119 lines
3.9 KiB
ReStructuredText
119 lines
3.9 KiB
ReStructuredText
.. _trusty_tee:
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Trusty TEE
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##########
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Introduction
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************
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`Trusty`_ is a set of software components supporting a Trusted Execution Environment (TEE).
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TEE is commonly known as an isolated processing environment in which applications can be
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securely executed irrespective of the rest of the system. For more information about TEE,
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please visit the `Trusted Execution Environment wiki page <https://en.wikipedia.org/wiki/Trusted_execution_environment>`_.
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Trusty consists of:
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1. An operating system (the Trusty OS) that runs on a processor intended to provide a TEE
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#. Drivers for the Android kernel (Linux) to facilitate communication with applications
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running under the Trusty OS
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#. A set of libraries for Android/Linux systems software to facilitate communication with
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trusted applications executed within the Trusty OS using the kernel drivers
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LK (`Little Kernel`_) is a tiny operating system suited for small embedded devices, bootloaders,
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and other environments where OS primitives such as threads, mutexes, and timers are needed, but
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there's a desire to keep things small and lightweight. LK has been chosen as the Trusty OS kernel.
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Trusty Architecture
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*******************
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.. figure:: images/trusty-arch.png
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:align: center
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:width: 800px
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:name: Trusty Architectural diagram
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.. note::
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Trusty OS is running in Secure World in the architecture drawing above.
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Trusty specific Hypercalls
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**************************
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There are a few :ref:`hypercall_apis` that are related to Trusty.
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.. doxygengroup:: trusty_hypercall
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:project: Project ACRN
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:content-only:
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Trusty Boot flow
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****************
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By design, the User OS bootloader (``UOS_Loader``) will trigger the Trusty boot process. The complete boot flow is illustrated below.
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.. graphviz:: images/trusty-boot-flow.dot
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:name: trusty-boot-flow
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:align: center
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:caption: Trusty Boot Flow
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As shown in the above figure, here are some details about the Trusty boot flow processing:
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1. UOS_Loader
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a. Load and verify Trusty image from virtual disk
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#. Allocate runtime memory for trusty
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#. Do ELF relocation of trusty image and get entry address
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#. Call ``hcall_initialize_trusty`` with trusty memory base and entry address
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#. ACRN (``hcall_initialize_trusty``)
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a. Save World context for Normal World
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#. Init World context for Secure World (RIP, RSP, EPT, etc.)
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#. Resume to Secure World
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#. Trusty
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a. Booting up
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#. Call ``hcall_world_switch`` to switch back to Normal World if boot completed
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#. ACRN (``hcall_world_switch``)
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a. Save World context for the World which caused this ``vmexit`` (Secure World)
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#. Restore World context for next World (Normal World (UOS_Loader))
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#. Resume to next World (UOS_Loader)
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#. UOS_Loader
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a. Continue to boot
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EPT Hierarchy
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*************
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As per the Trusty design, Trusty can access Normal World's memory, but Normal World cannot
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access Secure World's memory. Hence it means Secure World EPTP page table hierarchy
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must contain normal world GPA address space, while Trusty world's GPA address space
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must be removed from the Normal world EPTP page table hierarchy.
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Design
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======
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Put Secure World's GPA to very high position: 511 GB - 512 GB. The PML4/PDPT for Trusty
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World are separated from Normal World. PD/PT for low memory (< 511 GB) are shared in
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both Trusty World's EPT and Normal World's EPT. PD/PT for high memory (>= 511 GB) are
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valid for Trusty World's EPT only.
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Benefit
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=======
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This design will benefit the EPT changes of Normal World. There are requirement to
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modify Normal World's EPT during runtime such as increasing memory, changing attributes,
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etc. If such behavior happened, only PD and PT for Normal World need to
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be updated.
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.. figure:: images/ept-hierarchy.png
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:align: center
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:width: 800px
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:name: EPT hierarchy
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API
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===
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.. doxygengroup:: trusty_apis
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:project: Project ACRN
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:content-only:
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.. _Trusty: https://source.android.com/security/trusty/
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.. _Little Kernel: https://github.com/littlekernel/lk
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