TEXT   42

TEE subsystem

Guest on 1st July 2022 07:31:21 PM

  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.   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. AMD-TEE driver
  116. ==============
  117.  
  118. The AMD-TEE driver handles the communication with AMD's TEE environment. The
  119. TEE environment is provided by AMD Secure Processor.
  120.  
  121. The AMD Secure Processor (formerly called Platform Security Processor or PSP)
  122. is a dedicated processor that features ARM TrustZone technology, along with a
  123. software-based Trusted Execution Environment (TEE) designed to enable
  124. third-party Trusted Applications. This feature is currently enabled only for
  125. APUs.
  126.  
  127. The following picture shows a high level overview of AMD-TEE::
  128.  
  129.                                              |
  130.     x86                                      |
  131.                                              |
  132.  User space            (Kernel space)        |    AMD Secure Processor (PSP)
  133.  ~~~~~~~~~~            ~~~~~~~~~~~~~~        |    ~~~~~~~~~~~~~~~~~~~~~~~~~~
  134.                                              |
  135.  +--------+                                  |       +-------------+
  136.  | Client |                                  |       | Trusted     |
  137.  +--------+                                  |       | Application |
  138.      /\                                      |       +-------------+
  139.      ||                                      |             /\
  140.      ||                                      |             ||
  141.      ||                                      |             \/
  142.      ||                                      |         +----------+
  143.      ||                                      |         |   TEE    |
  144.      ||                                      |         | Internal |
  145.      \/                                      |         |   API    |
  146.  +---------+           +-----------+---------+         +----------+
  147.  | TEE     |           | TEE       | AMD-TEE |         | AMD-TEE  |
  148.  | Client  |           | subsystem | driver  |         | Trusted  |
  149.  | API     |           |           |         |         |   OS     |
  150.  +---------+-----------+----+------+---------+---------+----------+
  151.  |   Generic TEE API        |      | ASP     |      Mailbox       |
  152.  |   IOCTL (TEE_IOC_*)      |      | driver  | Register Protocol  |
  153.  +--------------------------+      +---------+--------------------+
  154.  
  155. At the lowest level (in x86), the AMD Secure Processor (ASP) driver uses the
  156. CPU to PSP mailbox regsister to submit commands to the PSP. The format of the
  157. command buffer is opaque to the ASP driver. It's role is to submit commands to
  158. the secure processor and return results to AMD-TEE driver. The interface
  159. between AMD-TEE driver and AMD Secure Processor driver can be found in [6].
  160.  
  161. The AMD-TEE driver packages the command buffer payload for processing in TEE.
  162. The command buffer format for the different TEE commands can be found in [7].
  163.  
  164. The TEE commands supported by AMD-TEE Trusted OS are:
  165. * TEE_CMD_ID_LOAD_TA          - loads a Trusted Application (TA) binary into
  166.                                 TEE environment.
  167. * TEE_CMD_ID_UNLOAD_TA        - unloads TA binary from TEE environment.
  168. * TEE_CMD_ID_OPEN_SESSION     - opens a session with a loaded TA.
  169. * TEE_CMD_ID_CLOSE_SESSION    - closes session with loaded TA
  170. * TEE_CMD_ID_INVOKE_CMD       - invokes a command with loaded TA
  171. * TEE_CMD_ID_MAP_SHARED_MEM   - maps shared memory
  172. * TEE_CMD_ID_UNMAP_SHARED_MEM - unmaps shared memory
  173.  
  174. AMD-TEE Trusted OS is the firmware running on AMD Secure Processor.
  175.  
  176. The AMD-TEE driver registers itself with TEE subsystem and implements the
  177. following driver function callbacks:
  178.  
  179. * get_version - returns the driver implementation id and capability.
  180. * open - sets up the driver context data structure.
  181. * release - frees up driver resources.
  182. * open_session - loads the TA binary and opens session with loaded TA.
  183. * close_session -  closes session with loaded TA and unloads it.
  184. * invoke_func - invokes a command with loaded TA.
  185.  
  186. cancel_req driver callback is not supported by AMD-TEE.
  187.  
  188. The GlobalPlatform TEE Client API [5] can be used by the user space (client) to
  189. talk to AMD's TEE. AMD's TEE provides a secure environment for loading, opening
  190. a session, invoking commands and clossing session with TA.
  191.  
  192. References
  193. ==========
  194.  
  195. [1] https://github.com/OP-TEE/optee_os
  196.  
  197. [2] http://infocenter.arm.com/help/topic/com.arm.doc.den0028a/index.html
  198.  
  199. [3] drivers/tee/optee/optee_smc.h
  200.  
  201. [4] drivers/tee/optee/optee_msg.h
  202.  
  203. [5] http://www.globalplatform.org/specificationsdevice.asp look for
  204.     "TEE Client API Specification v1.0" and click download.
  205.  
  206. [6] include/linux/psp-tee.h
  207.  
  208. [7] drivers/tee/amdtee/amdtee_if.h

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