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| 1 | +TEE subsystem |
| 2 | +This document describes the TEE subsystem in Linux. |
| 3 | + |
| 4 | +A TEE (Trusted Execution Environment) is a trusted OS running in some |
| 5 | +secure environment, for example, TrustZone on ARM CPUs, or a separate |
| 6 | +secure co-processor etc. A TEE driver handles the details needed to |
| 7 | +communicate with the TEE. |
| 8 | + |
| 9 | +This subsystem deals with: |
| 10 | + |
| 11 | +- Registration of TEE drivers |
| 12 | + |
| 13 | +- Managing shared memory between Linux and the TEE |
| 14 | + |
| 15 | +- Providing a generic API to the TEE |
| 16 | + |
| 17 | +The TEE interface |
| 18 | +================= |
| 19 | + |
| 20 | +include/uapi/linux/tee.h defines the generic interface to a TEE. |
| 21 | + |
| 22 | +User space (the client) connects to the driver by opening /dev/tee[0-9]* or |
| 23 | +/dev/teepriv[0-9]*. |
| 24 | + |
| 25 | +- TEE_IOC_SHM_ALLOC allocates shared memory and returns a file descriptor |
| 26 | + which user space can mmap. When user space doesn't need the file |
| 27 | + descriptor any more, it should be closed. When shared memory isn't needed |
| 28 | + any longer it should be unmapped with munmap() to allow the reuse of |
| 29 | + memory. |
| 30 | + |
| 31 | +- TEE_IOC_VERSION lets user space know which TEE this driver handles and |
| 32 | + the its capabilities. |
| 33 | + |
| 34 | +- TEE_IOC_OPEN_SESSION opens a new session to a Trusted Application. |
| 35 | + |
| 36 | +- TEE_IOC_INVOKE invokes a function in a Trusted Application. |
| 37 | + |
| 38 | +- TEE_IOC_CANCEL may cancel an ongoing TEE_IOC_OPEN_SESSION or TEE_IOC_INVOKE. |
| 39 | + |
| 40 | +- TEE_IOC_CLOSE_SESSION closes a session to a Trusted Application. |
| 41 | + |
| 42 | +There are two classes of clients, normal clients and supplicants. The latter is |
| 43 | +a helper process for the TEE to access resources in Linux, for example file |
| 44 | +system access. A normal client opens /dev/tee[0-9]* and a supplicant opens |
| 45 | +/dev/teepriv[0-9]. |
| 46 | + |
| 47 | +Much of the communication between clients and the TEE is opaque to the |
| 48 | +driver. The main job for the driver is to receive requests from the |
| 49 | +clients, forward them to the TEE and send back the results. In the case of |
| 50 | +supplicants the communication goes in the other direction, the TEE sends |
| 51 | +requests to the supplicant which then sends back the result. |
| 52 | + |
| 53 | +OP-TEE driver |
| 54 | +============= |
| 55 | + |
| 56 | +The OP-TEE driver handles OP-TEE [1] based TEEs. Currently it is only the ARM |
| 57 | +TrustZone based OP-TEE solution that is supported. |
| 58 | + |
| 59 | +Lowest level of communication with OP-TEE builds on ARM SMC Calling |
| 60 | +Convention (SMCCC) [2], which is the foundation for OP-TEE's SMC interface |
| 61 | +[3] used internally by the driver. Stacked on top of that is OP-TEE Message |
| 62 | +Protocol [4]. |
| 63 | + |
| 64 | +OP-TEE SMC interface provides the basic functions required by SMCCC and some |
| 65 | +additional functions specific for OP-TEE. The most interesting functions are: |
| 66 | + |
| 67 | +- OPTEE_SMC_FUNCID_CALLS_UID (part of SMCCC) returns the version information |
| 68 | + which is then returned by TEE_IOC_VERSION |
| 69 | + |
| 70 | +- OPTEE_SMC_CALL_GET_OS_UUID returns the particular OP-TEE implementation, used |
| 71 | + to tell, for instance, a TrustZone OP-TEE apart from an OP-TEE running on a |
| 72 | + separate secure co-processor. |
| 73 | + |
| 74 | +- OPTEE_SMC_CALL_WITH_ARG drives the OP-TEE message protocol |
| 75 | + |
| 76 | +- OPTEE_SMC_GET_SHM_CONFIG lets the driver and OP-TEE agree on which memory |
| 77 | + range to used for shared memory between Linux and OP-TEE. |
| 78 | + |
| 79 | +The GlobalPlatform TEE Client API [5] is implemented on top of the generic |
| 80 | +TEE API. |
| 81 | + |
| 82 | +Picture of the relationship between the different components in the |
| 83 | +OP-TEE architecture. |
| 84 | + |
| 85 | + User space Kernel Secure world |
| 86 | + ~~~~~~~~~~ ~~~~~~ ~~~~~~~~~~~~ |
| 87 | + +--------+ +-------------+ |
| 88 | + | Client | | Trusted | |
| 89 | + +--------+ | Application | |
| 90 | + /\ +-------------+ |
| 91 | + || +----------+ /\ |
| 92 | + || |tee- | || |
| 93 | + || |supplicant| \/ |
| 94 | + || +----------+ +-------------+ |
| 95 | + \/ /\ | TEE Internal| |
| 96 | + +-------+ || | API | |
| 97 | + + TEE | || +--------+--------+ +-------------+ |
| 98 | + | Client| || | TEE | OP-TEE | | OP-TEE | |
| 99 | + | API | \/ | subsys | driver | | Trusted OS | |
| 100 | + +-------+----------------+----+-------+----+-----------+-------------+ |
| 101 | + | Generic TEE API | | OP-TEE MSG | |
| 102 | + | IOCTL (TEE_IOC_*) | | SMCCC (OPTEE_SMC_CALL_*) | |
| 103 | + +-----------------------------+ +------------------------------+ |
| 104 | + |
| 105 | +RPC (Remote Procedure Call) are requests from secure world to kernel driver |
| 106 | +or tee-supplicant. An RPC is identified by a special range of SMCCC return |
| 107 | +values from OPTEE_SMC_CALL_WITH_ARG. RPC messages which are intended for the |
| 108 | +kernel are handled by the kernel driver. Other RPC messages will be forwarded to |
| 109 | +tee-supplicant without further involvement of the driver, except switching |
| 110 | +shared memory buffer representation. |
| 111 | + |
| 112 | +References: |
| 113 | +[1] https://github.com/OP-TEE/optee_os |
| 114 | +[2] http://infocenter.arm.com/help/topic/com.arm.doc.den0028a/index.html |
| 115 | +[3] drivers/tee/optee/optee_smc.h |
| 116 | +[4] drivers/tee/optee/optee_msg.h |
| 117 | +[5] http://www.globalplatform.org/specificationsdevice.asp look for |
| 118 | + "TEE Client API Specification v1.0" and click download. |
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