docs: add threat model for AP-RSS interface
Signed-off-by: Tamas Ban <tamas.ban@arm.com>
Change-Id: Ic818da12584503e1a96396c4b55a8db14ae7584a
diff --git a/docs/threat_model/threat_model_rss_interface.rst b/docs/threat_model/threat_model_rss_interface.rst
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+Threat Model for RSS - AP interface
+***********************************
+
+************
+Introduction
+************
+This document is an extension for the general TF-A threat-model. It considers
+those platforms where a Runtime Security Subsystem (RSS) is included in the SoC
+next to the Application Processor (AP).
+
+********************
+Target of Evaluation
+********************
+The scope of this threat model only includes the interface between the RSS and
+AP. Otherwise, the TF-A :ref:`Generic Threat Model` document is applicable for
+the AP core. The threat model for the RSS firmware will be provided by the RSS
+firmware project in the future.
+
+
+Data Flow Diagram
+=================
+This diagram is different only from the general TF-A data flow diagram in that
+it includes the RSS and highlights the interface between the AP and the RSS
+cores. The interface description only focuses on the AP-RSS interface the rest
+is the same as in the general TF-A threat-model document.
+
+.. uml:: ../resources/diagrams/plantuml/tfa_rss_dfd.puml
+ :caption: Figure 1: TF-A Data Flow Diagram including RSS
+
+.. table:: Table 1: TF-A - RSS data flow diagram
+
+ +-----------------+--------------------------------------------------------+
+ | Diagram Element | Description |
+ +=================+========================================================+
+ | DF7 | | Boot images interact with RSS over a communication |
+ | | channel to record boot measurements and get image |
+ | | verification keys. At runtime, BL31 obtains the |
+ | | realm world attestation signing key from RSS. |
+ +-----------------+--------------------------------------------------------+
+
+Threat Assessment
+=================
+For this section, please reference the Threat Assessment under the general TF-A
+threat-model document, :ref:`Generic Threat Model`. All the threats listed there
+are applicable for the AP core, here only the differences are highlighted.
+
+ - ID 11: The access to the communication interface between AP and RSS is
+ allowed only for firmware running at EL3. Accidentally exposing this
+ interface to NSCode can allow malicious code to interact with RSS and
+ gain access to sensitive data.
+ - ID 13: Relevant in the context of the realm attestation key, which can be
+ retrieved by BL31 through DF7. The RSS communication protocol layer
+ mitigates against this by clearing its internal buffer when reply is
+ received. The caller of the API must do the same if data is not needed
+ anymore.
+
+--------------
+
+*Copyright (c) 2022, Arm Limited. All rights reserved.*
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