Musca-B1 Secure Enclave Specifics

Introduction

The Musca-B1 System-on-Chip contains two subsystems:

  • SSE-200 subsystem to host the main application.

  • CryptoIsland-300 subsystem can be used as a Secure Enclave (mentioned as SE in the document).

If TF-M is built with default configuration to the MUSCA-B1 platform only the SSE-200 subsystem used. But if the FORWARD_PROT_MSG cmake flag is turned on, the TF-M instance running on SSE-200 will communicate with the SE.

For more information you can check the Secure Enclave design document.

System boot

The desired boot flow would be to start up SE first at power on as SE is the Root of Trust in the system, and SSE-200 should be started up by SE. But the current Musca-B1 DAPLink FW releases the SSE-200 subsystem from reset first, and it would require complex changes to modify this boot order. So an additional SSE-200 BL0 component was added to the system to imitate that SE is the subsystem started up first.

@startuml

title Implemented boot flow

start
:Power On button pressed.;
:DAPLink Starts up SSE-200.;
:SSE-200 BL0 starts up the SE (through SCC), then enters wait state.;
:SE starts to run its MCUBoot image and authenticates all images (SE TF-M
image and the combined SSE-200 image). If all images are valid, control jumps
to SE’s TF-M image.;
:SE's TF-M image starts up, when initialization finishes, SSE-200 is
virtually released from reset by sending the start address of the combined
SSE-200 image over MHU;
:SSE-200 TF-M image starts up and synchronizes with SE over MHU;
:SSE-200 and SE are ready to communicate;
stop

@enduml

Note

Without the SSE-200 BL0 component, the boot flow can be treated as a valid reference solution.

Build

To produce all the images, TF-M build needs to be executed twice:

  • One build needed to create the SSE-200 images (BL0 and the combined SSE-200 image containing TF-M and the non-secure application), target platform needs to be set to musca_b1/sse_200 and the FORWARD_PROT_MSG cmake flag also needs to be set.

  • One build needed to create the SE images (MCUBoot and TF-M), target platform needs to be set to musca_b1/secure_enclave.

The order of the two builds is indifferent. The BL2 setting is mandatory for both builds, but MCUBoot image is only built for the SE platform. The cmake setup for the two builds must have the same debug profile (eg. debug, release).

To create a unified hex file:

  • Windows:

    srec_cat.exe <SE build dir>\bin\bl2.bin -Binary -offset 0x1A020000 ^
                 <SE build dir>\bin\tfm_s_signed.bin -Binary -offset 0x1A200000 ^
                 <SSE-200 build dir>\bin\bl0.bin -Binary -offset 0x1A000000 ^
                 <SSE-200 build dir>\bin\tfm_s_ns_signed.bin -Binary -offset 0x1A260000 ^
                 -o tfm_sse200_w_se.hex -Intel
    
  • Linux:

    srec_cat <SE build dir>/bin/bl2.bin -Binary -offset 0x1A020000 \
             <SE build dir>/bin/tfm_s_signed.bin -Binary -offset 0x1A200000 \
             <SSE-200 build dir>/bin/bl0.bin -Binary -offset 0x1A000000 \
             <SSE-200 build dir>/bin/tfm_s_ns_signed.bin -Binary -offset 0x1A260000 \
             -o tfm_sse200_w_se.hex -Intel
    

Known limitations

  • Musca-B1 Secure Enclave cannot reset the whole SoC, only itself. So if SE does a system reset it will stuck in a state waiting for a handshake signal from SSE-200. (It will never come, as SSE-200 is not reseted in such a situation.)


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