docs: add threat model for AP-RSS interface

Signed-off-by: Tamas Ban <tamas.ban@arm.com>
Change-Id: Ic818da12584503e1a96396c4b55a8db14ae7584a

2 files changed, 61 insertions, 1 deletions

diff --git a/docs/threat_model/index.rst b/docs/threat_model/index.rst
index ad8b82ac0..9fd55a92c 100644
--- a/docs/threat_model/index.rst
+++ b/docs/threat_model/index.rst
@@ -16,7 +16,8 @@ based on the data flow diagram and potential threat mitigations.
    threat_model_spm
    threat_model_el3_spm
    threat_model_fvp_r
+   threat_model_rss_interface
 
 --------------
 
-*Copyright (c) 2021, Arm Limited and Contributors. All rights reserved.*
+*Copyright (c) 2021-2022, Arm Limited and Contributors. All rights reserved.*
diff --git a/docs/threat_model/threat_model_rss_interface.rst b/docs/threat_model/threat_model_rss_interface.rst
new file mode 100644
index 000000000..4bceb631c
--- /dev/null
+++ b/docs/threat_model/threat_model_rss_interface.rst
@@ -0,0 +1,59 @@
+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.*
\ No newline at end of file