365 lines
15 KiB
ReStructuredText
365 lines
15 KiB
ReStructuredText
=============
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TEE subsystem
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=============
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This document describes the TEE subsystem in Linux.
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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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This subsystem deals with:
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- Registration of TEE drivers
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- Managing shared memory between Linux and the TEE
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- Providing a generic API to the TEE
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The TEE interface
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=================
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include/uapi/linux/tee.h defines the generic interface to a TEE.
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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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- 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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- TEE_IOC_VERSION lets user space know which TEE this driver handles and
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its capabilities.
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- TEE_IOC_OPEN_SESSION opens a new session to a Trusted Application.
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- TEE_IOC_INVOKE invokes a function in a Trusted Application.
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- TEE_IOC_CANCEL may cancel an ongoing TEE_IOC_OPEN_SESSION or TEE_IOC_INVOKE.
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- TEE_IOC_CLOSE_SESSION closes a session to a Trusted Application.
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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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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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The TEE kernel interface
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========================
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Kernel provides a TEE bus infrastructure where a Trusted Application is
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represented as a device identified via Universally Unique Identifier (UUID) and
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client drivers register a table of supported device UUIDs.
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TEE bus infrastructure registers following APIs:
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match():
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iterates over the client driver UUID table to find a corresponding
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match for device UUID. If a match is found, then this particular device is
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probed via corresponding probe API registered by the client driver. This
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process happens whenever a device or a client driver is registered with TEE
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bus.
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uevent():
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notifies user-space (udev) whenever a new device is registered on
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TEE bus for auto-loading of modularized client drivers.
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TEE bus device enumeration is specific to underlying TEE implementation, so it
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is left open for TEE drivers to provide corresponding implementation.
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Then TEE client driver can talk to a matched Trusted Application using APIs
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listed in include/linux/tee_drv.h.
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TEE client driver example
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-------------------------
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Suppose a TEE client driver needs to communicate with a Trusted Application
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having UUID: ``ac6a4085-0e82-4c33-bf98-8eb8e118b6c2``, so driver registration
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snippet would look like::
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static const struct tee_client_device_id client_id_table[] = {
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{UUID_INIT(0xac6a4085, 0x0e82, 0x4c33,
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0xbf, 0x98, 0x8e, 0xb8, 0xe1, 0x18, 0xb6, 0xc2)},
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{}
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};
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MODULE_DEVICE_TABLE(tee, client_id_table);
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static struct tee_client_driver client_driver = {
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.id_table = client_id_table,
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.driver = {
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.name = DRIVER_NAME,
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.bus = &tee_bus_type,
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.probe = client_probe,
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.remove = client_remove,
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},
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};
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static int __init client_init(void)
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{
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return driver_register(&client_driver.driver);
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}
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static void __exit client_exit(void)
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{
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driver_unregister(&client_driver.driver);
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}
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module_init(client_init);
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module_exit(client_exit);
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OP-TEE driver
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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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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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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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- 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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- 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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- OPTEE_SMC_CALL_WITH_ARG drives the OP-TEE message protocol
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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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The GlobalPlatform TEE Client API [5] is implemented on top of the generic
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TEE API.
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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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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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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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OP-TEE device enumeration
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-------------------------
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OP-TEE provides a pseudo Trusted Application: drivers/tee/optee/device.c in
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order to support device enumeration. In other words, OP-TEE driver invokes this
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application to retrieve a list of Trusted Applications which can be registered
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as devices on the TEE bus.
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OP-TEE notifications
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--------------------
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There are two kinds of notifications that secure world can use to make
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normal world aware of some event.
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1. Synchronous notifications delivered with ``OPTEE_RPC_CMD_NOTIFICATION``
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using the ``OPTEE_RPC_NOTIFICATION_SEND`` parameter.
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2. Asynchronous notifications delivered with a combination of a non-secure
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edge-triggered interrupt and a fast call from the non-secure interrupt
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handler.
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Synchronous notifications are limited by depending on RPC for delivery,
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this is only usable when secure world is entered with a yielding call via
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``OPTEE_SMC_CALL_WITH_ARG``. This excludes such notifications from secure
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world interrupt handlers.
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An asynchronous notification is delivered via a non-secure edge-triggered
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interrupt to an interrupt handler registered in the OP-TEE driver. The
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actual notification value are retrieved with the fast call
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``OPTEE_SMC_GET_ASYNC_NOTIF_VALUE``. Note that one interrupt can represent
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multiple notifications.
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One notification value ``OPTEE_SMC_ASYNC_NOTIF_VALUE_DO_BOTTOM_HALF`` has a
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special meaning. When this value is received it means that normal world is
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supposed to make a yielding call ``OPTEE_MSG_CMD_DO_BOTTOM_HALF``. This
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call is done from the thread assisting the interrupt handler. This is a
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building block for OP-TEE OS in secure world to implement the top half and
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bottom half style of device drivers.
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OPTEE_INSECURE_LOAD_IMAGE Kconfig option
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----------------------------------------
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The OPTEE_INSECURE_LOAD_IMAGE Kconfig option enables the ability to load the
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BL32 OP-TEE image from the kernel after the kernel boots, rather than loading
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it from the firmware before the kernel boots. This also requires enabling the
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corresponding option in Trusted Firmware for Arm. The Trusted Firmware for Arm
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documentation [8] explains the security threat associated with enabling this as
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well as mitigations at the firmware and platform level.
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There are additional attack vectors/mitigations for the kernel that should be
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addressed when using this option.
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1. Boot chain security.
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* Attack vector: Replace the OP-TEE OS image in the rootfs to gain control of
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the system.
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* Mitigation: There must be boot chain security that verifies the kernel and
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rootfs, otherwise an attacker can modify the loaded OP-TEE binary by
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modifying it in the rootfs.
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2. Alternate boot modes.
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* Attack vector: Using an alternate boot mode (i.e. recovery mode), the
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OP-TEE driver isn't loaded, leaving the SMC hole open.
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* Mitigation: If there are alternate methods of booting the device, such as a
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recovery mode, it should be ensured that the same mitigations are applied
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in that mode.
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3. Attacks prior to SMC invocation.
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* Attack vector: Code that is executed prior to issuing the SMC call to load
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OP-TEE can be exploited to then load an alternate OS image.
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* Mitigation: The OP-TEE driver must be loaded before any potential attack
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vectors are opened up. This should include mounting of any modifiable
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filesystems, opening of network ports or communicating with external
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devices (e.g. USB).
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4. Blocking SMC call to load OP-TEE.
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* Attack vector: Prevent the driver from being probed, so the SMC call to
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load OP-TEE isn't executed when desired, leaving it open to being executed
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later and loading a modified OS.
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* Mitigation: It is recommended to build the OP-TEE driver as builtin driver
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rather than as a module to prevent exploits that may cause the module to
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not be loaded.
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AMD-TEE driver
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==============
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The AMD-TEE driver handles the communication with AMD's TEE environment. The
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TEE environment is provided by AMD Secure Processor.
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The AMD Secure Processor (formerly called Platform Security Processor or PSP)
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is a dedicated processor that features ARM TrustZone technology, along with a
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software-based Trusted Execution Environment (TEE) designed to enable
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third-party Trusted Applications. This feature is currently enabled only for
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APUs.
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The following picture shows a high level overview of AMD-TEE::
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x86 |
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User space (Kernel space) | AMD Secure Processor (PSP)
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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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|| | ||
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|| | \/
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|| | +----------+
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|| | | TEE |
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|| | | Internal |
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\/ | | API |
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+---------+ +-----------+---------+ +----------+
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| TEE | | TEE | AMD-TEE | | AMD-TEE |
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| Client | | subsystem | driver | | Trusted |
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| API | | | | | OS |
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+---------+-----------+----+------+---------+---------+----------+
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| Generic TEE API | | ASP | Mailbox |
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| IOCTL (TEE_IOC_*) | | driver | Register Protocol |
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+--------------------------+ +---------+--------------------+
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At the lowest level (in x86), the AMD Secure Processor (ASP) driver uses the
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CPU to PSP mailbox register to submit commands to the PSP. The format of the
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command buffer is opaque to the ASP driver. It's role is to submit commands to
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the secure processor and return results to AMD-TEE driver. The interface
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between AMD-TEE driver and AMD Secure Processor driver can be found in [6].
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The AMD-TEE driver packages the command buffer payload for processing in TEE.
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The command buffer format for the different TEE commands can be found in [7].
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The TEE commands supported by AMD-TEE Trusted OS are:
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* TEE_CMD_ID_LOAD_TA - loads a Trusted Application (TA) binary into
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TEE environment.
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* TEE_CMD_ID_UNLOAD_TA - unloads TA binary from TEE environment.
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* TEE_CMD_ID_OPEN_SESSION - opens a session with a loaded TA.
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* TEE_CMD_ID_CLOSE_SESSION - closes session with loaded TA
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* TEE_CMD_ID_INVOKE_CMD - invokes a command with loaded TA
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* TEE_CMD_ID_MAP_SHARED_MEM - maps shared memory
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* TEE_CMD_ID_UNMAP_SHARED_MEM - unmaps shared memory
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AMD-TEE Trusted OS is the firmware running on AMD Secure Processor.
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The AMD-TEE driver registers itself with TEE subsystem and implements the
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following driver function callbacks:
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* get_version - returns the driver implementation id and capability.
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* open - sets up the driver context data structure.
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* release - frees up driver resources.
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* open_session - loads the TA binary and opens session with loaded TA.
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* close_session - closes session with loaded TA and unloads it.
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* invoke_func - invokes a command with loaded TA.
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cancel_req driver callback is not supported by AMD-TEE.
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The GlobalPlatform TEE Client API [5] can be used by the user space (client) to
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talk to AMD's TEE. AMD's TEE provides a secure environment for loading, opening
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a session, invoking commands and closing session with TA.
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References
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==========
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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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[6] include/linux/psp-tee.h
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[7] drivers/tee/amdtee/amdtee_if.h
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[8] https://trustedfirmware-a.readthedocs.io/en/latest/threat_model/threat_model.html
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