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)
?
