10.3. Measured Boot using a Discrete TPM (PoC)

Measured Boot is the process of cryptographically measuring the code and critical data used at boot time, for example using a TPM, so that the security state can be attested later.

The current implementation of the driver included in TF-A supports several backends and each has a different means to store the measurements. This section focuses on the Discrete TPM backend, which stores measurements in a PCR within the TPM. This backend can be paired with the TCG event log to provide attestation of the measurements stored in the event log. See details in Measured Boot Design.

This section provides instructions to setup and build a proof of concept (PoC) that showcases the use of Measured Boot with a Discrete TPM interface.

Note

The instructions given in this document are meant to build a PoC to show how Measured Boot on TF-A can interact with a Discrete TPM interface. This PoC is platform specific, and uses a SPI based Discrete TPM, the Raspberry Pi communicates with the TPM via a GPIO pin bit-banged SPI interface. For other platforms, different may be required to interface with the hardware (e.g., different hardware communication protocols) and different TPM interfaces (e.g., FIFO vs CRB).

10.3.1. Components

Platform: The PoC is developed on the Raspberry Pi 3 (rpi3), due to quick driver development and the availability of GPIO pins to interface with a TPM expansion module. Measured boot capabilities using the TCG Event Log are ported to the Raspberry Pi 3 platform inside TF-A. This PoC specifically uses the Raspberry Pi 3 Model B V1.2, but this PoC is compatible with other Raspberry Pi 3 models.

Discrete TPM: The TPM chip selected is a breakout board compatible with the Raspberry Pi 3 GPIO pins. This PoC uses a SPI based LetsTrust TPM breakout board equipped with a Infineon Optiga™ SLB 9670 TPM 2.0 chip. Link to device: https://thepihut.com/products/letstrust-tpm-for-raspberry-pi

Note

If you have another TPM breakout board that uses the same Infineon Optiga™ SLB 9670 TPM 2.0 SPI based chip, it will also work. Ensure that the correct GPIO pins are utilized on the Raspberry Pi 3 to avoid communication issues, and possible hardware failures.

TF-A TPM Drivers: To interface with a physical (Discrete) TPM chip in TF-A, the PoC uses TF-A drivers that provide the command, interface, link, and platform layers required to send and receive data to and from the TPM. The drivers are located in TFA, and not in a SP, so that they may be used in early stages such as BL2, and in some cases, BL1. The design of the TPM Drivers is documented here: Discrete TPM drivers.

U-boot BL33: This PoC showcases measured boot up to BL33, and for simplicity uses a U-boot image for BL33, so that the image is measured and loaded. Currently U-boot does not have Discrete TPM support for the Raspberry Pi 3 platform so the boot flow ends here.

10.3.2. Building the PoC for the Raspberry Pi 3

Build instructions for U-Boot.bin for Raspberry Pi 3.

First, the build requires a BL33 firmware image that can be packaged and measured by TF-A.

U-boot can be built for the Raspberry Pi 3, but there are some changes to be made to allow the build to succeed. First Clone U-boot and enter the repo.

git clone https://github.com/u-boot/u-boot.git
cd u-boot

Now to switch to a specific tag v2024.04 for testing purposes, and then build the defconfig labelled rpi_3_b_plus_defconfig.

git checkout tags/v2024.04 -b tfa_dtpm_poc
make CROSS_COMPILE=aarch64-linux-gnu- rpi_3_b_plus_defconfig

Lastly open the .config and change CONFIG_TEXT_BASE and CONFIG_SYS_UBOOT_START to 0x11000000 to match the BL33 starting point.

vim .config
CONFIG_TEXT_BASE=0x11000000
CONFIG_SYS_UBOOT_START=0x11000000

To build the u-boot binary, use the following command.

make CROSS_COMPILE=aarch64-linux-gnu- -j$(nproc)

Build TF-A for Raspberry Pi 3 with Discrete TPM and Measured Boot.

Copy over the u-boot.bin file over to your TF-A working directory.

cp /path/to/u-boot/build/u-boot.bin /path/to/tfa/u-boot.bin

TF-A build command:

CROSS_COMPILE=aarch64-linux-gnu-      \
make PLAT=rpi3                        \
RPI3_PRELOADED_DTB_BASE=0x200000      \
BL33=u-boot.bin                       \
SUPPORT_VFP=1                         \
DEBUG=0                               \
MEASURED_BOOT=1                       \
DISCRETE_TPM=1                        \
MBOOT_TPM_HASH_ALG=sha256             \
TPM_INTERFACE=FIFO_SPI                \
MBEDTLS_DIR=/path/to/mbedtls/repo     \
LOG_LEVEL=40                          \
fip all

This build command is similar to the one provided in the TF-A Raspberry Pi 3 platform port, To learn more about the platform and its build options, visit Raspberry Pi 3.

RPI3_PRELOADED_DTB_BASE is given a different address to accommodate the larger BL1 and BL2 firmware sizes, this is to accommodate the TPM drivers that are packaged in BL1 and BL2 for this PoC.

BL33 is the non trusted firmware, in this case the U-Boot binary built earlier.

SUPPORT_VFP is enabled, allows Vector Floating Point operations in EL3.

MEASURED_BOOT is enabled to allow the Measured Boot flow.

DISCRETE_TPM=1 enables the build of Discrete TPM drivers.

MBOOT_TPM_HASH_ALG=sha256 sets the hash algorithm to sha256, this is the only algorithm supported by both TF-A Measured Boot and the SLB 9670 TPM 2.0.

TPM_INTERFACE=FIFO_SPI specifies the use of the FIFO SPI interface.

MBEDTLS_DIR is the path to your local mbedtls repo.

LOG_LEVEL=40 ensures that eventlog is printed at the end of BL1 and BL2.

Hardware Setup:

TPM Connection: Connect the LetsTrust TPM board to GPIO pins 17 - 26 on the 40-pin GPIO header on the Raspberry Pi board. The 2x5 header of the TPM module must be aligned to the pins in a specific orientation, match the 3v3 and RST pins from the TPM board to pins 17 and 18 respectively on the Raspberry Pi 3 header. See rpi3 pinout.

Serial Console: Establish a serial connection to the Raspberry Pi 3 to view serial output during the boot sequence. The GND, TXD, and RXD pins, which are labelled 6, 8, and 10 on the Raspberry Pi 3 header respectively, are the required pins to establish a serial connection. The recommended way to connect to the board from another system is to use a USB to serial TTL cable to output the serial console in a easy manner.

SD Card Setup: Format a SD Card as FAT32 with a default Raspbian installation that is similar to the default Raspberry Pi 3 boot partition, this partition will utilize the default files installed in the root directory with Rasbian such as:
bcm2710-rpi3-b.dtb
bootcode.bin
config.txt
fixup.dat
start.elf
Open config.txt and overwrite the file with the following lines:
arm_64bit=1
disable_commandline_tags=2
enable_uart=1
armstub=armstub8.bin
device_tree_address=0x200000
device_tree_end=0x210000
These configurations are required to enable uart, enable 64bit mode, use the build TF binary, and the modified rpi3 device tree address and size.

Copy armstub8.bin from the TF-A build path to the root folder of the SD card.

The SD Card is now ready to be booted.

10.3.3. Running the PoC for the Raspberry Pi 3

Insert the SD Card into the Raspberry Pi 3 SD card port and boot the system.

To access the serial console output from the Raspberry Pi 3 you can either:

Follow instructions to use PuTTY to connect to Raspberry Pi 3 serial console.
Use the linux screen command:
screen /dev/ttyUSB0 115200

Once booted the output from the serial console will look like this:

Raspberry Pi Bootcode

Read File: config.txt, 153

Read File: start.elf, 2975040 (bytes)

Read File: fixup.dat, 7265 (bytes)

MESS:00:00:01.170422:0: brfs: File read: /mfs/sd/config.txt
MESS:00:00:01.174630:0: brfs: File read: 153 bytes
MESS:00:00:01.211473:0: HDMI0:EDID error reading EDID block 0 attempt 0
MESS:00:00:01.217639:0: HDMI0:EDID error reading EDID block 0 attempt 1
MESS:00:00:01.223977:0: HDMI0:EDID error reading EDID block 0 attempt 2
MESS:00:00:01.230313:0: HDMI0:EDID error reading EDID block 0 attempt 3
MESS:00:00:01.236650:0: HDMI0:EDID error reading EDID block 0 attempt 4
MESS:00:00:01.242987:0: HDMI0:EDID error reading EDID block 0 attempt 5
MESS:00:00:01.249324:0: HDMI0:EDID error reading EDID block 0 attempt 6
MESS:00:00:01.255660:0: HDMI0:EDID error reading EDID block 0 attempt 7
MESS:00:00:01.261997:0: HDMI0:EDID error reading EDID block 0 attempt 8
MESS:00:00:01.268334:0: HDMI0:EDID error reading EDID block 0 attempt 9
MESS:00:00:01.274429:0: HDMI0:EDID giving up on reading EDID block 0
MESS:00:00:01.282647:0: brfs: File read: /mfs/sd/config.txt
MESS:00:00:01.286929:0: gpioman: gpioman_get_pin_num: pin LEDS_PWR_OK not defined
MESS:00:00:01.487295:0: gpioman: gpioman_get_pin_num: pin DISPLAY_DSI_PORT not defined
MESS:00:00:01.494853:0: gpioman: gpioman_get_pin_num: pin LEDS_PWR_OK not defined
MESS:00:00:01.500763:0: *** Restart logging
MESS:00:00:01.504638:0: brfs: File read: 153 bytes
MESS:00:00:01.510139:0: hdmi: HDMI0:EDID error reading EDID block 0 attempt 0
MESS:00:00:01.517254:0: hdmi: HDMI0:EDID error reading EDID block 0 attempt 1
MESS:00:00:01.524112:0: hdmi: HDMI0:EDID error reading EDID block 0 attempt 2
MESS:00:00:01.530970:0: hdmi: HDMI0:EDID error reading EDID block 0 attempt 3
MESS:00:00:01.537826:0: hdmi: HDMI0:EDID error reading EDID block 0 attempt 4
MESS:00:00:01.544685:0: hdmi: HDMI0:EDID error reading EDID block 0 attempt 5
MESS:00:00:01.551543:0: hdmi: HDMI0:EDID error reading EDID block 0 attempt 6
MESS:00:00:01.558399:0: hdmi: HDMI0:EDID error reading EDID block 0 attempt 7
MESS:00:00:01.565258:0: hdmi: HDMI0:EDID error reading EDID block 0 attempt 8
MESS:00:00:01.572116:0: hdmi: HDMI0:EDID error reading EDID block 0 attempt 9
MESS:00:00:01.578730:0: hdmi: HDMI0:EDID giving up on reading EDID block 0
MESS:00:00:01.584634:0: hdmi: HDMI0:EDID error reading EDID block 0 attempt 0
MESS:00:00:01.592427:0: hdmi: HDMI0:EDID error reading EDID block 0 attempt 1
MESS:00:00:01.599286:0: hdmi: HDMI0:EDID error reading EDID block 0 attempt 2
MESS:00:00:01.606142:0: hdmi: HDMI0:EDID error reading EDID block 0 attempt 3
MESS:00:00:01.613001:0: hdmi: HDMI0:EDID error reading EDID block 0 attempt 4
MESS:00:00:01.619858:0: hdmi: HDMI0:EDID error reading EDID block 0 attempt 5
MESS:00:00:01.626717:0: hdmi: HDMI0:EDID error reading EDID block 0 attempt 6
MESS:00:00:01.633575:0: hdmi: HDMI0:EDID error reading EDID block 0 attempt 7
MESS:00:00:01.640431:0: hdmi: HDMI0:EDID error reading EDID block 0 attempt 8
MESS:00:00:01.647288:0: hdmi: HDMI0:EDID error reading EDID block 0 attempt 9
MESS:00:00:01.653905:0: hdmi: HDMI0:EDID giving up on reading EDID block 0
MESS:00:00:01.659769:0: hdmi: HDMI:hdmi_get_state is deprecated, use hdmi_get_display_state instead
MESS:00:00:01.668264:0: HDMI0: hdmi_pixel_encoding: 162000000
MESS:00:00:01.673988:0: vec: vec_middleware_power_on: vec_base: 0x7e806000 rev-id 0x00002708 @ vec: 0x7e806100 @ 0x00000420 enc: 0x7e806060 @ 0x00000220 cgmsae: 0x7e80605c @ 0x00000000
MESS:00:00:01.880234:0: dtb_file 'bcm2710-rpi-3-b.dtb'
MESS:00:00:01.889713:0: brfs: File read: /mfs/sd/bcm2710-rpi-3-b.dtb
MESS:00:00:01.894375:0: Loaded 'bcm2710-rpi-3-b.dtb' to 0x200000 size 0x7cb2
MESS:00:00:01.915761:0: brfs: File read: 31922 bytes
MESS:00:00:02.007202:0: brfs: File read: /mfs/sd/config.txt
MESS:00:00:02.017277:0: brfs: File read: 153 bytes
MESS:00:00:02.020772:0: Failed to open command line file 'cmdline.txt'
MESS:00:00:02.042302:0: gpioman: gpioman_get_pin_num: pin EMMC_ENABLE not defined
MESS:00:00:02.398066:0: kernel=
MESS:00:00:02.455255:0: brfs: File read: /mfs/sd/armstub8.bin
MESS:00:00:02.459284:0: Loaded 'armstub8.bin' to 0x0 size 0xdbe74
MESS:00:00:02.465109:0: No compatible kernel found
MESS:00:00:02.469610:0: Device tree loaded to 0x200000 (size 0x823f)
MESS:00:00:02.476805:0: uart: Set PL011 baud rate to 103448.300000 Hz
MESS:00:00:02.483381:0: uart: Baud rate change done...
MESS:00:00:02.486793:0: uart: Baud rateNOTICE:  Booting Trusted Firmware
NOTICE:  BL1: v2.11.0(release):v2.11.0-187-g0cb1ddc9c-dirty
NOTICE:  BL1: Built : 10:57:10, Jul  9 2024
INFO:    BL1: RAM 0x100ee000 - 0x100f9000
INFO:    Using crypto library 'mbed TLS'
NOTICE:  TPM Chip: vendor-id 0xd1, device-id 0x0, revision-id: 0x16
NOTICE:  rpi3: Detected: Raspberry Pi 3 Model B (1GB, Sony, UK) [0x00a02082]
INFO:    BL1: Loading BL2
INFO:    Loading image id=1 at address 0x100b4000
INFO:    Image id=1 loaded: 0x100b4000 - 0x100c0281
INFO:    TCG_EfiSpecIDEvent:
INFO:      PCRIndex           : 0
INFO:      EventType          : 3
INFO:      Digest             : 00
INFO:          : 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
INFO:          : 00 00 00
INFO:      EventSize          : 33
INFO:      Signature          : Spec ID Event03
INFO:      PlatformClass      : 0
INFO:      SpecVersion        : 2.0.2
INFO:      UintnSize          : 1
INFO:      NumberOfAlgorithms : 1
INFO:      DigestSizes        :
INFO:        #0 AlgorithmId   : SHA256
INFO:           DigestSize    : 32
INFO:      VendorInfoSize     : 0
INFO:    PCR_Event2:
INFO:      PCRIndex           : 0
INFO:      EventType          : 3
INFO:      Digests Count      : 1
INFO:        #0 AlgorithmId   : SHA256
INFO:           Digest        : 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
INFO:          : 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
INFO:      EventSize          : 17
INFO:      Signature          : StartupLocality
INFO:      StartupLocality    : 0
INFO:    PCR_Event2:
INFO:      PCRIndex           : 0
INFO:      EventType          : 1
INFO:      Digests Count      : 1
INFO:        #0 AlgorithmId   : SHA256
INFO:           Digest        : 55 11 51 d8 8b 7f 41 d3 18 16 f2 e8 80 bf 80 fa
INFO:          : b4 03 6d 96 4c a0 0a 98 45 cf 25 2f 1e a9 09 3e
INFO:      EventSize          : 5
INFO:      Event              : BL_2
NOTICE:  BL1: Booting BL2
INFO:    Entry point address = 0x100b4000
INFO:    SPSR = 0x3c5
NOTICE:  BL2: v2.11.0(release):v2.11.0-187-g0cb1ddc9c-dirty
NOTICE:  BL2: Built : 10:56:39, Jul  9 2024
INFO:    Using crypto library 'mbed TLS'
NOTICE:  TPM Chip: vendor-id 0xd1, device-id 0x0, revision-id: 0x16
INFO:    BL2: Doing platform setup
INFO:    BL2: Loading image id 3
INFO:    Loading image id=3 at address 0x100e0000
INFO:    Image id=3 loaded: 0x100e0000 - 0x100e706b
INFO:    BL2: Loading image id 5
INFO:    Loading image id=5 at address 0x11000000
INFO:    Image id=5 loaded: 0x11000000 - 0x110a8ad8
INFO:    TCG_EfiSpecIDEvent:
INFO:      PCRIndex           : 0
INFO:      EventType          : 3
INFO:      Digest             : 00
INFO:          : 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
INFO:          : 00 00 00
INFO:      EventSize          : 33
INFO:      Signature          : Spec ID Event03
INFO:      PlatformClass      : 0
INFO:      SpecVersion        : 2.0.2
INFO:      UintnSize          : 1
INFO:      NumberOfAlgorithms : 1
INFO:      DigestSizes        :
INFO:        #0 AlgorithmId   : SHA256
INFO:           DigestSize    : 32
INFO:      VendorInfoSize     : 0
INFO:    PCR_Event2:
INFO:      PCRIndex           : 0
INFO:      EventType          : 3
INFO:      Digests Count      : 1
INFO:        #0 AlgorithmId   : SHA256
INFO:           Digest        : 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
INFO:          : 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
INFO:      EventSize          : 17
INFO:      Signature          : StartupLocality
INFO:      StartupLocality    : 0
INFO:    PCR_Event2:
INFO:      PCRIndex           : 0
INFO:      EventType          : 1
INFO:      Digests Count      : 1
INFO:        #0 AlgorithmId   : SHA256
INFO:           Digest        : 55 11 51 d8 8b 7f 41 d3 18 16 f2 e8 80 bf 80 fa
INFO:          : b4 03 6d 96 4c a0 0a 98 45 cf 25 2f 1e a9 09 3e
INFO:      EventSize          : 5
INFO:      Event              : BL_2
INFO:    PCR_Event2:
INFO:      PCRIndex           : 0
INFO:      EventType          : 1
INFO:      Digests Count      : 1
INFO:        #0 AlgorithmId   : SHA256
INFO:           Digest        : f3 00 5c ed a2 12 8b 76 b7 82 da c5 28 c3 02 52
INFO:          : 19 e4 3a 82 f2 3c ab 1e 0d 78 84 9c b5 fe e2 4f
INFO:      EventSize          : 14
INFO:      Event              : SECURE_RT_EL3
INFO:    PCR_Event2:
INFO:      PCRIndex           : 0
INFO:      EventType          : 1
INFO:      Digests Count      : 1
INFO:        #0 AlgorithmId   : SHA256
INFO:           Digest        : 90 28 81 42 12 b7 9b ca aa 0c 40 76 33 5a 69 71
INFO:          : b6 19 2b 90 f2 d2 69 b8 de 8e 6d 05 4d c2 73 f9
INFO:      EventSize          : 6
INFO:      Event              : BL_33
NOTICE:  BL1: Booting BL31
INFO:    Entry point address = 0x100e0000
INFO:    SPSR = 0x3cd
NOTICE:  BL31: v2.11.0(release):v2.11.0-187-g0cb1ddc9c-dirty
NOTICE:  BL31: Built : 10:56:58, Jul  9 2024
INFO:    rpi3: Checking DTB...
INFO:    rpi3: Reserved 0x10000000 - 0x10100000 in DTB
INFO:    BL31: Initializing runtime services
INFO:    BL31: Preparing for EL3 exit to normal world
INFO:    Entry point address = 0x11000000
INFO:    SPSR = 0x3c9


U-Boot 2024.04-g84314330-dirty (Apr 23 2024 - 15:41:54 -0500)

DRAM:  948 MiB
RPI 3 Model B (0xa02082)
Core:  68 devices, 14 uclasses, devicetree: embed
MMC:   mmc@7e202000: 0, mmc@7e300000: 1
Loading Environment from FAT... OK
In:    serial,usbkbd
Out:   serial,vidconsole
Err:   serial,vidconsole
Net:   No ethernet found.
starting USB...
Bus usb@7e980000: USB DWC2
scanning bus usb@7e980000 for devices...
Error: smsc95xx_eth No valid MAC address found.
2 USB Device(s) found
      scanning usb for storage devices... 0 Storage Device(s) found
Hit any key to stop autoboot:  2  1  0
Card did not respond to voltage select! : -110
No EFI system partition
No EFI system partition
Failed to persist EFI variables
No EFI system partition
Failed to persist EFI variables
No EFI system partition
Failed to persist EFI variables
Missing TPMv2 device for EFI_TCG_PROTOCOL
** Booting bootflow '<NULL>' with efi_mgr
Loading Boot0000 'mmc 0' failed
EFI boot manager: Cannot load any image
Boot failed (err=-14)
Card did not respond to voltage select! : -110
No ethernet found.
No ethernet found.
U-Boot>

10.3.4. Next steps for Discrete TPM and Measured Boot development

In order to automatically validate the workings of the Discrete TPM, the creation of test cases that compare the eventlog image hashes with what is stored in PCR0 are a great way to test the core functionality of the Discrete TPM in Measured Boot.

Development of Discrete TPM drivers such as a reference FIFO I2C, MMIO, and CRB drivers has not started, these drivers will allow a larger number of platform to use a Discrete TPM in TF-A.