AM2432: Validating secure boot SBL image with/without booting

Aravind kumar

Part Number: AM2432

Tool/software:

Hi,

We are planning to implement SBL OTA upgrade. But, got stuck with an issue related to validating SBL image signature before flashing into the bootflash (0x00_0000 boot address).

As I know in HS-SE (secure boot), AM243x will be using fused keys to validate SBL before booting the image.

Is there any possibility to validate encrypted SBL image with fused keys without booting from an specific offset address? Does ROM boot libraries can used for it? are there any other api's/methods to do it?

If above validating without boot is not possible. Is there anyway, that we can specify SBL boot offset address and trigger boot sequence in HS-SE mode. So, AM243x can boot from the specified offset address (where OTA SBL image is present in the bootflash memory)? Is there any api's to trigger boot sequence from an specific address. Just like loading application image from SBL?

Need information on above two questions. Please provide information on those.

Thanks

5 months ago

0 Prashant Shivhare 5 months ago

TI__Guru 50861 points

Hello,

You may use the following TISCI API for authenticating a signed image

https://software-dl.ti.com/tisci/esd/latest/2_tisci_msgs/security/PROC_BOOT.html?highlight=tisci_msg_proc_auth_boot#proc-boot-authenticate-image-and-configure-processor

This is what the SBL uses to validate the application image before booting it.

https://github.com/TexasInstruments/mcupsdk-core/blob/2252c873974d577912b02fdad718a05c2cb04e4d/source/drivers/bootloader/soc/am64x_am243x/bootloader_soc.c#L1234C9-L1234C32

Regards,

Prashant

0 Aravind kumar 5 months ago in reply to Prashant Shivhare

Prodigy 20 points

Thanks for replying back.

Looks like above api is used for HS-FS mode. What about HS-SE mode? How to validate SBL image in the boot flash, when trying to validate SBL OTA image store is a different location.

0 Prashant Shivhare 5 months ago in reply to Aravind kumar

TI__Guru 50861 points

Aravind kumar said:
Looks like above api is used for HS-FS mode. What about HS-SE mode?

It's for both HSFS and HSSE.

On HSFS devices, since there are no keys the authentication is skipped and only integrity check is performed. On HSSE devices, the image is fully authenticated with the fused keys.

0 Hong Zhang 5 months ago in reply to Prashant Shivhare

Intellectual 507 points

Hello Prashant,

The use case for the TISCI API that you pointed above is for SBL to authenticate the app image.

For our use case here, we are updating the SBL, so the process would be to store the SBL into RAM or a storage SPI device first, then we need to call an API to authenticate this new SBL image using the SMPK keys in the OTP. We would like to program the SBL into bootflash only after the image is verified to be signed with the proper SMPK keys in OTP.

Thanks,
Hong

0 Prashant Shivhare 5 months ago in reply to Hong Zhang

TI__Guru 50861 points

Hello,

Hong Zhang said:
The use case for the TISCI API that you pointed above is for SBL to authenticate the app image.

The TISCI_MSG_PROC_AUTH_BOOT is a generic API for authenticating a signed random binary. The SBL using it for authenticating the appimage is just a use case of it.

Since, the SBL is signed with the ROM X.509 certificate format, the API cannot be directly used to authenticate the SBL as it is. Instead, you may follow the below steps:

Think of the signed SBL (ROM X.509 certificate + Payloads) as a single binary (Y).
Sign the Y using the SYSFW signing script generating the signed binary Z (= X (SYSFW X.509 certificate) + Y).
Receive the Z.
Authenticate the Z using TISCI_MSG_PROC_AUTH_BOOT.
If successful then discard X and flash the Y at the required offsets.

Regards,

Prashant

0 Prashant Shivhare 5 months ago in reply to Prashant Shivhare

TI__Guru 50861 points

Just in case,

There is no way to validate the SBL as it is without actually letting ROM boot it. The best you can do is to make sure the signed SBL as a binary is received and flashed correctly using the previously described steps.

If you instead want to validate the SBL as it is, you may follow the below steps:

Receive the SBL image and flash it at the primary offset 0x0 of flash.
If the SBL image is corrupted or anything, the ROM will reject the image and fall backs to the redundant offset 0x400000 of flash.
The redundant offset should contain a valid SBL image which may be a OTA SBL to trigger the OTA update again.

0 Hong Zhang 5 months ago in reply to Prashant Shivhare

Intellectual 507 points

Hello Prashant,

Which is the SYSFW signing script that you referred in the below statement? Could you please help to provide the name, so that we are on the same page?

Sign the Y using the SYSFW signing script generating the signed binary Z (= X (SYSFW X.509 certificate) + Y).

Thanks,

Hong

0 Prashant Shivhare 5 months ago in reply to Hong Zhang

TI__Guru 50861 points

Hi Hong,

This is the script that is used to sign the application images (.appimage) in the MCU+ SDK. It is available at: <MCU+ SDK>/tools/boot/signing/appimage_x509_cert_gen.py.

More information available here:

https://software-dl.ti.com/mcu-plus-sdk/esd/AM243X/09_02_00_50/exports/docs/api_guide_am243x/TOOLS_BOOT.html#TOOLS_BOOT_SIGNING

Regards,

Prashant

0 Venkata Kishore Kajuluri 5 months ago in reply to Prashant Shivhare

Prodigy 50 points

HI, is the API Going to check only the signature? Does it try to validate the revision destination address etc? Does using the API mean we need to have same SVREV values for SBL and the appimage?

0 Prashant Shivhare 5 months ago in reply to Venkata Kishore Kajuluri

TI__Guru 50861 points

I am not sure at the moment if it checks the SWREV also. I will check with the team & get back to you.

0 Mukul Bhatnagar 4 months ago in reply to Prashant Shivhare

TI__Guru* 82035 points

Thank you for your patience , this has taken us long to follow up.
We finally got the confirmation from development team, and it was confirmed that we don’t check for SWREV value in the proc auth boot API

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AM2432: Validating secure boot SBL image with/without booting