STM32H7Sx - Meaning of FLASH_OBKCR.NEXTKL within HDPL_0

[Eliasvan]

What is the meaning of the FLASH_OBKCR.NEXTKL field while executing within HDPL_0 (e.g., during execution of ST RSS)?

This question might seem irrelevant to me as I'm a user and user code will never execute within HDPL_0, but it might be important from a threat modeling point of view: does ST (by means of its RSS code) have access to HDPL_x user-provisioned AHKs (Application Hardware Keys) where x >= 1?

The reference manual states the following:

5.9.18 FLASH option byte key control register (FLASH_OBKCR)
"NEXTKL[1:0]: Next key level
00: OBKINDEX represents the index of the option byte key stored for the hide protection level
indicated in SBS_HDPLSR.
01: OBKINDEX represents the index of the option byte key stored for the hide protection level
indicated in SBS_HDPLSR plus one (e.g. if HDPL=1 in SBS_HDPLR the key of level 2 is
selected).
10 or 11: reserved"

Does this mean ST RSS (HDPL_0) can access:

• HDPL_0 AHKs (when NEXTKL=00)
• HDPL_1 AHKs (when NEXTKL=01)

but not:

• HDPL_2 AHKs (no applicable NEXTKL value, candidate NEXTKL=10 is reserved)

?

[KDJEM.1]

Hello @Eliasvan ;

I will check this internally and I will come back to you as soon as possible.

Internal ticket number: 219312 (This is an internal tracking number and is not accessible or usable by customers) 

Thank you.

Kaouthar