Hi,
Thanks for the feedback, I will create a PR then.
Thomas
________________________________ From: Jerome Forissier Sent: Tuesday, April 08, 2025 14:32 To: Stauffer Thomas MTANA; op-tee@lists.trustedfirmware.org Subject: Re: EXTERNAL - Extract RPMB Key
Hi,
On 4/8/25 13:35, Stauffer Thomas MTANA wrote:
As far as I understand there are currently two possibilities to setup an RPMB with an appropriate key.
- Use a specialized OP-TEE with CFG_RPMB_WRITE_KEY=y during production in an environment were leaking the key is ok.
- Get the HUK, get the CID from the MMC, then use scripts/derive_rpmb_key.py to calculate the RPMB key and set e.g. via mmc tool.
I've the impression when I read through some message on https://github.com/OP-TEE/optee_os/issues, that 1 is not considered the best idea.
Correct. CFG_RPMB_WRITE_KEY was implemented as an quick and easy solution for a development environment, especially for CI with QEMU. Then scripts/derive_rpmb_key.py was introduced as a potentially better option for production, but I guess not all production environments are the same so it may have its shortcomings.
Currently I'm working on a iMX8M platform and using CFG_RPMB_WRITE_KEY also has the drawback to use the legacy RPMB interface, which makes tee-supplicant still necessary, which otherwise would not be needed.
Indeed. The ability to write the key was left out on purpose.
The problem at least on iMX8M is that its not easy possible to get the HUK, and the HUK also is different, depending if a device is in a closed state or not. To extract the whole HUK, even in a sensible environment during production, seems unnecessary.
What I did now, something very simple. I extended tee_rpmb_fs.c a little bit and made a function which allows to retrieve the RPMB key, I added also a tiny PTA, and a corresponding user space application to get the RPMB key, only as long CFG_INSECURE is set, therefore also only a specialized OP-TEE build can provide this functionality.
Someone can now decide to get the RPMB key either before the device is closed (for testing devices), or afterwards. Write then the RPMB key manually (e.g. via mmc tool) and/or also store it securly in a production data database.
What would be the advantages, compared to CFG_RPMB_WRITE_KEY=y
- HUK is still not leaked, as it may be used to derive other secrets
- derive_rpmb_key.py is not necessary
- It is guaranteed that the correct RPMB key is retrieved, 100% matches how OP-TEE was configured (e.g. with compat mode enabled or not)
- Because the RPMB key is known, one can manually still delete the RPMB and see how OP-TEE handles this situation, at least during development
- Testing this functionality also on an open device (which is currently not possible at least on iMX8M, due to plat_rpmb_key_is_ready)
My question is, if there would be any interest to open a PR, so other people also can use this?
I would encourage you to do so, especially since you have the code already. People using i.MX platforms will likely be interested and provide useful feedback -- I mean better than mine :)
Thanks, -- Jerome