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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
- 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.
- The TEE kernel interface
- ========================
- Kernel provides a TEE bus infrastructure where a Trusted Application is
- represented as a device identified via Universally Unique Identifier (UUID) and
- client drivers register a table of supported device UUIDs.
- TEE bus infrastructure registers following APIs:
- match():
- iterates over the client driver UUID table to find a corresponding
- match for device UUID. If a match is found, then this particular device is
- probed via corresponding probe API registered by the client driver. This
- process happens whenever a device or a client driver is registered with TEE
- bus.
- uevent():
- notifies user-space (udev) whenever a new device is registered on
- TEE bus for auto-loading of modularized client drivers.
- TEE bus device enumeration is specific to underlying TEE implementation, so it
- is left open for TEE drivers to provide corresponding implementation.
- Then TEE client driver can talk to a matched Trusted Application using APIs
- listed in include/linux/tee_drv.h.
- TEE client driver example
- -------------------------
- Suppose a TEE client driver needs to communicate with a Trusted Application
- having UUID: ``ac6a4085-0e82-4c33-bf98-8eb8e118b6c2``, so driver registration
- snippet would look like::
- static const struct tee_client_device_id client_id_table[] = {
- {UUID_INIT(0xac6a4085, 0x0e82, 0x4c33,
- 0xbf, 0x98, 0x8e, 0xb8, 0xe1, 0x18, 0xb6, 0xc2)},
- {}
- };
- MODULE_DEVICE_TABLE(tee, client_id_table);
- static struct tee_client_driver client_driver = {
- .id_table = client_id_table,
- .driver = {
- .name = DRIVER_NAME,
- .bus = &tee_bus_type,
- .probe = client_probe,
- .remove = client_remove,
- },
- };
- static int __init client_init(void)
- {
- return driver_register(&client_driver.driver);
- }
- static void __exit client_exit(void)
- {
- driver_unregister(&client_driver.driver);
- }
- module_init(client_init);
- module_exit(client_exit);
- 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.
- OP-TEE device enumeration
- -------------------------
- OP-TEE provides a pseudo Trusted Application: drivers/tee/optee/device.c in
- order to support device enumeration. In other words, OP-TEE driver invokes this
- application to retrieve a list of Trusted Applications which can be registered
- as devices on the TEE bus.
- AMD-TEE driver
- ==============
- The AMD-TEE driver handles the communication with AMD's TEE environment. The
- TEE environment is provided by AMD Secure Processor.
- The AMD Secure Processor (formerly called Platform Security Processor or PSP)
- is a dedicated processor that features ARM TrustZone technology, along with a
- software-based Trusted Execution Environment (TEE) designed to enable
- third-party Trusted Applications. This feature is currently enabled only for
- APUs.
- The following picture shows a high level overview of AMD-TEE::
- |
- x86 |
- |
- User space (Kernel space) | AMD Secure Processor (PSP)
- ~~~~~~~~~~ ~~~~~~~~~~~~~~ | ~~~~~~~~~~~~~~~~~~~~~~~~~~
- |
- +--------+ | +-------------+
- | Client | | | Trusted |
- +--------+ | | Application |
- /\ | +-------------+
- || | /\
- || | ||
- || | \/
- || | +----------+
- || | | TEE |
- || | | Internal |
- \/ | | API |
- +---------+ +-----------+---------+ +----------+
- | TEE | | TEE | AMD-TEE | | AMD-TEE |
- | Client | | subsystem | driver | | Trusted |
- | API | | | | | OS |
- +---------+-----------+----+------+---------+---------+----------+
- | Generic TEE API | | ASP | Mailbox |
- | IOCTL (TEE_IOC_*) | | driver | Register Protocol |
- +--------------------------+ +---------+--------------------+
- At the lowest level (in x86), the AMD Secure Processor (ASP) driver uses the
- CPU to PSP mailbox regsister to submit commands to the PSP. The format of the
- command buffer is opaque to the ASP driver. It's role is to submit commands to
- the secure processor and return results to AMD-TEE driver. The interface
- between AMD-TEE driver and AMD Secure Processor driver can be found in [6].
- The AMD-TEE driver packages the command buffer payload for processing in TEE.
- The command buffer format for the different TEE commands can be found in [7].
- The TEE commands supported by AMD-TEE Trusted OS are:
- * TEE_CMD_ID_LOAD_TA - loads a Trusted Application (TA) binary into
- TEE environment.
- * TEE_CMD_ID_UNLOAD_TA - unloads TA binary from TEE environment.
- * TEE_CMD_ID_OPEN_SESSION - opens a session with a loaded TA.
- * TEE_CMD_ID_CLOSE_SESSION - closes session with loaded TA
- * TEE_CMD_ID_INVOKE_CMD - invokes a command with loaded TA
- * TEE_CMD_ID_MAP_SHARED_MEM - maps shared memory
- * TEE_CMD_ID_UNMAP_SHARED_MEM - unmaps shared memory
- AMD-TEE Trusted OS is the firmware running on AMD Secure Processor.
- The AMD-TEE driver registers itself with TEE subsystem and implements the
- following driver function callbacks:
- * get_version - returns the driver implementation id and capability.
- * open - sets up the driver context data structure.
- * release - frees up driver resources.
- * open_session - loads the TA binary and opens session with loaded TA.
- * close_session - closes session with loaded TA and unloads it.
- * invoke_func - invokes a command with loaded TA.
- cancel_req driver callback is not supported by AMD-TEE.
- The GlobalPlatform TEE Client API [5] can be used by the user space (client) to
- talk to AMD's TEE. AMD's TEE provides a secure environment for loading, opening
- a session, invoking commands and clossing session with TA.
- 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.
- [6] include/linux/psp-tee.h
- [7] drivers/tee/amdtee/amdtee_if.h
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