Documentation/tee.txt

   1 =============
   2 TEE subsystem
   3 =============
   4
   5 This document describes the TEE subsystem in Linux.
   6
   7 A TEE (Trusted Execution Environment) is a trusted OS running in some
   8 secure environment, for example, TrustZone on ARM CPUs, or a separate
   9 secure co-processor etc. A TEE driver handles the details needed to
  10 communicate with the TEE.
  11
  12 This subsystem deals with:
  13
  14 - Registration of TEE drivers
  15
  16 - Managing shared memory between Linux and the TEE
  17
  18 - Providing a generic API to the TEE
  19
  20 The TEE interface
  21 =================
  22
  23 include/uapi/linux/tee.h defines the generic interface to a TEE.
  24
  25 User space (the client) connects to the driver by opening /dev/tee[0-9]* or
  26 /dev/teepriv[0-9]*.
  27
  28 - TEE_IOC_SHM_ALLOC allocates shared memory and returns a file descriptor
  29   which user space can mmap. When user space doesn't need the file
  30   descriptor any more, it should be closed. When shared memory isn't needed
  31   any longer it should be unmapped with munmap() to allow the reuse of
  32   memory.
  33
  34 - TEE_IOC_VERSION lets user space know which TEE this driver handles and
  35   the its capabilities.
  36
  37 - TEE_IOC_OPEN_SESSION opens a new session to a Trusted Application.
  38
  39 - TEE_IOC_INVOKE invokes a function in a Trusted Application.
  40
  41 - TEE_IOC_CANCEL may cancel an ongoing TEE_IOC_OPEN_SESSION or TEE_IOC_INVOKE.
  42
  43 - TEE_IOC_CLOSE_SESSION closes a session to a Trusted Application.
  44
  45 There are two classes of clients, normal clients and supplicants. The latter is
  46 a helper process for the TEE to access resources in Linux, for example file
  47 system access. A normal client opens /dev/tee[0-9]* and a supplicant opens
  48 /dev/teepriv[0-9].
  49
  50 Much of the communication between clients and the TEE is opaque to the
  51 driver. The main job for the driver is to receive requests from the
  52 clients, forward them to the TEE and send back the results. In the case of
  53 supplicants the communication goes in the other direction, the TEE sends
  54 requests to the supplicant which then sends back the result.
  55
  56 OP-TEE driver
  57 =============
  58
  59 The OP-TEE driver handles OP-TEE [1] based TEEs. Currently it is only the ARM
  60 TrustZone based OP-TEE solution that is supported.
  61
  62 Lowest level of communication with OP-TEE builds on ARM SMC Calling
  63 Convention (SMCCC) [2], which is the foundation for OP-TEE's SMC interface
  64 [3] used internally by the driver. Stacked on top of that is OP-TEE Message
  65 Protocol [4].
  66
  67 OP-TEE SMC interface provides the basic functions required by SMCCC and some
  68 additional functions specific for OP-TEE. The most interesting functions are:
  69
  70 - OPTEE_SMC_FUNCID_CALLS_UID (part of SMCCC) returns the version information
  71   which is then returned by TEE_IOC_VERSION
  72
  73 - OPTEE_SMC_CALL_GET_OS_UUID returns the particular OP-TEE implementation, used
  74   to tell, for instance, a TrustZone OP-TEE apart from an OP-TEE running on a
  75   separate secure co-processor.
  76
  77 - OPTEE_SMC_CALL_WITH_ARG drives the OP-TEE message protocol
  78
  79 - OPTEE_SMC_GET_SHM_CONFIG lets the driver and OP-TEE agree on which memory
  80   range to used for shared memory between Linux and OP-TEE.
  81
  82 The GlobalPlatform TEE Client API [5] is implemented on top of the generic
  83 TEE API.
  84
  85 Picture of the relationship between the different components in the
  86 OP-TEE architecture::
  87
  88       User space                  Kernel                   Secure world
  89       ~~~~~~~~~~                  ~~~~~~                   ~~~~~~~~~~~~
  90    +--------+                                             +-------------+
  91    | Client |                                             | Trusted     |
  92    +--------+                                             | Application |
  93       /\                                                  +-------------+
  94       || +----------+                                           /\
  95       || |tee-      |                                           ||
  96       || |supplicant|                                           \/
  97       || +----------+                                     +-------------+
  98       \/      /\                                          | TEE Internal|
  99    +-------+  ||                                          | API         |
 100    + TEE   |  ||            +--------+--------+           +-------------+
 101    | Client|  ||            | TEE    | OP-TEE |           | OP-TEE      |
 102    | API   |  \/            | subsys | driver |           | Trusted OS  |
 103    +-------+----------------+----+-------+----+-----------+-------------+
 104    |      Generic TEE API        |       |     OP-TEE MSG               |
 105    |      IOCTL (TEE_IOC_*)      |       |     SMCCC (OPTEE_SMC_CALL_*) |
 106    +-----------------------------+       +------------------------------+
 107
 108 RPC (Remote Procedure Call) are requests from secure world to kernel driver
 109 or tee-supplicant. An RPC is identified by a special range of SMCCC return
 110 values from OPTEE_SMC_CALL_WITH_ARG. RPC messages which are intended for the
 111 kernel are handled by the kernel driver. Other RPC messages will be forwarded to
 112 tee-supplicant without further involvement of the driver, except switching
 113 shared memory buffer representation.
 114
 115 References
 116 ==========
 117
 118 [1] https://github.com/OP-TEE/optee_os
 119
 120 [2] http://infocenter.arm.com/help/topic/com.arm.doc.den0028a/index.html
 121
 122 [3] drivers/tee/optee/optee_smc.h
 123
 124 [4] drivers/tee/optee/optee_msg.h
 125
 126 [5] http://www.globalplatform.org/specificationsdevice.asp look for
 127     "TEE Client API Specification v1.0" and click download.