Does Hardware TPM have TAB/RM component？

Question

Firstly I am not sure if this is the right place to ask this. If anyone has any idea of where this question belong please tell me.

Recently I was reading about TPM and TSS(TPM Software Stack). The diagram in this document -- TSS TAB and Resource Manager Specification shows the structure of the stack. In my understanding, the TCTI and stacks above is in the library to communicate with TPM; where TAB and stacks below TAB are in the real TPM or virtual TPM device. But I am not sure if my understanding is correct.

My question is: Is there a TAB/RM component (or component with similar responsibility) in a real TPM device? Or does tbs/some modules in OS take this resposibility?

Thank you!

Answer 1

No, the TAB/RM is a software component.

The TPM is a really constraint device with little memory (e.g. few key slots). To solve that problem, there is a TPM Access Broker/Resource Manager (TAB/RM) which automatically saves excess TPM data (encrypted) on your hard disk and loads it into the TPM if needed.

When you're on a linux machine, you might have noticed that there is a /dev/tpm0 and a /dev/tpmrm0. The tpm0 provides direct access while the tpmrm0 uses the TAB/RM (which is part of the linux kernel) to simplify your life. The interface is the same, btw, that is talking to the TAB/RM is like talking to a TPM with infinite resources :)

The TCTI, SAPI, ESAPI and FAPI layers are indeed libraries. They are all provided by this open source implementation.

For completeness sake: there is not only a in-kernel TAB/RM, but also a user space TAB/RM implementation doing the same thing. You can install it e.g. if your kernel is really old.