[PATCH v1 1/2] nvmem: bsec: Add support for OTP permanent write lock

[Sascha Hauer]

On Thu, Jan 30, 2025 at 01:08:13PM +0100, Oleksij Rempel wrote:
> From: Robin van der Gracht <robin at protonic.nl>
> 
> Introduce a mechanism to permanently lock OTP eFuses after programming by
> adding a new `writelock` parameter. When `writelock` is enabled, the
> driver:
> 
> - Programs the OTP fuse using `BSEC_SMC_PROG_OTP`.
> - If successful, triggers `BSEC_SMC_WRLOCK_OTP` (OP-TEE:
>   `STM32_SIP_SVC_BSEC_WRLOCK_OTP`) to permanently disable further
>   modifications to the OTP word.
> 
> Security Concern:
> Without this lock mechanism, an OTP word can still be altered by OR-ing
> additional bits onto the existing value, as STM32 BSEC OTP fuses only
> allow one-way bit transitions from 0 to 1. This is a potential security
> risk when dealing with keys or sensitive configuration values, as an
> attacker could modify certain OTP bits without fully replacing the
> original value.
> 
> Warning! Write lock is enabled globally per BSEC device:
> - While `writelock=1`, all writes via the BSEC device will be
>   permanently locked.
> - The user must avoid writing unintended values during this period,
>   as they will become irrevocable.
> 
> Example Use Case:
> To program and permanently lock an OTP word:
> bsec0.permanent_write_enable=1
> bsec0.writelock=1
> mw -l -d /dev/stm32-bsec 0x00000170+4 $some_data
> bsec0.permanent_write_enable=0
> bsec0.writelock=0

I don't really like this writelock approach. It makes it hard to write
something to OTP without locking and then lock it later. This can only
be done by writing the same data again, with writelock enabled this
time.

We have support for a protect operation, originally used for flashes.
This looks like a good match for this purpose. Could we use it here?

Sascha

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