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tpm.md

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TPM for IBM POWER9 - feasibility study

This document is an attempt to gather information about the issues faced with TPMs on POWER9 and what are the potential solutions to them.

Mind that thanks to POWER9 being more secure than any x86 by design, it is more important to have TPM support than to have a particularly secure or feature-full one. See platform comparison for more details.

Some definitions

  • TPM - Trusted Platform Module
  • dTPM - Discreet TPM module (a dedicated pluggable hardware module)
  • fTPM - Firmware-based TPM implementation
  • LPC - Low Pin Count (hardware interface)
  • I2C - Inter-Integrated Circuit (hardware interface)
  • SBE - Self-Boot Engine (part of POWER9 CPU and/or its firmware)
  • MCU - MicroController Unit (like a board connected via USB)
  • FPGA - Field-Programmable Gate Array (programmable integrated circuit)
  • BMC - Baseboard Management Controller ("service processor")

Problems

Talos II cannot generate LPC TPM cycles

A short overview of relationships between LPC and TPM on LPC can be found in this TPM publication:

The format of these TPM cycles is virtually identical to I/O LPC cycles
discussed earlier. The only visible difference between TPM and I/O cycles
is the value sent during the START phase of LPC transactions. Existing I/O
cycles are “target” cycles in LPC bus terminology and use 0x0 as their
4-bit START value. The new TPM cycles use the previously reserved value 0x5
as the start value.

To repeat: TPM cycles are implemented as I/O LPC cycles but use a different START nibble (5 instead of 0). The value of 5 is reserved in LPC specification and is claimed by TPM specification (Table 43). The same TPM specification mentions in an informative comment that:

For system Software, the TPM has a 64-bit address of 0x0000_0000_FED4_xxxx.
On LPC, the chipset passes the least significant 16 bits to the TPM.

Which allows generation of LPC cycles with arbitrary START nibble.

The problem is that LPC accesses on Talos II don't have this property and fix START nibble at 0, thus precluding a direct LPC TPM connection. See lpcdd.C in Hostboot, LPC cycles are abstracted away via more high-level memory-mapped I/O, where clients don't write START field themselves and have no control over its value. Hostboot hard-codes base addresses for different kinds of interactions (see examples in lpc.c, lpc_const.H and sbeConsole.H), which demonstrates lack of any pattern in a way that they are assigned, lack of a way to customize START field and absence of an address space dedicated to TPM.

Quick overview

Aspect\Approach Direct I2C BMC fTPM LPC adapter SPI adapter MCU dTPM LibreBMC SBE
TPM kind dTPM fTPM dTPM dTPM dTPM dTPM fTPM
Placement MCU BMC MCU MCU MCU BMC CPU
Fully open No Yes No No Yes Yes Yes
HW protected Yes No Yes Yes Yes Yes Yes
In stock Hardly (-1) - Yes (+1) Yes (+1) Hardly (-1) - -
Design HW Maybe (+/-1) No (+1) Yes (-1) Yes (-1) Partial (-.5) Partial (-.5) No (+1)
Manufacture HW Maybe (+/-1) No (+1) Yes (-1) Yes (-1) Yes (-1) Yes (-1) No (+1)
Needs OS driver No (+1) Yes (-1) Yes (-1) No (+1) Maybe (+/-1) Yes (-1) Yes (-1)
Score +2 / -2 +1 -2 0 -1.5 / -3.5 -2.5 +1

Score counting

Some rows in the table are informational and no score is assigned to them, the rest get points according to the following set of rules:

  • if aspect isn't applicable, then 0
  • if aspect is satisfied, then +1 or +0.5 if partially true
  • if aspect is lacking, then -1 or -0.5 if partially true
  • if aspects is marked as inverted, negate the value
Aspect Scored Description
TPM kind No Whether it's fTPM or dTPM
Placement No Where TPM functionality is located physically
Fully open No Whether involved hardware and software are fully open
HW protected No Protection against tampering is by design
In stock Yes If modules compatible with the approach are easy to find
Design HW Inverted Designing hardware is necessary
Manufacture HW Inverted Producing hardware is necessary
Needs OS driver Inverted Producing OS driver is necessary

Approaches

I2C TPM module

Hardware module with an uncommon interface (TPMs for PC mostly use LPC or SPI). Known chips of this kind are/were produced by Nuvoton, ST, Infineon, Atmel (AT97SC3205T).

  • Pros

    • Talos II has I2C header exposed on the board.
    • Best option if can be bought.

  • Cons

    • No datasheets for Nuvoton chips.
    • Limited or no availability of ready to use modules.
    • Might require design and production of new TPM module with I2C interface.

  • Risks

    • Hard to find and this might not improve in the future.

  • Implementation effort

  • Other notes

    • ST33 has Linux driver for TPM 2.0
    • Bought modules won't have OSS firmware.

Software TPM on BMC

TPM functionality exposed to the POWER9 CPU via LPC gadget device from BMC. That is OpenBMC is extended to provide fTPM though LPC bus, but do this with START field equal to 0. As secure as BMC is.

  • Pros

    • No additional hardware needed.
    • Open implementation.

  • Cons

    • Need to secure the BMC and manage that properly.

  • Risks

    • BMC might not have been designed to fulfill security requirements.
    • Still won't address LPC cycles issue?

  • Implementation effort

    • Implement fTPM in BMC.
    • LPC gadget BMC -> POWER.
    • No existing driver. Maybe need to hack up existing LPC driver to change the way LPC is accessed and translate START value.

  • Other notes

    • We could go full RO with coreboot if desired (by sacrificing some performance for MVPD).

IO LPC translator (IO TPM cycle - LPC TPM cycle)

New piece of hardware with FPGA that addresses LPC cycles issues.

Although might not actually need FPGA:

LAD[0]' = LAD[0] + ~LFRAME#
LAD[2]' = LAD[2] + ~LFRAME#

  • Pros

    • Conventional TPM modules.

  • Cons

    • Need to design and manufacture this hardware.

  • Risks

  • Implementation effort

    • Slightly easier than the fTPM on BMC - but not much.

  • Other notes

FPGA translator (SPI - LPC TPM cycle)

New piece of hardware that addresses LPC cycles issues by creating an SPI - LPC bridge, where LPC side is connected to the board and SPI TPM module is used.

  • Pros

    • Conventional TPM modules.
    • No need for a new driver.

  • Cons

    • Need to design and manufacture this hardware.

  • Risks

  • Implementation effort

    • Slightly easier than the fTPM on BMC - but not much.

  • Other notes

Open TPM on MCU (e.g. lpnTPM)

Find an existing open hardware TPM project based on an MCU and possibly extend it for I2C interface.

  • Pros

    • Open hardware.

  • Cons

    • Need to implement and manufacture TPM module.

  • Risks

    • I2C is not in the scope of lpnTPM as of now.

  • Implementation effort

    • Slightly easier than the fTPM on BMC - but not much.

  • Other notes

TPM on LibreBMC

Embed TPM into an alternative BMC hardware that's still in development. The BMC itself is implemented as a custom extension card defined by DC-SCM which needs a compatible motherboard.

Overall this doesn't look feasible. Kestrel on the other hand can be used with Talos II as is, but also lacks TPM and ASpeed BMC needs to be disabled after power on so it doesn't interfere. There might also be compatibility issues with coreboot port and this project is too in development. Still looks more realistic than LibreBMC.

  • Pros

    • Open hardware.

  • Cons

    • Project is in an early phase of development.
    • Can't be used with already existing boards.

  • Risks

    • Current design does not have TPM connector (because it's builtin?).

  • Implementation effort

  • Other notes

    • Maybe we can implement fTPM in FPGA BMC.

fTPM in SBE

Implement TPM on a chip that's used primarily for booting POWER9 processor.

  • Pros

    • SBE chip is an independent piece of hardware that's part of secure boot sequence for POWER9.

  • Cons

  • Risks

    • SBE SEEPROM may not have enough free space to include fTPM implementation.

  • Implementation effort

    • Will need a driver for the OS. If there is one for SBE communication from IBM, it still needs an update to support custom SBE firmware and extensions made to its protocol.

  • Other notes

    • We have not dealt with SBE much so far, but coreboot uses it less than Hostboot does, so we might be able to remove unused parts of sources.
    • Looks like sbe_seeprom_DD2 is compressed from 86 to 65 KiB, but compressed sbe_seeprom_AXONE is 150 KiB.
    • There are 96 KiB of RAM and 256 KiB of SEEPROM (227 KiB effectively due to ECC).
    • Stripped uncompressed x86-64 libtpm.a from MS compiled with -Os -g0 gives 257 KiB as a sum of text and data columns printed by size. There might be a chance it will fit when compressed. It's a bit larger for x86.
    • But there is also TPM NV memory that needs to be stored somewhere...

See also