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- 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
- the 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.
- 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.
- 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.
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