy = (unsigned int)data_rcvd;
      else
        to_copy = ep0_data_phase_length - ep0_data_phase_rcvd;
      memmove(ep0_data_phase_buffer, ep0_rx_buffer, to_copy);// copy received data to IO-buffer
      ep0_data_phase_buffer += (unsigned int)to_copy;// update global buffer pointer
      ep0_data_phase_rcvd += to_copy;           // update received counter
      *data_phase = 1;
      // stop transfer if received expected number of bytes
      // or received less then 0x40 bytes packet
      if ( (_DWORD)data_rcvd == 0x40 )
        end_of_transfer = ep0_data_phase_rcvd == ep0_data_phase_length;
      else
        end_of_transfer = 1;
  ...
}

But in our target SoCs zero length packets are processed in the same way as non-zero length packets.

unsigned int __fastcall usb_core_handle_usb_control_receive(unsigned int rx_buffer, int is_setup, unsigned int data_received, int *data_phase)
{
  ...
  // data packet processing starts here
  rx_buffer = ep0_data_phase_buffer;
  if ( !ep0_data_phase_buffer )
    return rx_buffer;
  if ( data_received + ep0_data_phase_rcvd > ep0_data_phase_length )
  {
    rx_buffer = sub_2E7C(128, 1);
reset_global_state:
    v10 = 0;
    ep0_data_phase_rcvd = 0;
    ep0_data_phase_length = 0;
    ep0_data_phase_buffer = 0;
    ep0_data_phase_if_num = -2;
LABEL_103:
    *v6 = v10;
    return rx_buffer;
  }
  if ( data_received >= ep0_data_phase_length - ep0_data_phase_rcvd )
    v7 = ep0_data_phase_length - ep0_data_phase_rcvd;
  else
    v7 = data_received;
  memmove(ep0_data_phase_buffer, rx_buffer_1, v7);
  end_of_transfer = data_received_1 != 0x40;  // in case of zero length packet `end_of_transfer` will be `true`
                                              // and global state will be reseted
  ep0_data_phase_buffer += v7;
  ep0_data_phase_rcvd += v7;
  rx_buffer = ep0_data_phase_rcvd;
  *v6 = 1;
  if ( rx_buffer == ep0_data_phase_length )
    end_of_transfer = 1;
  if ( end_of_transfer )
  {
    if ( (int)ep0_data_phase_if_num >= 0 && ep0_data_phase_if_num < ep_size )
    {
      v9 = *(void (**)(void))(ep_array[ep0_data_phase_if_num] + 36);
      if ( v9 )
      {
        v9();
        rx_buffer = usb_core_send_zlp();
      }
    }
    goto reset_global_state;
  }
  return rx_buffer;
}

As in the previous case, if we use normal OS with default USB stack, we can't avoid sending of zero length packets in status phase of standard device USB controll requests.

Important notes

  • Do not use any cables with embedded USB hubs (DCSD/Kong/Kanzi/etc.), as it might prevent a device from being recognized by the program. Normal USB-cables will do the trick just fine
  • This exploit demotes your device by default, so SWD-debugging will be available. But that also makes your device use development KBAG when decrypting Image3s. Keep that in mind if you're going to use this to decrypt firmware components

Authors

License

This project is licensed under the MIT License - see the LICENSE file for details

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checkm8 port for S5L8940X/S5L8942X/S5L8945X

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