docs: add threat model for AP-RSS interface

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

docs/resources/diagrams/plantuml/tfa_rss_dfd.puml

@@ -0,0 +1,77 @@
+/'
+ ' Copyright (c) 2021-2022, Arm Limited. All rights reserved.
+ '
+ ' SPDX-License-Identifier: BSD-3-Clause
+ '/
+
+/'
+TF-A Data Flow Diagram including RSS
+'/
+
+@startuml
+digraph tfa_dfd {
+
+    # Arrange nodes from left to right
+    rankdir="LR"
+
+    # Allow arrows to end on cluster boundaries
+    compound=true
+
+    # Default settings for edges and nodes
+    edge [minlen=2 color="#8c1b07"]
+    node [fillcolor="#ffb866" style=filled shape=box fixedsize=true width=1.6 height=0.7]
+
+    # Nodes outside of the trust boundary
+    nsec [label="Non-secure\nClients"]
+    sec [label="Secure\nClients"]
+    dbg [label="Debug & Trace"]
+    logs [label="Logs\n(UART)"]
+    nvm [label="Non-volatile\nMemory"]
+
+
+    # Trust boundary cluster
+    subgraph cluster_trusted{
+        graph [style=dashed color="#f22430"]
+
+        # HW IPs cluster
+        subgraph cluster_ip{
+            label ="Hardware IPs";
+            graph [style=filled color="#000000" fillcolor="#ffd29e"]
+
+            rank="same"
+            gic [label="GIC" width=1.2 height=0.5]
+            tzc [label="TZ\nController" width=1.2 height=0.5]
+            etc [label="..." shape=none style=none height=0.5]
+        }
+
+        # TF-A cluster
+        subgraph cluster_tfa{
+            label ="TF-A";
+            graph [style=filled color="#000000" fillcolor="#faf9cd"]
+
+            bl1 [label="Boot ROM\n(BL1)" fillcolor="#ddffb3"];
+            bl2 [label="Trusted Boot\nFirmware\n(BL2)" fillcolor="#ddffb3" height=1]
+            bl31 [label="TF-A Runtime\n(BL31)" fillcolor="#ddffb3"]
+        }
+
+        # RSS cluster
+        subgraph cluster_rss{
+            label ="RSS";
+            graph [style=filled color="#000000" fillcolor="#faf9cd"]
+
+            rss [label="Runtime Security\n\ Subsystem\n\ (RSS)" fillcolor="#ddffb3"]
+        }
+    }
+
+    # Interactions between nodes
+    nvm -> bl31 [lhead=cluster_tfa label="DF1"]
+    logs -> bl31 [dir="back" lhead=cluster_tfa label="DF2"]
+    dbg -> bl2 [dir="both" lhead=cluster_tfa label="DF3"]
+    sec -> bl2 [dir="both" lhead=cluster_tfa label="DF4"]
+    nsec -> bl1 [dir="both" lhead=cluster_tfa, label="DF5"]
+    bl2 ->  tzc [dir="both" ltail=cluster_tfa lhead=cluster_ip label="DF6" minlen=1]
+    bl31 -> rss [dir="both" ltail=cluster_tfa lhead=cluster_rss label="DF7" minlen=1]
+
+}
+
+@enduml

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.*

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.*