diff options
Diffstat (limited to 'docs/design_documents/measured_boot_poc.rst')
| -rw-r--r-- | docs/design_documents/measured_boot_poc.rst | 507 |
1 files changed, 507 insertions, 0 deletions
diff --git a/docs/design_documents/measured_boot_poc.rst b/docs/design_documents/measured_boot_poc.rst new file mode 100644 index 000000000..3ae539b5b --- /dev/null +++ b/docs/design_documents/measured_boot_poc.rst @@ -0,0 +1,507 @@ +Interaction between Measured Boot and an fTPM (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 Trusted Firmware-A +(TF-A) stores the measurements into a `TGC event log`_ in secure +memory. No other means of recording measurements (such as a discrete TPM) is +supported right now. + +The driver also provides mechanisms to pass the Event Log to normal world if +needed. + +This manual provides instructions to build a proof of concept (PoC) with the +sole intention of showing how Measured Boot can be used in conjunction with +a firmware TPM (fTPM) service implemented on top of OP-TEE. + +.. note:: + The instructions given in this document are meant to be used to build + a PoC to show how Measured Boot on TF-A can interact with a third + party (f)TPM service and they try to be as general as possible. Different + platforms might have different needs and configurations (e.g. different + SHA algorithms) and they might also use different types of TPM services + (or even a different type of service to provide the attestation) + and therefore the instuctions given here might not apply in such scenarios. + +Components +~~~~~~~~~~ + +The PoC is built on top of the `OP-TEE Toolkit`_, which has support to build +TF-A with support for Measured Boot enabled (and run it on a Foundation Model) +since commit cf56848. + +The aforementioned toolkit builds a set of images that contain all the components +needed to test that the Event Log was properly created. One of these images will +contain a third party fTPM service which in turn will be used to process the +Event Log. + +The reason to choose OP-TEE Toolkit to build our PoC around it is mostly +for convenience. As the fTPM service used is an OP-TEE TA, it was easy to add +build support for it to the toolkit and then build the PoC around it. + +The most relevant components installed in the image that are closely related to +Measured Boot/fTPM functionality are: + + - **OP-TEE**: As stated earlier, the fTPM service used in this PoC is built as an + OP-TEE TA and therefore we need to include the OP-TEE OS image. + Support to interfacing with Measured Boot was added to version 3.9.0 of + OP-TEE by implementing the ``PTA_SYSTEM_GET_TPM_EVENT_LOG`` syscall, which + allows the former to pass a copy of the Event Log to any TA requesting it. + OP-TEE knows the location of the Event Log by reading the DTB bindings + received from TF-A. Visit :ref:`DTB binding for Event Log properties` + for more details on this. + + - **fTPM Service**: We use a third party fTPM service in order to validate + the Measured Boot functionality. The chosen fTPM service is a sample + implementation for Aarch32 architecture included on the `ms-tpm-20-ref`_ + reference implementation from Microsoft. The service was updated in order + to extend the Measured Boot Event Log at boot up and it uses the + aforementioned ``PTA_SYSTEM_GET_TPM_EVENT_LOG`` call to retrieve a copy + of the former. + + .. note:: + Arm does not provide an fTPM implementation. The fTPM service used here + is a third party one which has been updated to support Measured Boot + service as provided by TF-A. As such, it is beyond the scope of this + manual to test and verify the correctness of the output generated by the + fTPM service. + + - **TPM Kernel module**: In order to interact with the fTPM service, we need + a kernel module to forward the request from user space to the secure world. + + - `tpm2-tools`_: This is a set of tools that allow to interact with the + fTPM service. We use this in order to read the PCRs with the measurements. + +Building the PoC for the Arm FVP platform +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ + +As mentioned before, this PoC is based on the OP-TEE Toolkit with some +extensions to enable Measured Boot and an fTPM service. Therefore, we can rely +on the instructions to build the original OP-TEE Toolkit. As a general rule, +the following steps should suffice: + +(1) Start by following the `Get and build the solution`_ instructions to build + the OP-TEE toolkit. On step 3, you need to get the manifest for FVP + platform from the main branch: + + .. code:: shell + + $ repo init -u https://github.com/OP-TEE/manifest.git -m fvp.xml + + Then proceed synching the repos as stated in step 3. Continue following + the instructions and stop before step 5. + +(2) Next you should obtain the `Armv8-A Foundation Platform (For Linux Hosts Only)`_. + The binary should be untar'ed to the root of the repo tree, i.e., like + this: ``<fvp-project>/Foundation_Platformpkg``. In the end, after cloning + all source code, getting the toolchains and "installing" + Foundation_Platformpkg, you should have a folder structure that looks like + this: + + .. code:: shell + + $ ls -la + total 80 + drwxrwxr-x 20 tf-a_user tf-a_user 4096 Jul 1 12:16 . + drwxr-xr-x 23 tf-a_user tf-a_user 4096 Jul 1 10:40 .. + drwxrwxr-x 12 tf-a_user tf-a_user 4096 Jul 1 10:45 build + drwxrwxr-x 16 tf-a_user tf-a_user 4096 Jul 1 12:16 buildroot + drwxrwxr-x 51 tf-a_user tf-a_user 4096 Jul 1 10:45 edk2 + drwxrwxr-x 6 tf-a_user tf-a_user 4096 Jul 1 12:14 edk2-platforms + drwxr-xr-x 7 tf-a_user tf-a_user 4096 Jul 1 10:52 Foundation_Platformpkg + drwxrwxr-x 17 tf-a_user tf-a_user 4096 Jul 2 10:40 grub + drwxrwxr-x 25 tf-a_user tf-a_user 4096 Jul 2 10:39 linux + drwxrwxr-x 15 tf-a_user tf-a_user 4096 Jul 1 10:45 mbedtls + drwxrwxr-x 6 tf-a_user tf-a_user 4096 Jul 1 10:45 ms-tpm-20-ref + drwxrwxr-x 8 tf-a_user tf-a_user 4096 Jul 1 10:45 optee_client + drwxrwxr-x 10 tf-a_user tf-a_user 4096 Jul 1 10:45 optee_examples + drwxrwxr-x 12 tf-a_user tf-a_user 4096 Jul 1 12:13 optee_os + drwxrwxr-x 8 tf-a_user tf-a_user 4096 Jul 1 10:45 optee_test + drwxrwxr-x 7 tf-a_user tf-a_user 4096 Jul 1 10:45 .repo + drwxrwxr-x 4 tf-a_user tf-a_user 4096 Jul 1 12:12 toolchains + drwxrwxr-x 21 tf-a_user tf-a_user 4096 Jul 1 12:15 trusted-firmware-a + +(3) Now enter into ``ms-tpm-20-ref`` and get its dependencies: + + .. code:: shell + + $ cd ms-tpm-20-ref + $ git submodule init + $ git submodule update + Submodule path 'external/wolfssl': checked out '9c87f979a7f1d3a6d786b260653d566c1d31a1c4' + +(4) Now, you should be able to continue with step 5 in "`Get and build the solution`_" + instructions. In order to enable support for Measured Boot, you need to + set the ``MEASURED_BOOT`` build option: + + .. code:: shell + + $ MEASURED_BOOT=y make -j `nproc` + + .. note:: + The build process will likely take a long time. It is strongly recommended to + pass the ``-j`` option to make to run the process faster. + + After this step, you should be ready to run the image. + +Running and using the PoC on the Armv8-A Foundation AEM FVP +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ + +With everything built, you can now run the image: + +.. code:: shell + + $ make run-only + +.. note:: + Using ``make run`` will build and run the image and it can be used instead + of simply ``make``. However, once the image is built, it is recommended to + use ``make run-only`` to avoid re-running all the building rules, which + would take time. + +When FVP is launched, two terminal windows will appear. ``FVP terminal_0`` +is the userspace terminal whereas ``FVP terminal_1`` is the counterpart for +the secure world (where TAs will print their logs, for instance). + +Log into the image shell with user ``root``, no password will be required. +Then we can issue the ``ftpm`` command, which is an alias that + +(1) loads the ftpm kernel module and + +(2) calls ``tpm2_pcrread``, which will access the fTPM service to read the + PCRs. + +When loading the ftpm kernel module, the fTPM TA is loaded into the secure +world. This TA then requests a copy of the Event Log generated during the +booting process so it can retrieve all the entries on the log and record them +first thing. + +.. note:: + For this PoC, nothing loaded after BL33 and NT_FW_CONFIG is recorded + in the Event Log. + +The secure world terminal should show the debug logs for the fTPM service, +including all the measurements available in the Event Log as they are being +processed: + +.. code:: shell + + M/TA: Preparing to extend the following TPM Event Log: + M/TA: TCG_EfiSpecIDEvent: + M/TA: PCRIndex : 0 + M/TA: EventType : 3 + M/TA: Digest : 00 + M/TA: : 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 + M/TA: : 00 00 00 + M/TA: EventSize : 33 + M/TA: Signature : Spec ID Event03 + M/TA: PlatformClass : 0 + M/TA: SpecVersion : 2.0.2 + M/TA: UintnSize : 1 + M/TA: NumberOfAlgorithms : 1 + M/TA: DigestSizes : + M/TA: #0 AlgorithmId : SHA256 + M/TA: DigestSize : 32 + M/TA: VendorInfoSize : 0 + M/TA: PCR_Event2: + M/TA: PCRIndex : 0 + M/TA: EventType : 3 + M/TA: Digests Count : 1 + M/TA: #0 AlgorithmId : SHA256 + M/TA: Digest : 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 + M/TA: : 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 + M/TA: EventSize : 17 + M/TA: Signature : StartupLocality + M/TA: StartupLocality : 0 + M/TA: PCR_Event2: + M/TA: PCRIndex : 0 + M/TA: EventType : 1 + M/TA: Digests Count : 1 + M/TA: #0 AlgorithmId : SHA256 + M/TA: Digest : 58 26 32 6e 64 45 64 da 45 de 35 db 96 fd ed 63 + M/TA: : 2a 6a d4 0d aa 94 b0 b1 55 e4 72 e7 1f 0a e0 d5 + M/TA: EventSize : 5 + M/TA: Event : BL_2 + M/TA: PCR_Event2: + M/TA: PCRIndex : 0 + M/TA: EventType : 1 + M/TA: Digests Count : 1 + M/TA: #0 AlgorithmId : SHA256 + M/TA: Digest : cf f9 7d a3 5c 73 ac cb 7b a0 25 80 6a 6e 50 a5 + M/TA: : 6b 2e d2 8c c9 36 92 7d 46 c5 b9 c3 a4 6c 51 7c + M/TA: EventSize : 6 + M/TA: Event : BL_31 + M/TA: PCR_Event2: + M/TA: PCRIndex : 0 + M/TA: EventType : 1 + M/TA: Digests Count : 1 + M/TA: #0 AlgorithmId : SHA256 + M/TA: Digest : 23 b0 a3 5d 54 d9 43 1a 5c b9 89 63 1c da 06 c2 + M/TA: : e5 de e7 7e 99 17 52 12 7d f7 45 ca 4f 4a 39 c0 + M/TA: EventSize : 10 + M/TA: Event : HW_CONFIG + M/TA: PCR_Event2: + M/TA: PCRIndex : 0 + M/TA: EventType : 1 + M/TA: Digests Count : 1 + M/TA: #0 AlgorithmId : SHA256 + M/TA: Digest : 4e e4 8e 5a e6 50 ed e0 b5 a3 54 8a 1f d6 0e 8a + M/TA: : ea 0e 71 75 0e a4 3f 82 76 ce af cd 7c b0 91 e0 + M/TA: EventSize : 14 + M/TA: Event : SOC_FW_CONFIG + M/TA: PCR_Event2: + M/TA: PCRIndex : 0 + M/TA: EventType : 1 + M/TA: Digests Count : 1 + M/TA: #0 AlgorithmId : SHA256 + M/TA: Digest : 01 b0 80 47 a1 ce 86 cd df 89 d2 1f 2e fc 6c 22 + M/TA: : f8 19 ec 6e 1e ec 73 ba 5a be d0 96 e3 5f 6d 75 + M/TA: EventSize : 6 + M/TA: Event : BL_32 + M/TA: PCR_Event2: + M/TA: PCRIndex : 0 + M/TA: EventType : 1 + M/TA: Digests Count : 1 + M/TA: #0 AlgorithmId : SHA256 + M/TA: Digest : 5d c6 ef 35 5a 90 81 b4 37 e6 3b 52 da 92 ab 8e + M/TA: : d9 6e 93 98 2d 40 87 96 1b 5a a7 ee f1 f4 40 63 + M/TA: EventSize : 18 + M/TA: Event : BL32_EXTRA1_IMAGE + M/TA: PCR_Event2: + M/TA: PCRIndex : 0 + M/TA: EventType : 1 + M/TA: Digests Count : 1 + M/TA: #0 AlgorithmId : SHA256 + M/TA: Digest : 39 b7 13 b9 93 db 32 2f 1b 48 30 eb 2c f2 5c 25 + M/TA: : 00 0f 38 dc 8e c8 02 cd 79 f2 48 d2 2c 25 ab e2 + M/TA: EventSize : 6 + M/TA: Event : BL_33 + M/TA: PCR_Event2: + M/TA: PCRIndex : 0 + M/TA: EventType : 1 + M/TA: Digests Count : 1 + M/TA: #0 AlgorithmId : SHA256 + M/TA: Digest : 25 10 60 5d d4 bc 9d 82 7a 16 9f 8a cc 47 95 a6 + M/TA: : fd ca a0 c1 2b c9 99 8f 51 20 ff c6 ed 74 68 5a + M/TA: EventSize : 13 + M/TA: Event : NT_FW_CONFIG + +These logs correspond to the measurements stored by TF-A during the measured +boot process and therefore, they should match the logs dumped by the former +during the boot up process. These can be seen on the terminal_0: + +.. code:: shell + + NOTICE: Booting Trusted Firmware + NOTICE: BL1: v2.5(release):v2.5 + NOTICE: BL1: Built : 10:41:20, Jul 2 2021 + NOTICE: BL1: Booting BL2 + NOTICE: BL2: v2.5(release):v2.5 + NOTICE: BL2: Built : 10:41:20, Jul 2 2021 + NOTICE: TCG_EfiSpecIDEvent: + NOTICE: PCRIndex : 0 + NOTICE: EventType : 3 + NOTICE: Digest : 00 + NOTICE: : 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 + NOTICE: : 00 00 00 + NOTICE: EventSize : 33 + NOTICE: Signature : Spec ID Event03 + NOTICE: PlatformClass : 0 + NOTICE: SpecVersion : 2.0.2 + NOTICE: UintnSize : 1 + NOTICE: NumberOfAlgorithms : 1 + NOTICE: DigestSizes : + NOTICE: #0 AlgorithmId : SHA256 + NOTICE: DigestSize : 32 + NOTICE: VendorInfoSize : 0 + NOTICE: PCR_Event2: + NOTICE: PCRIndex : 0 + NOTICE: EventType : 3 + NOTICE: Digests Count : 1 + NOTICE: #0 AlgorithmId : SHA256 + NOTICE: Digest : 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 + NOTICE: : 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 + NOTICE: EventSize : 17 + NOTICE: Signature : StartupLocality + NOTICE: StartupLocality : 0 + NOTICE: PCR_Event2: + NOTICE: PCRIndex : 0 + NOTICE: EventType : 1 + NOTICE: Digests Count : 1 + NOTICE: #0 AlgorithmId : SHA256 + NOTICE: Digest : 58 26 32 6e 64 45 64 da 45 de 35 db 96 fd ed 63 + NOTICE: : 2a 6a d4 0d aa 94 b0 b1 55 e4 72 e7 1f 0a e0 d5 + NOTICE: EventSize : 5 + NOTICE: Event : BL_2 + NOTICE: PCR_Event2: + NOTICE: PCRIndex : 0 + NOTICE: EventType : 1 + NOTICE: Digests Count : 1 + NOTICE: #0 AlgorithmId : SHA256 + NOTICE: Digest : cf f9 7d a3 5c 73 ac cb 7b a0 25 80 6a 6e 50 a5 + NOTICE: : 6b 2e d2 8c c9 36 92 7d 46 c5 b9 c3 a4 6c 51 7c + NOTICE: EventSize : 6 + NOTICE: Event : BL_31 + NOTICE: PCR_Event2: + NOTICE: PCRIndex : 0 + NOTICE: EventType : 1 + NOTICE: Digests Count : 1 + NOTICE: #0 AlgorithmId : SHA256 + NOTICE: Digest : 23 b0 a3 5d 54 d9 43 1a 5c b9 89 63 1c da 06 c2 + NOTICE: : e5 de e7 7e 99 17 52 12 7d f7 45 ca 4f 4a 39 c0 + NOTICE: EventSize : 10 + NOTICE: Event : HW_CONFIG + NOTICE: PCR_Event2: + NOTICE: PCRIndex : 0 + NOTICE: EventType : 1 + NOTICE: Digests Count : 1 + NOTICE: #0 AlgorithmId : SHA256 + NOTICE: Digest : 4e e4 8e 5a e6 50 ed e0 b5 a3 54 8a 1f d6 0e 8a + NOTICE: : ea 0e 71 75 0e a4 3f 82 76 ce af cd 7c b0 91 e0 + NOTICE: EventSize : 14 + NOTICE: Event : SOC_FW_CONFIG + NOTICE: PCR_Event2: + NOTICE: PCRIndex : 0 + NOTICE: EventType : 1 + NOTICE: Digests Count : 1 + NOTICE: #0 AlgorithmId : SHA256 + NOTICE: Digest : 01 b0 80 47 a1 ce 86 cd df 89 d2 1f 2e fc 6c 22 + NOTICE: : f8 19 ec 6e 1e ec 73 ba 5a be d0 96 e3 5f 6d 75 + NOTICE: EventSize : 6 + NOTICE: Event : BL_32 + NOTICE: PCR_Event2: + NOTICE: PCRIndex : 0 + NOTICE: EventType : 1 + NOTICE: Digests Count : 1 + NOTICE: #0 AlgorithmId : SHA256 + NOTICE: Digest : 5d c6 ef 35 5a 90 81 b4 37 e6 3b 52 da 92 ab 8e + NOTICE: : d9 6e 93 98 2d 40 87 96 1b 5a a7 ee f1 f4 40 63 + NOTICE: EventSize : 18 + NOTICE: Event : BL32_EXTRA1_IMAGE + NOTICE: PCR_Event2: + NOTICE: PCRIndex : 0 + NOTICE: EventType : 1 + NOTICE: Digests Count : 1 + NOTICE: #0 AlgorithmId : SHA256 + NOTICE: Digest : 39 b7 13 b9 93 db 32 2f 1b 48 30 eb 2c f2 5c 25 + NOTICE: : 00 0f 38 dc 8e c8 02 cd 79 f2 48 d2 2c 25 ab e2 + NOTICE: EventSize : 6 + NOTICE: Event : BL_33 + NOTICE: PCR_Event2: + NOTICE: PCRIndex : 0 + NOTICE: EventType : 1 + NOTICE: Digests Count : 1 + NOTICE: #0 AlgorithmId : SHA256 + NOTICE: Digest : 25 10 60 5d d4 bc 9d 82 7a 16 9f 8a cc 47 95 a6 + NOTICE: : fd ca a0 c1 2b c9 99 8f 51 20 ff c6 ed 74 68 5a + NOTICE: EventSize : 13 + NOTICE: Event : NT_FW_CONFIG + NOTICE: BL1: Booting BL31 + NOTICE: BL31: v2.5(release):v2.5 + NOTICE: BL31: Built : 10:41:20, Jul 2 2021 + +Following up with the fTPM startup process, we can see that all the +measurements in the Event Log are extended and recorded in the appropriate PCR: + +.. code:: shell + + M/TA: TPM2_PCR_EXTEND_COMMAND returned value: + M/TA: ret_tag = 0x8002, size = 0x00000013, rc = 0x00000000 + M/TA: TPM2_PCR_EXTEND_COMMAND returned value: + M/TA: ret_tag = 0x8002, size = 0x00000013, rc = 0x00000000 + M/TA: TPM2_PCR_EXTEND_COMMAND returned value: + M/TA: ret_tag = 0x8002, size = 0x00000013, rc = 0x00000000 + M/TA: TPM2_PCR_EXTEND_COMMAND returned value: + M/TA: ret_tag = 0x8002, size = 0x00000013, rc = 0x00000000 + M/TA: TPM2_PCR_EXTEND_COMMAND returned value: + M/TA: ret_tag = 0x8002, size = 0x00000013, rc = 0x00000000 + M/TA: TPM2_PCR_EXTEND_COMMAND returned value: + M/TA: ret_tag = 0x8002, size = 0x00000013, rc = 0x00000000 + M/TA: TPM2_PCR_EXTEND_COMMAND returned value: + M/TA: ret_tag = 0x8002, size = 0x00000013, rc = 0x00000000 + M/TA: TPM2_PCR_EXTEND_COMMAND returned value: + M/TA: ret_tag = 0x8002, size = 0x00000013, rc = 0x00000000 + M/TA: TPM2_PCR_EXTEND_COMMAND returned value: + M/TA: ret_tag = 0x8002, size = 0x00000013, rc = 0x00000000 + M/TA: 9 Event logs processed + +After the fTPM TA is loaded, the call to ``insmod`` issued by the ``ftpm`` +alias to load the ftpm kernel module returns, and then the TPM PCRs are read +by means of ``tpm_pcrread`` command. Note that we are only interested in the +SHA256 logs here, as this is the algorithm we used on TF-A for the measurements +(see the field ``AlgorithmId`` on the logs above): + +.. code:: shell + + sha256: + 0 : 0xA6EB3A7417B8CFA9EBA2E7C22AD5A4C03CDB8F3FBDD7667F9C3EF2EA285A8C9F + 1 : 0x0000000000000000000000000000000000000000000000000000000000000000 + 2 : 0x0000000000000000000000000000000000000000000000000000000000000000 + 3 : 0x0000000000000000000000000000000000000000000000000000000000000000 + 4 : 0x0000000000000000000000000000000000000000000000000000000000000000 + 5 : 0x0000000000000000000000000000000000000000000000000000000000000000 + 6 : 0x0000000000000000000000000000000000000000000000000000000000000000 + 7 : 0x0000000000000000000000000000000000000000000000000000000000000000 + 8 : 0x0000000000000000000000000000000000000000000000000000000000000000 + 9 : 0x0000000000000000000000000000000000000000000000000000000000000000 + 10: 0x0000000000000000000000000000000000000000000000000000000000000000 + 11: 0x0000000000000000000000000000000000000000000000000000000000000000 + 12: 0x0000000000000000000000000000000000000000000000000000000000000000 + 13: 0x0000000000000000000000000000000000000000000000000000000000000000 + 14: 0x0000000000000000000000000000000000000000000000000000000000000000 + 15: 0x0000000000000000000000000000000000000000000000000000000000000000 + 16: 0x0000000000000000000000000000000000000000000000000000000000000000 + 17: 0xFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFF + 18: 0xFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFF + 19: 0xFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFF + 20: 0xFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFF + 21: 0xFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFF + 22: 0xFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFF + 23: 0x0000000000000000000000000000000000000000000000000000000000000000 + +In this PoC we are only interested in PCR0, which must be non-null. This is +because the boot process records all the images in this PCR (see field ``PCRIndex`` +on the Event Log above). The rest of the records must be 0 at this point. + +.. note:: + The fTPM service used has support only for 16 PCRs, therefore the content + of PCRs above 15 can be ignored. + +.. note:: + As stated earlier, Arm does not provide an fTPM implementation and therefore + we do not validate here if the content of PCR0 is correct or not. For this + PoC, we are only focused on the fact that the event log could be passed to a third + party fTPM and its records were properly extended. + +Fine-tuning the fTPM TA +~~~~~~~~~~~~~~~~~~~~~~~ + +As stated earlier, the OP-TEE Toolkit includes support to build a third party fTPM +service. The build options for this service are tailored for the PoC and defined in +the build environment variable ``FTPM_FLAGS`` (see ``<toolkit_home>/build/common.mk``) +but they can be modified if needed to better adapt it to a specific scenario. + +The most relevant options for Measured Boot support are: + + - **CFG_TA_DEBUG**: Enables debug logs in the Terminal_1 console. + - **CFG_TEE_TA_LOG_LEVEL**: Defines the log level used for the debug messages. + - **CFG_TA_MEASURED_BOOT**: Enables support for measured boot on the fTPM. + - **CFG_TA_EVENT_LOG_SIZE**: Defines the size, in bytes, of the larger event log that + the fTPM is able to store, as this buffer is allocated at build time. This must be at + least the same as the size of the event log generated by TF-A. If this build option + is not defined, the fTPM falls back to a default value of 1024 bytes, which is enough + for this PoC, so this variable is not defined in FTPM_FLAGS. + +-------------- + +*Copyright (c) 2021, Arm Limited. All rights reserved.* + +.. _OP-TEE Toolkit: https://github.com/OP-TEE/build +.. _ms-tpm-20-ref: https://github.com/microsoft/ms-tpm-20-ref +.. _Get and build the solution: https://optee.readthedocs.io/en/latest/building/gits/build.html#get-and-build-the-solution +.. _Armv8-A Foundation Platform (For Linux Hosts Only): https://developer.arm.com/tools-and-software/simulation-models/fixed-virtual-platforms/arm-ecosystem-models +.. _tpm2-tools: https://github.com/tpm2-software/tpm2-tools +.. _TGC event log: https://trustedcomputinggroup.org/resource/tcg-efi-platform-specification/ |