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Generic Threat Model

Introduction

This document provides a generic threat model for TF-A firmware.

Note

This threat model doesn't consider Root and Realm worlds introduced by :ref:`Realm Management Extension (RME)`.

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Target of Evaluation

In this threat model, the target of evaluation is the Trusted Firmware for A-class Processors (TF-A). This includes the boot ROM (BL1), the trusted boot firmware (BL2) and the runtime EL3 firmware (BL31) as shown on Figure 1. Everything else on Figure 1 is outside of the scope of the evaluation.

TF-A can be configured in various ways. In this threat model we consider only the most basic configuration. To that end we make the following assumptions:

All TF-A images are run from either ROM or on-chip trusted SRAM. This means TF-A is not vulnerable to an attacker that can probe or tamper with off-chip memory.
Trusted boot is enabled. This means an attacker can't boot arbitrary images that are not approved by platform providers.
There is no Secure-EL2. We don't consider threats that may come with Secure-EL2 software.
Measured boot is disabled. We do not consider the threats nor the mitigations that may come with it.
No experimental features are enabled. We do not consider threats that may come from them.

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.

System Message: ERROR/3 (<stdin>, line 55)

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.. uml:: ../resources/diagrams/plantuml/tfa_dfd.puml
  :caption: Figure 1: TF-A Data Flow Diagram

Table 1: TF-A Data Flow Diagram Description
Diagram Element	Description
DF1	At boot time, images are loaded from non-volatile memory and verified by TF-A boot firmware. These images include TF-A BL2 and BL31 images, as well as other secure and non-secure images.
DF2	TF-A log system framework outputs debug messages over a UART interface.
DF3	Debug and trace IP on a platform can allow access to registers and memory of TF-A.
DF4	Secure world software (e.g. trusted OS) interact with TF-A through SMC call interface and/or shared memory.
DF5	Non-secure world software (e.g. rich OS) interact with TF-A through SMC call interface and/or shared memory.
DF6	This path represents the interaction between TF-A and various hardware IPs such as TrustZone controller and GIC. At boot time TF-A configures/initializes the IPs and interacts with them at runtime through interrupts and registers.

Threat Analysis

In this section we identify and provide assessment of potential threats to TF-A firmware. The threats are identified for each diagram element on the data flow diagram above.

For each threat, we identify the asset that is under threat, the threat agent and the threat type. Each threat is given a risk rating that represents the impact and likelihood of that threat. We also discuss potential mitigations.

Assets

We have identified the following assets for TF-A:

Table 2: TF-A Assets
Asset	Description
Sensitive Data	These include sensitive data that an attacker must not be able to tamper with (e.g. the Root of Trust Public Key) or see (e.g. secure logs, debugging information such as crash reports).
Code Execution	This represents the requirement that the platform should run only TF-A code approved by the platform provider.
Availability	This represents the requirement that TF-A services should always be available for use.

Threat Agents

To understand the attack surface, it is important to identify potential attackers, i.e. attack entry points. The following threat agents are in scope of this threat model.

Table 3: Threat Agents
Threat Agent	Description
NSCode	Malicious or faulty code running in the Non-secure world, including NS-EL0 NS-EL1 and NS-EL2 levels
SecCode	Malicious or faulty code running in the secure world, including S-EL0 and S-EL1 levels
AppDebug	Physical attacker using debug signals to access TF-A resources
PhysicalAccess	Physical attacker having access to external device communication bus and to external flash communication bus using common hardware

Note

In this threat model an advanced physical attacker that has the capability to tamper with a hardware (e.g. "rewiring" a chip using a focused ion beam (FIB) workstation or decapsulate the chip using chemicals) is considered out-of-scope.

Threat Types

In this threat model we categorize threats using the STRIDE threat analysis technique. In this technique a threat is categorized as one or more of these types: Spoofing, Tampering, Repudiation, Information disclosure, Denial of service or Elevation of privilege.

Threat Risk Ratings

For each threat identified, a risk rating that ranges from informational to critical is given based on the likelihood of the threat occuring if a mitigation is not in place, and the impact of the threat (i.e. how severe the consequences could be). Table 4 explains each rating in terms of score, impact and likelihood.

Table 4: Rating and score as applied to impact and likelihood
Rating (Score)	Impact	Likelihood
Critical (5)	Extreme impact to entire organization if exploited.	Threat is almost certain to be exploited. Knowledge of the threat and how to exploit it are in the public domain.
High (4)	Major impact to entire organization or single line of business if exploited	Threat is relatively easy to detect and exploit by an attacker with little skill.
Medium (3)	Noticeable impact to line of business if exploited.	A knowledgeable insider or expert attacker could exploit the threat without much difficulty.
Low (2)	Minor damage if exploited or could be used in conjunction with other vulnerabilities to perform a more serious attack	Exploiting the threat would require considerable expertise and resources
Informational (1)	Poor programming practice or poor design decision that may not represent an immediate risk on its own, but may have security implications if multiplied and/or combined with other threats.	Threat is not likely to be exploited on its own, but may be used to gain information for launching another attack

Aggregate risk scores are assigned to identified threats; specifically, the impact score multiplied by the likelihood score. For example, a threat with high likelihood and low impact would have an aggregate risk score of eight (8); that is, four (4) for high likelihood multiplied by two (2) for low impact. The aggregate risk score determines the finding's overall risk level, as shown in the following table.

Table 5: Overall risk levels and corresponding aggregate scores
Overall Risk Level	Aggregate Risk Score (Impact multiplied by Likelihood)
Critical	20–25
High	12–19
Medium	6–11
Low	2–5
Informational	1

The likelihood and impact of a threat depends on the target environment in which TF-A is running. For example, attacks that require physical access are unlikely in server environments while they are more common in Internet of Things(IoT) environments. In this threat model we consider three target environments: Internet of Things(IoT), Mobile and Server.

Threat Assessment

The following threats were identified by applying STRIDE analysis on each diagram element of the data flow diagram.

For each threat, we strive to indicate whether the mitigations are currently implemented or not. However, the answer to this question is not always straight forward. Some mitigations are partially implemented in the generic code but also rely on the platform code to implement some bits of it. This threat model aims to be platform-independent and it is important to keep in mind that such threats only get mitigated if the platform code properly fulfills its responsibilities.

Also, some mitigations require enabling specific features, which must be explicitly turned on via a build flag.

These are highlighted in the Mitigations implemented? box.

ID	01
Threat	An attacker can mangle firmware images to execute arbitrary code Some TF-A images are loaded from external storage. It is possible for an attacker to access the external flash memory and change its contents physically, through the Rich OS, or using the updating mechanism to modify the non-volatile images to execute arbitrary code.
Diagram Elements	DF1, DF4, DF5
Affected TF-A Components	BL2, BL31
Assets	Code Execution
Threat Agent	PhysicalAccess, NSCode, SecCode
Threat Type	Tampering, Elevation of Privilege
Application	Server	IoT	Mobile
Impact	Critical (5)	Critical (5)	Critical (5)
Likelihood	Critical (5)	Critical (5)	Critical (5)
Total Risk Rating	Critical (25)	Critical (25)	Critical (25)
Mitigations	1) Implement the Trusted Board Boot (TBB) feature which prevents malicious firmware from running on the platform by authenticating all firmware images. 2) Perform extra checks on unauthenticated data, such as FIP metadata, prior to use.
Mitigations implemented?	1) Yes, provided that the `TRUSTED_BOARD_BOOT` build option is set to 1. 2) Yes.

ID	02
Threat	An attacker may attempt to boot outdated, potentially vulnerable firmware image When updating firmware, an attacker may attempt to rollback to an older version that has unfixed vulnerabilities.
Diagram Elements	DF1, DF4, DF5
Affected TF-A Components	BL2, BL31
Assets	Code Execution
Threat Agent	PhysicalAccess, NSCode, SecCode
Threat Type	Tampering
Application	Server	IoT	Mobile
Impact	Critical (5)	Critical (5)	Critical (5)
Likelihood	Critical (5)	Critical (5)	Critical (5)
Total Risk Rating	Critical (25)	Critical (25)	Critical (25)
Mitigations	Implement anti-rollback protection using non-volatile counters (NV counters) as required by TBBR-Client specification.
Mitigations implemented?	Yes / Platform specific. After a firmware image is validated, the image revision number taken from a certificate extension field is compared with the corresponding NV counter stored in hardware to make sure the new counter value is larger than the current counter value. Platforms must implement this protection using platform specific hardware NV counters.

ID	03
Threat	An attacker can use Time-of-Check-Time-of-Use (TOCTOU) attack to bypass image authentication during the boot process Time-of-Check-Time-of-Use (TOCTOU) threats occur when the security check is produced before the time the resource is accessed. If an attacker is sitting in the middle of the off-chip images, they could change the binary containing executable code right after the integrity and authentication check has been performed.
Diagram Elements	DF1
Affected TF-A Components	BL1, BL2
Assets	Code Execution, Sensitive Data
Threat Agent	PhysicalAccess
Threat Type	Elevation of Privilege
Application	Server	IoT	Mobile
Impact	N/A	Critical (5)	Critical (5)
Likelihood	N/A	Medium (3)	Medium (3)
Total Risk Rating	N/A	High (15)	High (15)
Mitigations	Copy image to on-chip memory before authenticating it.
Mitigations implemented?	Platform specific. The list of images to load and their location is platform specific. Platforms are responsible for arranging images to be loaded in on-chip memory.

ID	04
Threat	An attacker with physical access can execute arbitrary image by bypassing the signature verification stage using glitching techniques Glitching (Fault injection) attacks attempt to put a hardware into a undefined state by manipulating an environmental variable such as power supply. TF-A relies on a chain of trust that starts with the ROTPK, which is the key stored inside the chip and the root of all validation processes. If an attacker can break this chain of trust, they could execute arbitrary code on the device. This could be achieved with physical access to the device by attacking the normal execution flow of the process using glitching techniques that target points where the image is validated against the signature.
Diagram Elements	DF1
Affected TF-A Components	BL1, BL2
Assets	Code Execution
Threat Agent	PhysicalAccess
Threat Type	Tampering, Elevation of Privilege
Application	Server	IoT	Mobile
Impact	N/A	Critical (5)	Critical (5)
Likelihood	N/A	Medium (3)	Medium (3)
Total Risk Rating	N/A	High (15)	High (15)
Mitigations	Mechanisms to detect clock glitch and power variations.
Mitigations implemented?	No. The most effective mitigation is adding glitching detection and mitigation circuit at the hardware level. However, software techniques, such as adding redundant checks when performing conditional branches that are security sensitive, can be used to harden TF-A against such attacks. At the moment TF-A doesn't implement such mitigations.

ID	05
Threat	Information leak via UART logs such as crashes During the development stages of software it is common to include crash reports with detailed information of the CPU state including current values of the registers, privilege level and stack dumps. This information is useful when debugging problems before releasing the production version, but it could be used by an attacker to develop a working exploit if left in the production version.
Diagram Elements	DF2
Affected TF-A Components	BL1, BL2, BL31
Assets	Sensitive Data
Threat Agent	AppDebug
Threat Type	Information Disclosure
Application	Server	IoT	Mobile
Impact	N/A	Low (2)	Low (2)
Likelihood	N/A	High (4)	High (4)
Total Risk Rating	N/A	Medium (8)	Medium (8)
Mitigations	Remove crash reports in production releases.
Mitigations implemented?	Yes / Platform Specific. Requires the right build options to be used. Crash reporting is only enabled for debug builds by default, see `CRASH_REPORTING` build option. The log level can be tuned at build time, from very verbose to no output at all. See `LOG_LEVEL` build option. By default, release builds are a lot less verbose than debug ones but still produce some output. Messages produced by the platform code should use the appropriate level of verbosity so as not to leak sensitive information in production builds.

ID	06
Threat	An attacker can read sensitive data and execute arbitrary code through the external debug and trace interface Arm processors include hardware-assisted debug and trace features that can be controlled without the need for software operating on the platform. If left enabled without authentication, this feature can be used by an attacker to inspect and modify TF-A registers and memory allowing the attacker to read sensitive data and execute arbitrary code.
Diagram Elements	DF3
Affected TF-A Components	BL1, BL2, BL31
Assets	Code Execution, Sensitive Data
Threat Agent	AppDebug
Threat Type	Tampering, Information Disclosure, Elevation of privilege
Application	Server	IoT	Mobile
Impact	N/A	High (4)	High (4)
Likelihood	N/A	Critical (5)	Critical (5)
Total Risk Rating	N/A	Critical (20)	Critical (20)
Mitigations	Disable the debug and trace capability for production releases or enable proper debug authentication as recommended by [DEN0034].
Mitigations implemented?	Platform specific. Configuration of debug and trace capabilities is entirely platform specific.

ID	07
Threat	An attacker can perform a denial-of-service attack by using a broken SMC call that causes the system to reboot or enter into unknown state. Secure and non-secure clients access TF-A services through SMC calls. Malicious code can attempt to place the TF-A runtime into an inconsistent state by calling unimplemented SMC call or by passing invalid arguments.
Diagram Elements	DF4, DF5
Affected TF-A Components	BL31
Assets	Availability
Threat Agent	NSCode, SecCode
Threat Type	Denial of Service
Application	Server	IoT	Mobile
Impact	Medium (3)	Medium (3)	Medium (3)
Likelihood	High (4)	High (4)	High (4)
Total Risk Rating	High (12)	High (12)	High (12)
Mitigations	Validate SMC function ids and arguments before using them.
Mitigations implemented?	Yes / Platform specific. For standard services, all input is validated. Platforms that implement SiP services must also validate SMC call arguments.

ID	08
Threat	Memory corruption due to memory overflows and lack of boundary checking when accessing resources could allow an attacker to execute arbitrary code, modify some state variable to change the normal flow of the program, or leak sensitive information Like in other software, TF-A has multiple points where memory corruption security errors can arise. Some of the errors include integer overflow, buffer overflow, incorrect array boundary checks, and incorrect error management. Improper use of asserts instead of proper input validations might also result in these kinds of errors in release builds.
Diagram Elements	DF4, DF5
Affected TF-A Components	BL1, BL2, BL31
Assets	Code Execution, Sensitive Data
Threat Agent	NSCode, SecCode
Threat Type	Tampering, Information Disclosure, Elevation of Privilege
Application	Server	IoT	Mobile
Impact	Critical (5)	Critical (5)	Critical (5)
Likelihood	Medium (3	Medium (3)	Medium (3)
Total Risk Rating	High (15)	High (15)	High (15)
Mitigations	1) Use proper input validation. 2) Code reviews, testing.
Mitigations implemented?	1) Yes. Data received from normal world, such as addresses and sizes identifying memory regions, are sanitized before being used. These security checks make sure that the normal world software does not access memory beyond its limit. By default asserts are only used to check for programming errors in debug builds. Other types of errors are handled through condition checks that remain enabled in release builds. See TF-A error handling policy. TF-A provides an option to use asserts in release builds, however we recommend using proper runtime checks instead of relying on asserts in release builds. 2) Yes. TF-A uses a combination of manual code reviews and automated program analysis and testing to detect and fix memory corruption bugs. All TF-A code including platform code go through manual code reviews. Additionally, static code analysis is performed using Coverity Scan on all TF-A code. The code is also tested with Trusted Firmware-A Tests on Juno and FVP platforms.

ID	09
Threat	Improperly handled SMC calls can leak register contents When switching between worlds, TF-A register state can leak to software in different security contexts.
Diagram Elements	DF4, DF5
Affected TF-A Components	BL31
Assets	Sensitive Data
Threat Agent	NSCode, SecCode
Threat Type	Information Disclosure
Application	Server	IoT	Mobile
Impact	Medium (3)	Medium (3)	Medium (3)
Likelihood	High (4)	High (4)	High (4)
Total Risk Rating	High (12)	High (12)	High (12)
Mitigations	Save and restore registers when switching contexts.
Mitigations implemented?	Yes. This is the default behaviour in TF-A. Build options are also provided to save/restore additional registers such as floating-point registers. These should be enabled if required.

ID	10
Threat	SMC calls can leak sensitive information from TF-A memory via microarchitectural side channels Microarchitectural side-channel attacks such as Spectre can be used to leak data across security boundaries. An attacker might attempt to use this kind of attack to leak sensitive data from TF-A memory.
Diagram Elements	DF4, DF5
Affected TF-A Components	BL31
Assets	Sensitive Data
Threat Agent	SecCode, NSCode
Threat Type	Information Disclosure
Application	Server	IoT	Mobile
Impact	Medium (3)	Medium (3)	Medium (3)
Likelihood	Medium (3)	Medium (3)	Medium (3)
Total Risk Rating	Medium (9)	Medium (9)	Medium (9)
Mitigations	Enable appropriate side-channel protections.
Mitigations implemented?	Yes / Platform specific. TF-A implements software mitigations for Spectre type attacks as recommended by Cache Speculation Side-channels for the generic code. SiPs should implement similar mitigations for code that is deemed to be vulnerable to such attacks.

ID	11
Threat	Misconfiguration of the Memory Management Unit (MMU) may allow a normal world software to access sensitive data or execute arbitrary code A misconfiguration of the MMU could lead to an open door for software running in the normal world to access sensitive data or even execute code if the proper security mechanisms are not in place.
Diagram Elements	DF5, DF6
Affected TF-A Components	BL1, BL2, BL31
Assets	Sensitive Data, Code execution
Threat Agent	NSCode
Threat Type	Information Disclosure, Elevation of Privilege
Application	Server	IoT	Mobile
Impact	Critical (5)	Critical (5)	Critical (5)
Likelihood	High (4)	High (4)	High (4)
Total Risk Rating	Critical (20)	Critical (20)	Critical (20)
Mitigations	When configuring access permissions, the principle of least privilege ought to be enforced. This means we should not grant more privileges than strictly needed, e.g. code should be read-only executable, read-only data should be read-only execute-never, and so on.
Mitigations implemented?	Platform specific. MMU configuration is platform specific, therefore platforms need to make sure that the correct attributes are assigned to memory regions. TF-A provides a library which abstracts the low-level details of MMU configuration. It provides well-defined and tested APIs. Platforms are encouraged to use it to limit the risk of misconfiguration.

ID	12
Threat	Incorrect configuration of Performance Monitor Unit (PMU) counters can allow an attacker to mount side-channel attacks using information exposed by the counters Non-secure software can configure PMU registers to count events at any exception level and in both Secure and Non-secure states. This allows a Non-secure software (or a lower-level Secure software) to potentially carry out side-channel timing attacks against TF-A.
Diagram Elements	DF5, DF6
Affected TF-A Components	BL31
Assets	Sensitive Data
Threat Agent	NSCode
Threat Type	Information Disclosure
Impact	Medium (3)	Medium (3)	Medium (3)
Likelihood	Low (2)	Low (2)	Low (2)
Total Risk Rating	Medium (6)	Medium (6)	Medium (6)
Mitigations	Follow mitigation strategies as described in Secure Development Guidelines.
Mitigations implemented?	Yes / platform specific. General events and cycle counting in the Secure world is prohibited by default when applicable. However, on some implementations (e.g. PMUv3) Secure world event counting depends on external debug interface signals, i.e. Secure world event counting is enabled if external debug is enabled. Configuration of debug signals is platform specific, therefore platforms need to make sure that external debug is disabled in production or proper debug authentication is in place. This should be the case if threat #06 is properly mitigated.

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