1=============
2TEE subsystem
3=============
4
5This document describes the TEE subsystem in Linux.
6
7A TEE (Trusted Execution Environment) is a trusted OS running in some
8secure environment, for example, TrustZone on ARM CPUs, or a separate
9secure co-processor etc. A TEE driver handles the details needed to
10communicate with the TEE.
11
12This 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
20The TEE interface
21=================
22
23include/uapi/linux/tee.h defines the generic interface to a TEE.
24
25User 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
45There are two classes of clients, normal clients and supplicants. The latter is
46a helper process for the TEE to access resources in Linux, for example file
47system access. A normal client opens /dev/tee[0-9]* and a supplicant opens
48/dev/teepriv[0-9].
49
50Much of the communication between clients and the TEE is opaque to the
51driver. The main job for the driver is to receive requests from the
52clients, forward them to the TEE and send back the results. In the case of
53supplicants the communication goes in the other direction, the TEE sends
54requests to the supplicant which then sends back the result.
55
56OP-TEE driver
57=============
58
59The OP-TEE driver handles OP-TEE [1] based TEEs. Currently it is only the ARM
60TrustZone based OP-TEE solution that is supported.
61
62Lowest level of communication with OP-TEE builds on ARM SMC Calling
63Convention (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
65Protocol [4].
66
67OP-TEE SMC interface provides the basic functions required by SMCCC and some
68additional 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
82The GlobalPlatform TEE Client API [5] is implemented on top of the generic
83TEE API.
84
85Picture of the relationship between the different components in the
86OP-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
108RPC (Remote Procedure Call) are requests from secure world to kernel driver
109or tee-supplicant. An RPC is identified by a special range of SMCCC return
110values from OPTEE_SMC_CALL_WITH_ARG. RPC messages which are intended for the
111kernel are handled by the kernel driver. Other RPC messages will be forwarded to
112tee-supplicant without further involvement of the driver, except switching
113shared memory buffer representation.
114
115References
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.
128