[PATCH v8 4/4] Documentation: tee subsystem and op-tee driver

[Randy Dunlap]

On 02/11/16 09:14, Jens Wiklander wrote:
> Signed-off-by: Jens Wiklander <jens.wiklander at linaro.org>
> ---
>  Documentation/00-INDEX |   2 +
>  Documentation/tee.txt  | 117 +++++++++++++++++++++++++++++++++++++++++++++++++
>  MAINTAINERS            |   1 +
>  3 files changed, 120 insertions(+)
>  create mode 100644 Documentation/tee.txt

> diff --git a/Documentation/tee.txt b/Documentation/tee.txt
> new file mode 100644
> index 0000000..4ac91d2
> --- /dev/null
> +++ b/Documentation/tee.txt
> @@ -0,0 +1,117 @@
> +TEE subsystem
> +This document describes the TEE subsystem in Linux

                                                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 anymore

                                                                           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

end with '.'

> +
> +- TEE_IOC_INVOKE invokes a function in a Trusted Application

ditto

> +
> +- TEE_IOC_CANCEL may cancel an ongoing TEE_IOC_OPEN_SESSION or TEE_IOC_INVOKE

ditto

> +
> +- TEE_IOC_CLOSE_SESSION closes a session to a Trusted Application

ditto

> +
> +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.


-- 
~Randy