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bb4d7d7195
Added a threat model for PSA firmware update as well as TBBR FWU aka firmware recovery. Change-Id: I2396e13144076d7294f61f6817e1a8646225c6c2 Signed-off-by: Manish V Badarkhe <Manish.Badarkhe@arm.com>
103 lines
4.8 KiB
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103 lines
4.8 KiB
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Threat Model for TF-A with PSA FWU or TBBR FWU support
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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Introduction
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************
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This document provides a threat model of TF-A firmware for platforms with
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the feature PSA firmware update or TBBR firmware update or both enabled.
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To understand the design of the firmware update refer
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:ref:`Firmware Update (FWU)`.
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Although it is a separate document, it references the :ref:`Generic Threat
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Model` in a number of places, as some of the contents are applicable to this
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threat model.
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Target of Evaluation
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********************
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In this threat model, the target of evaluation is the Trusted Firmware for
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A-class Processors (TF-A) when PSA FWU support is enabled or TBBR FWU mode
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is enabled. This includes the boot ROM (BL1), the trusted boot firmware (BL2).
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Threat Assessment
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*****************
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For this section, please reference the Threat Assessment under the
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:ref:`Generic Threat Model`. Here only the differences are highlighted.
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PSA FWU
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*******
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Threats to be Mitigated by the Boot Firmware
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--------------------------------------------
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The following table analyses the :ref:`Boot Firmware Threats` in the context
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of this threat model. Only additional details are pointed out.
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+----+-------------+-------------------------------------------------------+
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| ID | Applicable? | Comments |
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+====+=============+=======================================================+
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| 01 | Yes | | Attacker can use arbitrary images to update the |
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| | | system. |
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+----+-------------+-------------------------------------------------------+
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| 02 | Yes | | Attacker tries to update the system with the |
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| | | vulnerable/older firmware. |
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+----+-------------+-------------------------------------------------------+
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| 03 | Yes | |
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+----+-------------+-------------------------------------------------------+
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| 04 | Yes | |
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+----+-------------+-------------------------------------------------------+
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Threats to be mitigated by platform design
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------------------------------------------
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PSA FWU is driven by metadata stored in non-volatile storage. This metadata
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is not cryptographically signed. Also, depending on the hardware design,
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it may be stored in untrusted storage, which makes it possible for software
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outside of TF-A security boundary or for a physical attacker to modify it
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in order to change the behaviour of the FWU process.
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Below we provide some possible FWU metadata corruption scenarios:
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1. The FWU metadata includes the firmware bank for booting; the attacker
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tries to modify it to prevent the execution of the updated firmware.
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2. The FWU metadata features a field indicating the firmware's status, either
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in trial run or accepted run. The attacker tries to manipulate this field,
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ensuring the updated firmware consistently runs in trial mode, with the
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intention of preventing the anti-rollback update.
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By design, no software mitigations exist to prevent this. The safeguarding
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of FWU metadata relies on the platform's hardware design to mitigate potential
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attacks on it, if this is a concern in the platform's threat model.
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For example, FWU metadata may be stored in secure storage under exclusive
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access from secure software, protecting it from physical, unauthenticated
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accesses and from non-secure software accesses.
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TBBR FWU - Firmware Recovery
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****************************
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Threats to be Mitigated by the Boot Firmware
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--------------------------------------------
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The following table analyses the :ref:`Boot Firmware Threats` in the context
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of this threat model. Only additional details are pointed out.
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+----+-------------+-------------------------------------------------------+
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| ID | Applicable? | Comments |
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+====+=============+=======================================================+
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| 01 | Yes | | Attacker can use arbitrary images to recover the |
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| | | system. |
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+----+-------------+-------------------------------------------------------+
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| 02 | Yes | | Attacker tries to recover the system with the |
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| | | vulnerable/older firmware. |
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+----+-------------+-------------------------------------------------------+
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| 03 | Yes | |
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+----+-------------+-------------------------------------------------------+
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| 04 | Yes | |
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+----+-------------+-------------------------------------------------------+
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--------------
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*Copyright (c) 2024, Arm Limited. All rights reserved.*
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