=============

  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
    the 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.
  
  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.

  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.