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 the 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