5 This document describes the TEE subsystem in Linux.
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.
12 This subsystem deals with:
14 - Registration of TEE drivers
16 - Managing shared memory between Linux and the TEE
18 - Providing a generic API to the TEE
23 include/uapi/linux/tee.h defines the generic interface to a TEE.
25 User space (the client) connects to the driver by opening /dev/tee[0-9]* or
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
34 - TEE_IOC_VERSION lets user space know which TEE this driver handles and
37 - TEE_IOC_OPEN_SESSION opens a new session to a Trusted Application.
39 - TEE_IOC_INVOKE invokes a function in a Trusted Application.
41 - TEE_IOC_CANCEL may cancel an ongoing TEE_IOC_OPEN_SESSION or TEE_IOC_INVOKE.
43 - TEE_IOC_CLOSE_SESSION closes a session to a Trusted Application.
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
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.
56 The TEE kernel interface
57 ========================
59 Kernel provides a TEE bus infrastructure where a Trusted Application is
60 represented as a device identified via Universally Unique Identifier (UUID) and
61 client drivers register a table of supported device UUIDs.
63 TEE bus infrastructure registers following APIs:
66 iterates over the client driver UUID table to find a corresponding
67 match for device UUID. If a match is found, then this particular device is
68 probed via corresponding probe API registered by the client driver. This
69 process happens whenever a device or a client driver is registered with TEE
73 notifies user-space (udev) whenever a new device is registered on
74 TEE bus for auto-loading of modularized client drivers.
76 TEE bus device enumeration is specific to underlying TEE implementation, so it
77 is left open for TEE drivers to provide corresponding implementation.
79 Then TEE client driver can talk to a matched Trusted Application using APIs
80 listed in include/linux/tee_drv.h.
82 TEE client driver example
83 -------------------------
85 Suppose a TEE client driver needs to communicate with a Trusted Application
86 having UUID: ``ac6a4085-0e82-4c33-bf98-8eb8e118b6c2``, so driver registration
87 snippet would look like::
89 static const struct tee_client_device_id client_id_table[] = {
90 {UUID_INIT(0xac6a4085, 0x0e82, 0x4c33,
91 0xbf, 0x98, 0x8e, 0xb8, 0xe1, 0x18, 0xb6, 0xc2)},
95 MODULE_DEVICE_TABLE(tee, client_id_table);
97 static struct tee_client_driver client_driver = {
98 .id_table = client_id_table,
101 .bus = &tee_bus_type,
102 .probe = client_probe,
103 .remove = client_remove,
107 static int __init client_init(void)
109 return driver_register(&client_driver.driver);
112 static void __exit client_exit(void)
114 driver_unregister(&client_driver.driver);
117 module_init(client_init);
118 module_exit(client_exit);
123 The OP-TEE driver handles OP-TEE [1] based TEEs. Currently it is only the ARM
124 TrustZone based OP-TEE solution that is supported.
126 Lowest level of communication with OP-TEE builds on ARM SMC Calling
127 Convention (SMCCC) [2], which is the foundation for OP-TEE's SMC interface
128 [3] used internally by the driver. Stacked on top of that is OP-TEE Message
131 OP-TEE SMC interface provides the basic functions required by SMCCC and some
132 additional functions specific for OP-TEE. The most interesting functions are:
134 - OPTEE_SMC_FUNCID_CALLS_UID (part of SMCCC) returns the version information
135 which is then returned by TEE_IOC_VERSION
137 - OPTEE_SMC_CALL_GET_OS_UUID returns the particular OP-TEE implementation, used
138 to tell, for instance, a TrustZone OP-TEE apart from an OP-TEE running on a
139 separate secure co-processor.
141 - OPTEE_SMC_CALL_WITH_ARG drives the OP-TEE message protocol
143 - OPTEE_SMC_GET_SHM_CONFIG lets the driver and OP-TEE agree on which memory
144 range to used for shared memory between Linux and OP-TEE.
146 The GlobalPlatform TEE Client API [5] is implemented on top of the generic
149 Picture of the relationship between the different components in the
150 OP-TEE architecture::
152 User space Kernel Secure world
153 ~~~~~~~~~~ ~~~~~~ ~~~~~~~~~~~~
154 +--------+ +-------------+
155 | Client | | Trusted |
156 +--------+ | Application |
161 || +----------+ +-------------+
162 \/ /\ | TEE Internal|
164 + TEE | || +--------+--------+ +-------------+
165 | Client| || | TEE | OP-TEE | | OP-TEE |
166 | API | \/ | subsys | driver | | Trusted OS |
167 +-------+----------------+----+-------+----+-----------+-------------+
168 | Generic TEE API | | OP-TEE MSG |
169 | IOCTL (TEE_IOC_*) | | SMCCC (OPTEE_SMC_CALL_*) |
170 +-----------------------------+ +------------------------------+
172 RPC (Remote Procedure Call) are requests from secure world to kernel driver
173 or tee-supplicant. An RPC is identified by a special range of SMCCC return
174 values from OPTEE_SMC_CALL_WITH_ARG. RPC messages which are intended for the
175 kernel are handled by the kernel driver. Other RPC messages will be forwarded to
176 tee-supplicant without further involvement of the driver, except switching
177 shared memory buffer representation.
179 OP-TEE device enumeration
180 -------------------------
182 OP-TEE provides a pseudo Trusted Application: drivers/tee/optee/device.c in
183 order to support device enumeration. In other words, OP-TEE driver invokes this
184 application to retrieve a list of Trusted Applications which can be registered
185 as devices on the TEE bus.
190 There are two kinds of notifications that secure world can use to make
191 normal world aware of some event.
193 1. Synchronous notifications delivered with ``OPTEE_RPC_CMD_NOTIFICATION``
194 using the ``OPTEE_RPC_NOTIFICATION_SEND`` parameter.
195 2. Asynchronous notifications delivered with a combination of a non-secure
196 edge-triggered interrupt and a fast call from the non-secure interrupt
199 Synchronous notifications are limited by depending on RPC for delivery,
200 this is only usable when secure world is entered with a yielding call via
201 ``OPTEE_SMC_CALL_WITH_ARG``. This excludes such notifications from secure
202 world interrupt handlers.
204 An asynchronous notification is delivered via a non-secure edge-triggered
205 interrupt to an interrupt handler registered in the OP-TEE driver. The
206 actual notification value are retrieved with the fast call
207 ``OPTEE_SMC_GET_ASYNC_NOTIF_VALUE``. Note that one interrupt can represent
208 multiple notifications.
210 One notification value ``OPTEE_SMC_ASYNC_NOTIF_VALUE_DO_BOTTOM_HALF`` has a
211 special meaning. When this value is received it means that normal world is
212 supposed to make a yielding call ``OPTEE_MSG_CMD_DO_BOTTOM_HALF``. This
213 call is done from the thread assisting the interrupt handler. This is a
214 building block for OP-TEE OS in secure world to implement the top half and
215 bottom half style of device drivers.
220 The AMD-TEE driver handles the communication with AMD's TEE environment. The
221 TEE environment is provided by AMD Secure Processor.
223 The AMD Secure Processor (formerly called Platform Security Processor or PSP)
224 is a dedicated processor that features ARM TrustZone technology, along with a
225 software-based Trusted Execution Environment (TEE) designed to enable
226 third-party Trusted Applications. This feature is currently enabled only for
229 The following picture shows a high level overview of AMD-TEE::
234 User space (Kernel space) | AMD Secure Processor (PSP)
235 ~~~~~~~~~~ ~~~~~~~~~~~~~~ | ~~~~~~~~~~~~~~~~~~~~~~~~~~
237 +--------+ | +-------------+
238 | Client | | | Trusted |
239 +--------+ | | Application |
248 +---------+ +-----------+---------+ +----------+
249 | TEE | | TEE | AMD-TEE | | AMD-TEE |
250 | Client | | subsystem | driver | | Trusted |
252 +---------+-----------+----+------+---------+---------+----------+
253 | Generic TEE API | | ASP | Mailbox |
254 | IOCTL (TEE_IOC_*) | | driver | Register Protocol |
255 +--------------------------+ +---------+--------------------+
257 At the lowest level (in x86), the AMD Secure Processor (ASP) driver uses the
258 CPU to PSP mailbox regsister to submit commands to the PSP. The format of the
259 command buffer is opaque to the ASP driver. It's role is to submit commands to
260 the secure processor and return results to AMD-TEE driver. The interface
261 between AMD-TEE driver and AMD Secure Processor driver can be found in [6].
263 The AMD-TEE driver packages the command buffer payload for processing in TEE.
264 The command buffer format for the different TEE commands can be found in [7].
266 The TEE commands supported by AMD-TEE Trusted OS are:
268 * TEE_CMD_ID_LOAD_TA - loads a Trusted Application (TA) binary into
270 * TEE_CMD_ID_UNLOAD_TA - unloads TA binary from TEE environment.
271 * TEE_CMD_ID_OPEN_SESSION - opens a session with a loaded TA.
272 * TEE_CMD_ID_CLOSE_SESSION - closes session with loaded TA
273 * TEE_CMD_ID_INVOKE_CMD - invokes a command with loaded TA
274 * TEE_CMD_ID_MAP_SHARED_MEM - maps shared memory
275 * TEE_CMD_ID_UNMAP_SHARED_MEM - unmaps shared memory
277 AMD-TEE Trusted OS is the firmware running on AMD Secure Processor.
279 The AMD-TEE driver registers itself with TEE subsystem and implements the
280 following driver function callbacks:
282 * get_version - returns the driver implementation id and capability.
283 * open - sets up the driver context data structure.
284 * release - frees up driver resources.
285 * open_session - loads the TA binary and opens session with loaded TA.
286 * close_session - closes session with loaded TA and unloads it.
287 * invoke_func - invokes a command with loaded TA.
289 cancel_req driver callback is not supported by AMD-TEE.
291 The GlobalPlatform TEE Client API [5] can be used by the user space (client) to
292 talk to AMD's TEE. AMD's TEE provides a secure environment for loading, opening
293 a session, invoking commands and clossing session with TA.
298 [1] https://github.com/OP-TEE/optee_os
300 [2] http://infocenter.arm.com/help/topic/com.arm.doc.den0028a/index.html
302 [3] drivers/tee/optee/optee_smc.h
304 [4] drivers/tee/optee/optee_msg.h
306 [5] http://www.globalplatform.org/specificationsdevice.asp look for
307 "TEE Client API Specification v1.0" and click download.
309 [6] include/linux/psp-tee.h
311 [7] drivers/tee/amdtee/amdtee_if.h