Documentation: Destage TEE subsystem documentation

Add a separate documentation directory for TEE subsystem since it is a
standalone subsystem which already offers devices consumed by multiple
different subsystem drivers.

Split overall TEE subsystem documentation modularly where:
- The userspace API has been moved to Documentation/userspace-api/tee.rst.
- The driver API has been moved to Documentation/driver-api/tee.rst.
- The first module covers the overview of TEE subsystem.
- The further modules are dedicated to different TEE implementations like:
- OP-TEE
- AMD-TEE
- and so on for future TEE implementation support.

Acked-by: Rijo Thomas <[e-mail]>
Acked-by: Jens Wiklander <[e-mail]>
Signed-off-by: Sumit Garg <[e-mail]>
Signed-off-by: Jonathan Corbet <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]

Documentation/driver-api/index.rst

···
    hte/index
    wmi
    dpll
+   tee
 
 .. only::  subproject and html
 

Documentation/driver-api/tee.rst

···
+.. SPDX-License-Identifier: GPL-2.0
+
+===============================================
+TEE (Trusted Execution Environment) driver API
+===============================================
+
+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);

Documentation/security/keys/trusted-encrypted.rst

···
      (2) TEE
 
          TEEs have well-documented, standardized client interface and APIs. For
-         more details refer to ``Documentation/staging/tee.rst``.
+         more details refer to ``Documentation/driver-api/tee.rst``.
 
      (3) CAAM
 

Documentation/staging/index.rst

···
    rpmsg
    speculation
    static-keys
-   tee
    xz

Documentation/staging/tee.rst

···
-=============
-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

Documentation/subsystem-apis.rst

···
    misc-devices/index
    peci/index
    wmi/index
+   tee/index

Documentation/tee/amd-tee.rst

···
+.. SPDX-License-Identifier: GPL-2.0
+
+=============================================
+AMD-TEE (AMD's Trusted Execution Environment)
+=============================================
+
+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 [1].
+
+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 [2].
+
+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 [3] 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] include/linux/psp-tee.h
+
+[2] drivers/tee/amdtee/amdtee_if.h
+
+[3] http://www.globalplatform.org/specificationsdevice.asp look for
+    "TEE Client API Specification v1.0" and click download.

Documentation/tee/index.rst

···
+.. SPDX-License-Identifier: GPL-2.0
+
+=============
+TEE Subsystem
+=============
+
+.. toctree::
+   :maxdepth: 1
+
+   tee
+   op-tee
+   amd-tee
+
+.. only::  subproject and html
+
+   Indices
+   =======
+
+   * :ref:`genindex`

Documentation/tee/op-tee.rst

···
+.. SPDX-License-Identifier: GPL-2.0
+
+====================================================
+OP-TEE (Open Portable Trusted Execution Environment)
+====================================================
+
+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 [6] 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.
+
+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] https://trustedfirmware-a.readthedocs.io/en/latest/threat_model/threat_model.html

Documentation/tee/tee.rst

···
+.. SPDX-License-Identifier: GPL-2.0
+
+===================================
+TEE (Trusted Execution Environment)
+===================================
+
+This document describes the TEE subsystem in Linux.
+
+Overview
+========
+
+A TEE 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

Documentation/userspace-api/index.rst

···
    sysfs-platform_profile
    vduse
    futex2
+   tee
 
 .. only::  subproject and html
 

Documentation/userspace-api/tee.rst

···
+.. SPDX-License-Identifier: GPL-2.0
+.. tee:
+
+==================================================
+TEE (Trusted Execution Environment) Userspace API
+==================================================
+
+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.

MAINTAINERS

···
 R:	Sumit Garg <[e-mail]>
 L:	[e-mail]
 S:	Maintained
-F:	Documentation/staging/tee.rst
+F:	Documentation/driver-api/tee.rst
+F:	Documentation/tee/
+F:	Documentation/userspace-api/tee.rst
 F:	drivers/tee/
 F:	include/linux/tee_drv.h
 F:	include/uapi/linux/tee.h

drivers/tee/optee/Kconfig

···
 	  https://trustedfirmware-a.readthedocs.io/en/latest/threat_model/threat_model.html
 
 	  Additional documentation on kernel security risks are at
-	  Documentation/staging/tee.rst.
+	  Documentation/tee/op-tee.rst.