docs(threat model): add TF-A threat model

This is the first release of the public Trusted
Firmware A class threat model. This release
provides the baseline for future updates to be
applied as required by developments to the
TF-A code base.
Signed-off-by: Zelalem Aweke <zelalem.aweke@arm.com>
Change-Id: I3c9aadc46196837679f0b1377bec9ed4fc42ff11

docs/_static/css/custom.css

+/*
+ * Copyright (c) 2021, Arm Limited. All rights reserved.
+ *
+ * SPDX-License-Identifier: BSD-3-Clause
+ */
+
+/*
+ * Set the white-space property of tables to normal.
+ * With this setting sequences of whitespace inside
+ * a table will collapse into a single whitespace,
+ * and text will wrap when necessary.
+ */
+.wy-table-responsive table td {
+white-space: normal;
+}

docs/conf.py

 # -*- coding: utf-8 -*-
 #
-# Copyright (c) 2019, Arm Limited. All rights reserved.
+# Copyright (c) 2019-2021, Arm Limited. All rights reserved.
 #
 # SPDX-License-Identifier: BSD-3-Clause
 #
@@ -76,6 +76,14 @@ html_theme_options = {
     'style_external_links': True # Display an icon next to external links
 }
 
+# Path to _static directory
+html_static_path = ['_static']
+
+# Path to css file relative to html_static_path
+html_css_files = [
+    'css/custom.css',
+]
+
 # -- Options for autosectionlabel --------------------------------------------
 
 # Only generate automatic section labels for document titles

docs/index.rst

@@ -15,6 +15,7 @@ Trusted Firmware-A Documentation
    perf/index
    security_advisories/index
    design_documents/index
+   threat_model/index
    change-log
    change-log-upcoming
    glossary
@@ -83,7 +84,7 @@ have previously been raised against the software.
 
 --------------
 
-*Copyright (c) 2013-2020, Arm Limited and Contributors. All rights reserved.*
+*Copyright (c) 2013-2021, Arm Limited and Contributors. All rights reserved.*
 
 .. _Armv7-A and Armv8-A: https://developer.arm.com/products/architecture/a-profile
 .. _Secure Monitor: http://www.arm.com/products/processors/technologies/trustzone/tee-smc.php

docs/resources/diagrams/plantuml/tfa_dfd.puml

+/'
+ ' Copyright (c) 2021, Arm Limited. All rights reserved.
+ '
+ ' SPDX-License-Identifier: BSD-3-Clause
+ '/
+
+/'
+TF-A Data Flow Diagram
+'/
+
+@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"]
+        }
+    }
+
+    # 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]
+}
+
+@enduml

docs/threat_model/index.rst

+Threat Model
+=============
+
+.. toctree::
+   :maxdepth: 1
+   :caption: Contents
+   :numbered:
+
+   threat_model
+
+--------------
+
+*Copyright (c) 2021, Arm Limited and Contributors. All rights reserved.*