Hi Raghu,
Thanks a lot for the review comments and feedback!
On 2/25/20 3:45 AM, Raghu Krishnamurthy via TF-A wrote:
The patch stack looks great! I do have a suggestion for the long term evolution/future work on root's of trust and cert_create. It would be great to generalize "dual-root" to "multi-root". It is conceivable that firmware supporting secure partitions/SPCI etc, could move to having multiple root's of trust. Here we have the silicon provider, and the platform owner as two roots, but a more complex system could contain silicon firmware, platform firmware and multiple secure partitions, each signed by different entities. To remove the signing dependencies between each of these entities, we could have a ROTPK for each of these entities and the same solution used here, can be applied to solve multiple root's of trust and is summarized very well by your statement -"As long as there is a defined contract between BL2 and the (P)(*)ROTK-rooted images as to how/where to securely get this key or hash, there should not be any need for the two vendors to do any cross-signing."
To do this, If possible, we should start moving away from tables such as
the ones in cert_create/src/dualroot/cot.c and have platforms provide the certificate dependencies and keys used to sign them in a config file. This achieves two things: it makes cert_create independent of the cert chain a platform wants to use and the number of roots of trust, and, makes cert_create more usable by reducing the number of command line arguments to be provided, which is a long list today. You could potentially use the config file provided to auto generate the tbbr_cot.c file being linked into the firmware too.
Let me know what you think.
I agree with everything you said above. Indeed, there's no reason to stop at 2 roots of trust and as you pointed out there are real use cases to enable more. The dualroot chain of trust is only the first step in that direction and is a useful way to experiment with extending the TBBR implementation and chain of trust, while addressing a real use case.
Also completely agree with the lack of flexibility of the hard-coded chain of trust in cert_create/src/dualroot/cot.c and drivers/auth/dualroot/cot.c for that matter. In fact, we (at Arm) are thinking along the same lines as you and have had similar ideas boiling for some time. We are making gradual changes to introduce more flexibility into TF-A, not just for the chain of trust, but for any platform-specific data.
Maybe you've seen the recent FConf framework patches, which is a key piece into enabling platform layers to move platform-specific data into configuration files. There is still work to do but down the line we are already thinking about moving the chain of trust description into such a configuration file.
Moving the CoT into a configuration file has many advantages:
- It could constitute the single input source for the chain of trust, serving both the cert_create tool and the firmware. Today, the CoT is described and duplicated in both places and there is really no good reason to keep things like that IMO. As you said, we could auto-generate the tbbr_cot.c file and build into the firmware, or even have the firmware dynamically parse the configuration file at runtime and extract its CoT.
- It could simplify the description of the CoT. Today, I think that the C data structures in tbbr_cot.c are quite complex and not straight-forward to understand at first. If we described them using some configuration language, I believe we could abstract some of these details away or at least organize them in a more intuitive way.
Regards, Sandrine
Thanks Sandrine! Agree with what you have said below and good to know you're thinking along similar lines. Patch stack looks great otherwise!
-Raghu
On 2/25/20 12:11 AM, Sandrine Bailleux wrote:
Hi Raghu,
Thanks a lot for the review comments and feedback!
On 2/25/20 3:45 AM, Raghu Krishnamurthy via TF-A wrote:
The patch stack looks great! I do have a suggestion for the long term evolution/future work on root's of trust and cert_create. It would be great to generalize "dual-root" to "multi-root". It is conceivable that firmware supporting secure partitions/SPCI etc, could move to having multiple root's of trust. Here we have the silicon provider, and the platform owner as two roots, but a more complex system could contain silicon firmware, platform firmware and multiple secure partitions, each signed by different entities. To remove the signing dependencies between each of these entities, we could have a ROTPK for each of these entities and the same solution used here, can be applied to solve multiple root's of trust and is summarized very well by your statement -"As long as there is a defined contract between BL2 and the (P)(*)ROTK-rooted images as to how/where to securely get this key or hash, there should not be any need for the two vendors to do any cross-signing."
To do this, If possible, we should start moving away from tables such as
the ones in cert_create/src/dualroot/cot.c and have platforms provide the certificate dependencies and keys used to sign them in a config file. This achieves two things: it makes cert_create independent of the cert chain a platform wants to use and the number of roots of trust, and, makes cert_create more usable by reducing the number of command line arguments to be provided, which is a long list today. You could potentially use the config file provided to auto generate the tbbr_cot.c file being linked into the firmware too.
Let me know what you think.
I agree with everything you said above. Indeed, there's no reason to stop at 2 roots of trust and as you pointed out there are real use cases to enable more. The dualroot chain of trust is only the first step in that direction and is a useful way to experiment with extending the TBBR implementation and chain of trust, while addressing a real use case.
Also completely agree with the lack of flexibility of the hard-coded chain of trust in cert_create/src/dualroot/cot.c and drivers/auth/dualroot/cot.c for that matter. In fact, we (at Arm) are thinking along the same lines as you and have had similar ideas boiling for some time. We are making gradual changes to introduce more flexibility into TF-A, not just for the chain of trust, but for any platform-specific data.
Maybe you've seen the recent FConf framework patches, which is a key piece into enabling platform layers to move platform-specific data into configuration files. There is still work to do but down the line we are already thinking about moving the chain of trust description into such a configuration file.
Moving the CoT into a configuration file has many advantages:
- It could constitute the single input source for the chain of trust,
serving both the cert_create tool and the firmware. Today, the CoT is described and duplicated in both places and there is really no good reason to keep things like that IMO. As you said, we could auto-generate the tbbr_cot.c file and build into the firmware, or even have the firmware dynamically parse the configuration file at runtime and extract its CoT.
- It could simplify the description of the CoT. Today, I think that the
C data structures in tbbr_cot.c are quite complex and not straight-forward to understand at first. If we described them using some configuration language, I believe we could abstract some of these details away or at least organize them in a more intuitive way.
Regards, Sandrine
tf-a@lists.trustedfirmware.org
-
Raghu Krishnamurthy -
Sandrine Bailleux