=============
TEE subsystem
=============

This document describes the TEE subsystem in Linux.

A TEE (Trusted Execution Environment) is a trusted OS running in some
secure environment, for example, TrustZone on ARM CPUs, or a separate
secure co-processor etc. A TEE driver handles the details needed to
communicate with the TEE.

This subsystem deals with:

- Registration of TEE drivers

- Managing shared memory between Linux and the TEE

- Providing a generic API to the TEE

The TEE interface
=================

include/uapi/linux/tee.h defines the generic interface to a TEE.

User space (the client) connects to the driver by opening /dev/tee[0-9]* or
/dev/teepriv[0-9]*.

- TEE_IOC_SHM_ALLOC allocates shared memory and returns a file descriptor
  which user space can mmap. When user space doesn't need the file
  descriptor any more, it should be closed. When shared memory isn't needed
  any longer it should be unmapped with munmap() to allow the reuse of
  memory.

- TEE_IOC_VERSION lets user space know which TEE this driver handles and
  its capabilities.

- TEE_IOC_OPEN_SESSION opens a new session to a Trusted Application.

- TEE_IOC_INVOKE invokes a function in a Trusted Application.

- TEE_IOC_CANCEL may cancel an ongoing TEE_IOC_OPEN_SESSION or TEE_IOC_INVOKE.

- TEE_IOC_CLOSE_SESSION closes a session to a Trusted Application.

There are two classes of clients, normal clients and supplicants. The latter is
a helper process for the TEE to access resources in Linux, for example file
system access. A normal client opens /dev/tee[0-9]* and a supplicant opens
/dev/teepriv[0-9].

Much of the communication between clients and the TEE is opaque to the
driver. The main job for the driver is to receive requests from the
clients, forward them to the TEE and send back the results. In the case of
supplicants the communication goes in the other direction, the TEE sends
requests to the supplicant which then sends back the result.

The TEE kernel interface
========================

Kernel provides a TEE bus infrastructure where a Trusted Application is
represented as a device identified via Universally Unique Identifier (UUID) and
client drivers register a table of supported device UUIDs.

TEE bus infrastructure registers following APIs:

match():
  iterates over the client driver UUID table to find a corresponding
  match for device UUID. If a match is found, then this particular device is
  probed via corresponding probe API registered by the client driver. This
  process happens whenever a device or a client driver is registered with TEE
  bus.

uevent():
  notifies user-space (udev) whenever a new device is registered on
  TEE bus for auto-loading of modularized client drivers.

TEE bus device enumeration is specific to underlying TEE implementation, so it
is left open for TEE drivers to provide corresponding implementation.

Then TEE client driver can talk to a matched Trusted Application using APIs
listed in include/linux/tee_drv.h.

TEE client driver example
-------------------------

Suppose a TEE client driver needs to communicate with a Trusted Application
having UUID: ``ac6a4085-0e82-4c33-bf98-8eb8e118b6c2``, so driver registration
snippet would look like::

	static const struct tee_client_device_id client_id_table[] = {
		{UUID_INIT(0xac6a4085, 0x0e82, 0x4c33,
			   0xbf, 0x98, 0x8e, 0xb8, 0xe1, 0x18, 0xb6, 0xc2)},
		{}
	};

	MODULE_DEVICE_TABLE(tee, client_id_table);

	static struct tee_client_driver client_driver = {
		.id_table	= client_id_table,
		.driver		= {
			.name		= DRIVER_NAME,
			.bus		= &tee_bus_type,
			.probe		= client_probe,
			.remove		= client_remove,
		},
	};

	static int __init client_init(void)
	{
		return driver_register(&client_driver.driver);
	}

	static void __exit client_exit(void)
	{
		driver_unregister(&client_driver.driver);
	}

	module_init(client_init);
	module_exit(client_exit);

OP-TEE driver
=============

The OP-TEE driver handles OP-TEE [1] based TEEs. Currently it is only the ARM
TrustZone based OP-TEE solution that is supported.

Lowest level of communication with OP-TEE builds on ARM SMC Calling
Convention (SMCCC) [2], which is the foundation for OP-TEE's SMC interface
[3] used internally by the driver. Stacked on top of that is OP-TEE Message
Protocol [4].

OP-TEE SMC interface provides the basic functions required by SMCCC and some
additional functions specific for OP-TEE. The most interesting functions are:

- OPTEE_SMC_FUNCID_CALLS_UID (part of SMCCC) returns the version information
  which is then returned by TEE_IOC_VERSION

- OPTEE_SMC_CALL_GET_OS_UUID returns the particular OP-TEE implementation, used
  to tell, for instance, a TrustZone OP-TEE apart from an OP-TEE running on a
  separate secure co-processor.

- OPTEE_SMC_CALL_WITH_ARG drives the OP-TEE message protocol

- OPTEE_SMC_GET_SHM_CONFIG lets the driver and OP-TEE agree on which memory
  range to used for shared memory between Linux and OP-TEE.

The GlobalPlatform TEE Client API [5] is implemented on top of the generic
TEE API.

Picture of the relationship between the different components in the
OP-TEE architecture::

      User space                  Kernel                   Secure world
      ~~~~~~~~~~                  ~~~~~~                   ~~~~~~~~~~~~
   +--------+                                             +-------------+
   | Client |                                             | Trusted     |
   +--------+                                             | Application |
      /\                                                  +-------------+
      || +----------+                                           /\
      || |tee-      |                                           ||
      || |supplicant|                                           \/
      || +----------+                                     +-------------+
      \/      /\                                          | TEE Internal|
   +-------+  ||                                          | API         |
   + TEE   |  ||            +--------+--------+           +-------------+
   | Client|  ||            | TEE    | OP-TEE |           | OP-TEE      |
   | API   |  \/            | subsys | driver |           | Trusted OS  |
   +-------+----------------+----+-------+----+-----------+-------------+
   |      Generic TEE API        |       |     OP-TEE MSG               |
   |      IOCTL (TEE_IOC_*)      |       |     SMCCC (OPTEE_SMC_CALL_*) |
   +-----------------------------+       +------------------------------+

RPC (Remote Procedure Call) are requests from secure world to kernel driver
or tee-supplicant. An RPC is identified by a special range of SMCCC return
values from OPTEE_SMC_CALL_WITH_ARG. RPC messages which are intended for the
kernel are handled by the kernel driver. Other RPC messages will be forwarded to
tee-supplicant without further involvement of the driver, except switching
shared memory buffer representation.

OP-TEE device enumeration
-------------------------

OP-TEE provides a pseudo Trusted Application: drivers/tee/optee/device.c in
order to support device enumeration. In other words, OP-TEE driver invokes this
application to retrieve a list of Trusted Applications which can be registered
as devices on the TEE bus.

OP-TEE notifications
--------------------

There are two kinds of notifications that secure world can use to make
normal world aware of some event.

1. Synchronous notifications delivered with ``OPTEE_RPC_CMD_NOTIFICATION``
   using the ``OPTEE_RPC_NOTIFICATION_SEND`` parameter.
2. Asynchronous notifications delivered with a combination of a non-secure
   edge-triggered interrupt and a fast call from the non-secure interrupt
   handler.

Synchronous notifications are limited by depending on RPC for delivery,
this is only usable when secure world is entered with a yielding call via
``OPTEE_SMC_CALL_WITH_ARG``. This excludes such notifications from secure
world interrupt handlers.

An asynchronous notification is delivered via a non-secure edge-triggered
interrupt to an interrupt handler registered in the OP-TEE driver. The
actual notification value are retrieved with the fast call
``OPTEE_SMC_GET_ASYNC_NOTIF_VALUE``. Note that one interrupt can represent
multiple notifications.

One notification value ``OPTEE_SMC_ASYNC_NOTIF_VALUE_DO_BOTTOM_HALF`` has a
special meaning. When this value is received it means that normal world is
supposed to make a yielding call ``OPTEE_MSG_CMD_DO_BOTTOM_HALF``. This
call is done from the thread assisting the interrupt handler. This is a
building block for OP-TEE OS in secure world to implement the top half and
bottom half style of device drivers.

OPTEE_INSECURE_LOAD_IMAGE Kconfig option
----------------------------------------

The OPTEE_INSECURE_LOAD_IMAGE Kconfig option enables the ability to load the
BL32 OP-TEE image from the kernel after the kernel boots, rather than loading
it from the firmware before the kernel boots. This also requires enabling the
corresponding option in Trusted Firmware for Arm. The Trusted Firmware for Arm
documentation [8] explains the security threat associated with enabling this as
well as mitigations at the firmware and platform level.

There are additional attack vectors/mitigations for the kernel that should be
addressed when using this option.

1. Boot chain security.

   * Attack vector: Replace the OP-TEE OS image in the rootfs to gain control of
     the system.

   * Mitigation: There must be boot chain security that verifies the kernel and
     rootfs, otherwise an attacker can modify the loaded OP-TEE binary by
     modifying it in the rootfs.

2. Alternate boot modes.

   * Attack vector: Using an alternate boot mode (i.e. recovery mode), the
     OP-TEE driver isn't loaded, leaving the SMC hole open.

   * Mitigation: If there are alternate methods of booting the device, such as a
     recovery mode, it should be ensured that the same mitigations are applied
     in that mode.

3. Attacks prior to SMC invocation.

   * Attack vector: Code that is executed prior to issuing the SMC call to load
     OP-TEE can be exploited to then load an alternate OS image.

   * Mitigation: The OP-TEE driver must be loaded before any potential attack
     vectors are opened up. This should include mounting of any modifiable
     filesystems, opening of network ports or communicating with external
     devices (e.g. USB).

4. Blocking SMC call to load OP-TEE.

   * Attack vector: Prevent the driver from being probed, so the SMC call to
     load OP-TEE isn't executed when desired, leaving it open to being executed
     later and loading a modified OS.

   * Mitigation: It is recommended to build the OP-TEE driver as builtin driver
     rather than as a module to prevent exploits that may cause the module to
     not be loaded.

AMD-TEE driver
==============

The AMD-TEE driver handles the communication with AMD's TEE environment. The
TEE environment is provided by AMD Secure Processor.

The AMD Secure Processor (formerly called Platform Security Processor or PSP)
is a dedicated processor that features ARM TrustZone technology, along with a
software-based Trusted Execution Environment (TEE) designed to enable
third-party Trusted Applications. This feature is currently enabled only for
APUs.

The following picture shows a high level overview of AMD-TEE::

                                             |
    x86                                      |
                                             |
 User space            (Kernel space)        |    AMD Secure Processor (PSP)
 ~~~~~~~~~~            ~~~~~~~~~~~~~~        |    ~~~~~~~~~~~~~~~~~~~~~~~~~~
                                             |
 +--------+                                  |       +-------------+
 | Client |                                  |       | Trusted     |
 +--------+                                  |       | Application |
     /\                                      |       +-------------+
     ||                                      |             /\
     ||                                      |             ||
     ||                                      |             \/
     ||                                      |         +----------+
     ||                                      |         |   TEE    |
     ||                                      |         | Internal |
     \/                                      |         |   API    |
 +---------+           +-----------+---------+         +----------+
 | TEE     |           | TEE       | AMD-TEE |         | AMD-TEE  |
 | Client  |           | subsystem | driver  |         | Trusted  |
 | API     |           |           |         |         |   OS     |
 +---------+-----------+----+------+---------+---------+----------+
 |   Generic TEE API        |      | ASP     |      Mailbox       |
 |   IOCTL (TEE_IOC_*)      |      | driver  | Register Protocol  |
 +--------------------------+      +---------+--------------------+

At the lowest level (in x86), the AMD Secure Processor (ASP) driver uses the
CPU to PSP mailbox register to submit commands to the PSP. The format of the
command buffer is opaque to the ASP driver. It's role is to submit commands to
the secure processor and return results to AMD-TEE driver. The interface
between AMD-TEE driver and AMD Secure Processor driver can be found in [6].

The AMD-TEE driver packages the command buffer payload for processing in TEE.
The command buffer format for the different TEE commands can be found in [7].

The TEE commands supported by AMD-TEE Trusted OS are:

* TEE_CMD_ID_LOAD_TA          - loads a Trusted Application (TA) binary into
                                TEE environment.
* TEE_CMD_ID_UNLOAD_TA        - unloads TA binary from TEE environment.
* TEE_CMD_ID_OPEN_SESSION     - opens a session with a loaded TA.
* TEE_CMD_ID_CLOSE_SESSION    - closes session with loaded TA
* TEE_CMD_ID_INVOKE_CMD       - invokes a command with loaded TA
* TEE_CMD_ID_MAP_SHARED_MEM   - maps shared memory
* TEE_CMD_ID_UNMAP_SHARED_MEM - unmaps shared memory

AMD-TEE Trusted OS is the firmware running on AMD Secure Processor.

The AMD-TEE driver registers itself with TEE subsystem and implements the
following driver function callbacks:

* get_version - returns the driver implementation id and capability.
* open - sets up the driver context data structure.
* release - frees up driver resources.
* open_session - loads the TA binary and opens session with loaded TA.
* close_session -  closes session with loaded TA and unloads it.
* invoke_func - invokes a command with loaded TA.

cancel_req driver callback is not supported by AMD-TEE.

The GlobalPlatform TEE Client API [5] can be used by the user space (client) to
talk to AMD's TEE. AMD's TEE provides a secure environment for loading, opening
a session, invoking commands and closing session with TA.

References
==========

[1] https://github.com/OP-TEE/optee_os

[2] http://infocenter.arm.com/help/topic/com.arm.doc.den0028a/index.html

[3] drivers/tee/optee/optee_smc.h

[4] drivers/tee/optee/optee_msg.h

[5] http://www.globalplatform.org/specificationsdevice.asp look for
    "TEE Client API Specification v1.0" and click download.

[6] include/linux/psp-tee.h

[7] drivers/tee/amdtee/amdtee_if.h

[8] https://trustedfirmware-a.readthedocs.io/en/latest/threat_model/threat_model.html