1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
|
=============
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.
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
|
