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+Threat Model for TF-A with Arm CCA support
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+Introduction
+************
+
+This document provides a threat model of TF-A firmware for platforms with Arm
+Realm Management Extension (RME) support which implement Arm Confidential
+Compute Architecture (Arm CCA).
+
+Although it is a separate document, it references the :ref:`Generic Threat
+Model` in a number of places, as some of the contents is commonly applicable to
+TF-A with or without Arm CCA support.
+
+Target of Evaluation
+********************
+
+In this threat model, the target of evaluation is the Trusted Firmware for
+A-class Processors (TF-A) with RME support and Arm CCA support. This includes
+the boot ROM (BL1), the trusted boot firmware (BL2) and the runtime EL3 firmware
+(BL31).
+
+Assumptions
+===========
+
+We make the following assumptions:
+
+- :ref:`Realm Management Extension (RME)` is enabled on the platform.
+
+- Arm CCA Hardware Enforced Security (HES) is available on the platform, as
+  recommended by `Arm CCA security model`_:
+
+    *[R0004] Arm strongly recommends that all implementations of CCA utilize*
+    *hardware enforced security (CCA HES).*
+
+- All TF-A images run from on-chip memory. Data used by these images also live
+  in on-chip memory. This means TF-A is not vulnerable to an attacker that can
+  probe or tamper with off-chip memory.
+
+  These are requirements of the `Arm CCA security model`_:
+
+    *[R0147] Monitor code executes entirely from on-chip memory.*
+
+    *[R0149] Any monitor data that may affect the CCA security guarantee, other*
+    *than GPT, is either held in on-chip memory, or in external memory but with*
+    *additional integrity protection.*
+
+  Note that this threat model hardens *[R0149]* requirement by forbidding to
+  hold data in external memory, even if it is integrity-protected - except for
+  GPT data.
+
+- TF-A BL1 image is immutable and thus implicitly trusted. It runs from
+  read-only memory or write-protected memory. This could be on-chip ROM, on-chip
+  OTP, locked on-chip flash, or write-protected on-chip RAM for example.
+
+  This is a requirement of the `Arm CCA security model`_:
+
+    *[R0158] Arm recommends that all initial boot code is immutable on a*
+    *secured system.*
+
+    *[R0050] If all or part of initial boot code is instantiated in on-chip*
+    *memory then other trusted subsystems or application PE cannot modify that*
+    *code before it has been executed.*
+
+- Trusted boot and measured boot are enabled. This means an attacker can't boot
+  arbitrary images that are not approved by platform providers.
+
+  These are requirements of the `Arm CCA security model`_:
+
+    *[R0048] A secured system can only load authorized CCA firmware.*
+
+    *[R0079] All Monitor firmware loaded by PE initial boot is measured and*
+    *verified as outlined in Verified boot.*
+
+- No experimental features are enabled. These are typically incomplete features,
+  which need more time to stabilize. Thus, we do not consider threats that may
+  come from them. It is not recommended to use these features in production
+  builds.
+
+Data Flow Diagram
+=================
+
+Figure 1 shows a high-level data flow diagram for TF-A. The diagram shows a
+model of the different components of a TF-A-based system and their interactions
+with TF-A. A description of each diagram element is given on Table 1. On the
+diagram, the red broken lines indicate trust boundaries. Components outside of
+the broken lines are considered untrusted by TF-A.
+
+.. uml:: ../resources/diagrams/plantuml/tfa_arm_cca_dfd.puml
+  :caption: Figure 1: Data Flow Diagram
+
+.. table:: Table 1: Data Flow Diagram Description
+
+  +-----------------+--------------------------------------------------------+
+  | Diagram Element | Description                                            |
+  +=================+========================================================+
+  |       DF1       | | Refer to DF1 description in the                      |
+  |                 |   :ref:`Generic Threat Model`. Additionally TF-A       |
+  |                 |   loads realm images.                                  |
+  +-----------------+--------------------------------------------------------+
+  |     DF2-DF6     | | Refer to DF2-DF6 descriptions in the                 |
+  |                 |   :ref:`Generic Threat Model`.                         |
+  +-----------------+--------------------------------------------------------+
+  |       DF7       | | Boot images interact with Arm CCA HES to record boot |
+  |                 |   measurements and retrieve data used for AP images    |
+  |                 |   authentication.                                      |
+  |                 |                                                        |
+  |                 | | The runtime firmware interacts with Arm CCA HES to   |
+  |                 |   obtain sensitive attestation data for the realm      |
+  |                 |   world.                                               |
+  +-----------------+--------------------------------------------------------+
+  |       DF8       | | Realm world software (e.g. TF-RMM) interact with     |
+  |                 |   TF-A through SMC call interface and/or shared        |
+  |                 |   memory.                                              |
+  +-----------------+--------------------------------------------------------+
+
+Threat Analysis
+***************
+
+In this threat model, we use the same method to analyse threats as in the
+:ref:`Generic Threat Model`. This section only points out differences where
+applicable.
+
+- There is an additional threat agent: *RealmCode*. It takes the form of
+  malicious or faulty code running in the realm world, including R-EL2, R-EL1
+  and R-EL0 levels.
+
+- At this time we only consider the ``Server`` target environment. New threats
+  identified in this threat model will only be given a risk rating for this
+  environment. Other environments may be added in a future revision
+
+Threat Assessment
+=================
+
+General Threats for All Firmware Images
+---------------------------------------
+
+The following table analyses the :ref:`General Threats` in the context of this
+threat model. Only deltas are pointed out.
+
+  +----+-------------+-------------------------------------------------------+
+  | ID | Applicable? | Comments                                              |
+  +====+=============+=======================================================+
+  | 05 |     Yes     |                                                       |
+  +----+-------------+-------------------------------------------------------+
+  | 06 |     Yes     |                                                       |
+  +----+-------------+-------------------------------------------------------+
+  | 08 |     Yes     | Additional diagram element: DF8.                      |
+  |    |             |                                                       |
+  |    |             | Additional threat agent: RealmCode.                   |
+  +----+-------------+-------------------------------------------------------+
+  | 11 |     Yes     | | Misconfiguration of the Memory Management Unit      |
+  |    |             |   (MMU) may allow a **normal/secure/realm** world     |
+  |    |             |   software to access sensitive data, execute arbitrary|
+  |    |             |   code or access otherwise restricted HW interface.   |
+  |    |             |                                                       |
+  |    |             | | **Note that on RME systems, MMU configuration also  |
+  |    |             |   includes Granule Protection Tables (GPT) setup.**   |
+  |    |             |                                                       |
+  |    |             | | Additional diagram elements: DF4, DF7, DF8.         |
+  |    |             |                                                       |
+  |    |             | | Additional threat agents: SecCode, RealmCode.       |
+  +----+-------------+-------------------------------------------------------+
+  | 13 |     Yes     | Additional diagram element: DF8.                      |
+  |    |             |                                                       |
+  |    |             | Additional threat agent: RealmCode.                   |
+  +----+-------------+-------------------------------------------------------+
+  | 15 |     Yes     | Additional diagram element: DF8.                      |
+  |    |             |                                                       |
+  |    |             | Additional threat agent: RealmCode.                   |
+  +----+-------------+-------------------------------------------------------+
+
+Threats to be Mitigated by the Boot Firmware
+--------------------------------------------
+
+The following table analyses the :ref:`Boot Firmware Threats` in the context of
+this threat model. Only deltas are pointed out.
+
+  +----+-------------+-------------------------------------------------------+
+  | ID | Applicable? | Comments                                              |
+  +====+=============+=======================================================+
+  | 01 |     Yes     | Additional diagram element: DF8.                      |
+  |    |             |                                                       |
+  |    |             | Additional threat agent: RealmCode.                   |
+  +----+-------------+-------------------------------------------------------+
+  | 02 |     Yes     | Additional diagram element: DF8.                      |
+  |    |             |                                                       |
+  |    |             | Additional threat agent: RealmCode.                   |
+  +----+-------------+-------------------------------------------------------+
+  | 03 |     Yes     |                                                       |
+  +----+-------------+-------------------------------------------------------+
+  | 04 |     Yes     |                                                       |
+  +----+-------------+-------------------------------------------------------+
+
+Threats to be Mitigated by the Runtime EL3 Firmware
+---------------------------------------------------
+
+The following table analyses the :ref:`Runtime Firmware Threats` in the context
+of this threat model. Only deltas are pointed out.
+
+  +----+-------------+-------------------------------------------------------+
+  | ID | Applicable? | Comments                                              |
+  +====+=============+=======================================================+
+  | 07 |     Yes     | Additional diagram element: DF8.                      |
+  |    |             |                                                       |
+  |    |             | Additional threat agent: RealmCode.                   |
+  +----+-------------+-------------------------------------------------------+
+  | 09 |     Yes     | Additional diagram element: DF8.                      |
+  |    |             |                                                       |
+  |    |             | Additional threat agent: RealmCode.                   |
+  +----+-------------+-------------------------------------------------------+
+  | 10 |     Yes     | Additional diagram element: DF8.                      |
+  |    |             |                                                       |
+  |    |             | Additional threat agent: RealmCode.                   |
+  +----+-------------+-------------------------------------------------------+
+  | 12 |     Yes     | Additional diagram element: DF8.                      |
+  |    |             |                                                       |
+  |    |             | Additional threat agent: RealmCode.                   |
+  +----+-------------+-------------------------------------------------------+
+  | 14 |     Yes     |                                                       |
+  +----+-------------+-------------------------------------------------------+
+
+*Copyright (c) 2023, Arm Limited. All rights reserved.*
+
+.. _Arm CCA Security Model: https://developer.arm.com/documentation/DEN0096/A_a