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docs(threat-model): add a threat model for TF-A with Arm CCA

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Arm Confidential Compute Architecture (Arm CCA) support, underpinned by
Arm Realm Management Extension (RME) support, brings in a few important
software and hardware architectural changes in TF-A, which warrants a
new security analysis of the code base. Results of this analysis are
captured in a new threat model document, provided in this patch.

The main changes introduced in TF-A to support Arm CCA / RME are:

 - Presence of a new threat agent: realm world clients.

 - Availability of Arm CCA Hardware Enforced Security (HES) to support
   measured boot and trusted boot.

 - Configuration of the Granule Protection Tables (GPT) for
   inter-world memory protection.

This is only an initial version of the threat model and we expect to
enrich it in the future.

Signed-off-by: Sandrine Bailleux <sandrine.bailleux@arm.com>
Co-authored-by: Manish Pandey <manish.pandey2@arm.com>
Change-Id: Iab84dc724df694511508f90dc76b6d469c4cccd5
This commit is contained in:
Sandrine Bailleux 2023-09-06 16:11:12 +02:00
parent dcbf3a1173
commit 446354122c
4 changed files with 317 additions and 0 deletions
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82
docs/resources/diagrams/plantuml/tfa_arm_cca_dfd.puml Normal file
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@ -0,0 +1,82 @@
/'
' Copyright (c) 2023, Arm Limited. All rights reserved.
'
' SPDX-License-Identifier: BSD-3-Clause
'/
/'
TF-A with Arm CCA 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
realm [label="Realm\nClients"]
nsec [label="Non-secure\nClients"]
sec [label="Secure\nClients"]
dbg [label="Debug & Trace"]
uart [label="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]
mmu [label="MMU" 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"]
}
# HES cluster
subgraph cluster_hes{
label ="Arm CCA HES";
graph [style=filled color="#000000" fillcolor="#ffd29e"]
hes [label="Hardware\nEnforced Security"]
}
}
# Interactions between nodes
# -- The following lines are copied from tfa_dfd.puml and must not be
# changed, at the risk of invalidating DF* references.
nvm -> bl31 [lhead=cluster_tfa label="DF1"]
uart -> bl31 [dir="both" 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 -> mmu [dir="both" ltail=cluster_tfa lhead=cluster_ip label="DF6"]
# -- The following lines are new for Arm CCA DFD.
bl2 -> hes [dir="both" ltail=cluster_tfa lhead=cluster_hes label="DF7"]
realm -> bl2 [dir="both" lhead=cluster_tfa label="DF8"]
}
@enduml

1
docs/threat_model/index.rst
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@ -35,6 +35,7 @@ data flow diagram, as well as a list of threats we have identified using the
threat_model_el3_spm
threat_model_fvp_r
threat_model_rss_interface
threat_model_arm_cca
--------------

9
docs/threat_model/threat_model.rst
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@ -36,6 +36,9 @@ assumptions:
- There are no Root and Realm worlds. These are introduced by :ref:`Realm
Management Extension (RME)`.
The :ref:`Threat Model for TF-A with Arm CCA support` covers these types of
configurations.
- No experimental features are enabled. We do not consider threats that may come
from them.
@ -274,6 +277,8 @@ some threats are confined in specific images, while others apply to each of
them. To help developers implement mitigations in the right place, threats below
are categorized based on the firmware image that should mitigate them.
.. _General Threats:
General Threats for All Firmware Images
---------------------------------------
@ -608,6 +613,8 @@ General Threats for All Firmware Images
+------------------------+-----------------------------------------------------+
.. _Boot Firmware Threats:
Threats to be Mitigated by the Boot Firmware
--------------------------------------------
@ -842,6 +849,8 @@ nonetheless once execution has reached the runtime EL3 firmware.
since the |SRTM| includes all secure world components.
.. _Runtime Firmware Threats:
Threats to be Mitigated by the Runtime EL3 Firmware
---------------------------------------------------

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docs/threat_model/threat_model_arm_cca.rst Normal file
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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