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+TEE subsystem
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+This document describes the TEE subsystem in Linux.
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+
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+A TEE (Trusted Execution Environment) is a trusted OS running in some
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+secure environment, for example, TrustZone on ARM CPUs, or a separate
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+secure co-processor etc. A TEE driver handles the details needed to
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+communicate with the TEE.
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+
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+This subsystem deals with:
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+
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+- Registration of TEE drivers
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+
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+- Managing shared memory between Linux and the TEE
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+
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+- Providing a generic API to the TEE
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+
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+The TEE interface
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+=================
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+
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+include/uapi/linux/tee.h defines the generic interface to a TEE.
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+
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+User space (the client) connects to the driver by opening /dev/tee[0-9]* or
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+/dev/teepriv[0-9]*.
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+
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+- TEE_IOC_SHM_ALLOC allocates shared memory and returns a file descriptor
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+ which user space can mmap. When user space doesn't need the file
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+ descriptor any more, it should be closed. When shared memory isn't needed
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+ any longer it should be unmapped with munmap() to allow the reuse of
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+ memory.
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+
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+- TEE_IOC_VERSION lets user space know which TEE this driver handles and
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+ the its capabilities.
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+
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+- TEE_IOC_OPEN_SESSION opens a new session to a Trusted Application.
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+
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+- TEE_IOC_INVOKE invokes a function in a Trusted Application.
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+
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+- TEE_IOC_CANCEL may cancel an ongoing TEE_IOC_OPEN_SESSION or TEE_IOC_INVOKE.
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+
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+- TEE_IOC_CLOSE_SESSION closes a session to a Trusted Application.
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+
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+There are two classes of clients, normal clients and supplicants. The latter is
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+a helper process for the TEE to access resources in Linux, for example file
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+system access. A normal client opens /dev/tee[0-9]* and a supplicant opens
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+/dev/teepriv[0-9].
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+
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+Much of the communication between clients and the TEE is opaque to the
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+driver. The main job for the driver is to receive requests from the
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+clients, forward them to the TEE and send back the results. In the case of
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+supplicants the communication goes in the other direction, the TEE sends
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+requests to the supplicant which then sends back the result.
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+
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+OP-TEE driver
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+=============
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+
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+The OP-TEE driver handles OP-TEE [1] based TEEs. Currently it is only the ARM
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+TrustZone based OP-TEE solution that is supported.
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+
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+Lowest level of communication with OP-TEE builds on ARM SMC Calling
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+Convention (SMCCC) [2], which is the foundation for OP-TEE's SMC interface
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+[3] used internally by the driver. Stacked on top of that is OP-TEE Message
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+Protocol [4].
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+
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+OP-TEE SMC interface provides the basic functions required by SMCCC and some
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+additional functions specific for OP-TEE. The most interesting functions are:
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+
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+- OPTEE_SMC_FUNCID_CALLS_UID (part of SMCCC) returns the version information
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+ which is then returned by TEE_IOC_VERSION
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+
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+- OPTEE_SMC_CALL_GET_OS_UUID returns the particular OP-TEE implementation, used
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+ to tell, for instance, a TrustZone OP-TEE apart from an OP-TEE running on a
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+ separate secure co-processor.
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+
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+- OPTEE_SMC_CALL_WITH_ARG drives the OP-TEE message protocol
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+
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+- OPTEE_SMC_GET_SHM_CONFIG lets the driver and OP-TEE agree on which memory
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+ range to used for shared memory between Linux and OP-TEE.
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+
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+The GlobalPlatform TEE Client API [5] is implemented on top of the generic
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+TEE API.
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+
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+Picture of the relationship between the different components in the
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+OP-TEE architecture.
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+
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+ User space Kernel Secure world
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+ ~~~~~~~~~~ ~~~~~~ ~~~~~~~~~~~~
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+ +--------+ +-------------+
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+ | Client | | Trusted |
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+ +--------+ | Application |
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+ /\ +-------------+
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+ || +----------+ /\
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+ || |tee- | ||
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+ || |supplicant| \/
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+ || +----------+ +-------------+
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+ \/ /\ | TEE Internal|
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+ +-------+ || | API |
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+ + TEE | || +--------+--------+ +-------------+
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+ | Client| || | TEE | OP-TEE | | OP-TEE |
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+ | API | \/ | subsys | driver | | Trusted OS |
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+ +-------+----------------+----+-------+----+-----------+-------------+
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+ | Generic TEE API | | OP-TEE MSG |
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+ | IOCTL (TEE_IOC_*) | | SMCCC (OPTEE_SMC_CALL_*) |
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+ +-----------------------------+ +------------------------------+
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+
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+RPC (Remote Procedure Call) are requests from secure world to kernel driver
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+or tee-supplicant. An RPC is identified by a special range of SMCCC return
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+values from OPTEE_SMC_CALL_WITH_ARG. RPC messages which are intended for the
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+kernel are handled by the kernel driver. Other RPC messages will be forwarded to
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+tee-supplicant without further involvement of the driver, except switching
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+shared memory buffer representation.
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+
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+References:
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+[1] https://github.com/OP-TEE/optee_os
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+[2] http://infocenter.arm.com/help/topic/com.arm.doc.den0028a/index.html
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+[3] drivers/tee/optee/optee_smc.h
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+[4] drivers/tee/optee/optee_msg.h
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+[5] http://www.globalplatform.org/specificationsdevice.asp look for
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+ "TEE Client API Specification v1.0" and click download.
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Jens Wiklander