The document discusses Trusted Execution Environments (TEEs) and running the Open Portable Trusted Execution Environment (OP-TEE) trusted operating system on RISC-V. It provides an overview of TEEs, describes OP-TEE and the requirements to implement it on RISC-V, including developing a boot sequence, kernel driver, and libraries. The document also compares TEE implementations on ARM TrustZone and Intel SGX and covers memory mapping when running OP-TEE on ARM-based boards.

1. TEE to Run Trusted OS
on RISC-V and Related
Technologies
1
National Institute of Advanced Industrial Science and Technology(AIST)
Kuniyasu Suzaki, Akira Tsukamoto

2. Contents
• What is TEE?
– RISC-V TEE
• Trusted OS
– “OP-TEE”
– Requirement to implement OP-TEE
2
The difference from Micro51 workshop.
Software view (RISC-V day Tokyo), Hardware view (Micro51 workshop)
Slides will be opened at https://www.slideshare.net/suzaki

3. HIEE: Hardware-assisted Isolated Execution Environments*
• HIEE is used for important process which is independent of OS.
• Current CPU has HIEE.
– X86 SMM: System Management Mode
• Used by BIOS/UEFI.
– Intel’s ME: Management Engine.
• Run MINIX. Used for remote power control.
– Intel SGX: Software Guard execution
– ARM Trust Zone
3
* F.Zhang and H.Zhang, SoK：A Study of Using Hardware-assisted Isolated Execution
Environments for Security[HASP16]
They are not programmable for a user.
They are programmable for a user.
They are used for TEE.

4. What is TEE?
• TEE: Trusted Execution Environment.
– TEE separates computing world into “normal” and “secure”.
• Secure world is used to run a critical code (e.g., authentication, DRM, etc).
– GlobalPlatform defines TEE specification.
• https://globalplatform.org/technical-committees/trusted-execution-environment-tee-committee/
4
CPU
TEE
Secure World Normal World
App
Normal OS

5. Privileges for TEE
• Global Platform's TEE specification assumes plural privileges on both
worlds to run application on a OS.
– Normal world runs normal applications on a normal OS.
– Secure world runs trusted applications (TAs) on a trusted OS.
• ARM Trust Zone offers same privileges to normal and secure world.
• Intel SGX has only one privilege (enclave).
– Enclave is different from Ring Architecture.
5
CPU
Trusted OS
Secure World Normal World
App
Normal OS (e.g., Linux)
Trusted Application (TA)
POSIX APIGP TEE API

6. TEE projects on RISC-V
• Sanctum [MIT project, USENIX Sec’16]
• KeyStone [UCB project]
– https://keystone-enclave.org/
• MultiZone [Hex-Five product]
– https://hex-five.com/
• TEE WG of RISC-V Foundation
6
Intel SGX type
One special privilege (Enclave)
ARM Trust Zone type
plural privileges
easy to implement GlobalPlatform
style Trusted OS

7. ARM
Trusted OS on ARM Trust Zone
• GlobalPlatform model
7
Secure world Normal world
Normal Applications
Normal OS
NIC
Core
Trusted OS
Trusted Applications (TA)
Core
Secure
Mode
Hard
eMMCUART
Exception Level
EL0: User
EL1: Privilege
EL2: Hypervisor
EL3: Secure
SMC (Secure Monitor Call)
instruction
Memory
Static allocate
Dynamic allocate

8. Boot Sequence on ARM Trust Zone
• BL: Boot Loader
• EL: Exception Level
8
Power on
BL1:
BootROM
BL2:
Trusted Boot
Firmware
Secure World Normal World
BL31:
Secure
Monitor
BL32:
Trusted OS BL33:
Normal OS
EL3:Secure EL2:Hypervisor EL1:Priviledge EL0:User EL3:Secure EL2:Hypervisor EL1:Priviledge EL0:User

9. Trusted OS
• Trusted OS is not a normal OS
– Trusted OS is TCB (Trusted Computing Base). It must be secure
(small).
– No POSIX API, No dynamic link library
• TA becomes a static linked binary.
• Trusted OS needs the help of normal OS
– Because Trusted OS has no File System, no device driver (except
some special devices, e.g., UART)
• When a TA want to save a data, the data is encrypted and saved on FS of
normal OS.
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10. Trusted OS
• Open Source Trusted OS
– OP-TEE (Linaro) https://github.com/OP-TEE
– Open-TEE (Aalto University[TrustCom15]) https://open-tee.github.io/
– Trusty (Google) https://source.android.com/security/trusty/index.html
– SierraTEE (Sierra) https://www.sierraware.com/open-source-ARM-TrustZone.html
• Enterprise Trusted OS
– Apple’s Secure Enclave
– Qualcomm’s QSEE
– Samsung’s Knox https://www.samsungknox.com/en
– Trastonic Kinibi and MobiCore
10

11. How to run a TA on OP-TEE
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Secure World Normal World
Secure Monitor
Exception Level
EL0: User
EL1: Privilege
EL2: Hypervisor
EL3: Secure
Linux
with op-tee driver
TA
with
libutee.a
OP-TEE
(signature check)
(dispatch by UUID)
TA-Client TEE-Supplicant
/dev/tee0 /dev/teeprev0
Load a TA
Use a TA Helper of TAsignatureMajor Roll
• TA-Client loads a TA on OP-TEE
– TA needs a signature to load on OP-TEE
– TA has a UUID to communicate with TA-
Client and TEE-Supplicant
• TA-Client sends a request to TA
• TA replies an answer to TA-Client
• TA sends a request to TEE-
Supplicant (e.g., to save an
encrypted data on File System)
TEE internal API only (No POSIX sys call)
TA
libtee.so

12. Memory Map of OP-TEE
• ARM Hikey Board 2GB
• Software size: Our experience
– Secure world
• Secure Monitor 33KB
• OP-TEE 281KB
• TA 1,200KB
– Normal World (on Linux)
• TA-Client 17KB
• TEE-Supplicant 197KB
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0x7FFFFFFF (2GB)
OP-TEE & TA
0x7F000000
16MB
0x7EC0000
Shared Memory 4 MB
0x7E80000
Linux Kernel
with SMC driver
4 MB
0x7E00000
8 MB
Secure Data
Path Buffer
Reserved
Secure
World
Normal
World

13. OP-TEE on RISC-V using seL4
• Rahul Mahadev’s Google Summer of Code
• http://mahadevrahul.blogspot.com/
– The Trust Zone features and secure monitor
are implemented as a seL4 library.
– OP-TEE is paravirtualized, all calls referencing
ARM Trusted Firmware and secure monitor are
replaced with new calls.
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seL4
Rich OS
(Linux) Library to
emulate
Trust Zone
VMM
Paravirtualized OP-TEE
TAApp

14. Conclusions
• My talk is software view of TEE
• When we implement OP-TEE on RISC-V, we must develop
– Boot sequence: Trusted Boot Firmware, Secure Monitor
– Linux kernel driver
– Libraries (libutee.a for TA and libtee.so for Linux Apps)
– Linux application to assist TA (TEE-supplicant)
14
See Micro51 RISC-V workshop for hardware view
Slide of RISC-V Day Tokyo 2018 and Micro51 RISC-V workshop will be opened at
https://www.slideshare.net/suzaki