Trusted Firmware-M Generic Threat Model

Introduction

This document introduces a generic threat model of Trusted Firmware-M (TF-M). This generic threat model provides an overall analysis of TF-M implementation and identifies general threats and mitigation.

There is also a dedicated document for physical attack mitigations which can be found here.

Note

If you think a security vulnerability is found, please follow Trustedfirmware.org [Security-Incident-Process] to contact TF-M security team.

Scope

TF-M supports diverse models and topologies. It also implements multiple isolation levels. Each case may focus on different target of evaluation (TOE) and identify different assets and threats. TF-M implementation consists of several secure services, defined as Root of Trust (RoT) service. Those RoT services belong to diverse RoT (Application RoT or PSA RoT) and access different assets and hardware. Therefore each RoT service may require a dedicated threat model.

The analysis on specific models, topologies or RoT services may be covered in dedicated threat model documents. Those threat models are out of the scope of this document.

Methodology

The threat modeling in this document follows the process listed below to build up the threat model.

• Target of Evaluation (TOE)

• Assets identification

• Data Flow Diagram (DFD)

• Threats Prioritization

• Threats identification

TOE is the entity on which threat modeling is performed. The logic behind this process is to firstly investigate the TOE which could be a system, solution or use case. This first step helps to identify the assets to be protected in TOE.

According to TOE and assets, Trust Boundaries can be determined. The Data Flow Diagram (DFD) across Trust Boundaries is then defined to help identify the threats.

Those threats should be prioritized based on a specific group of principals and metrics. The principals and metrics should also be specified.

Target of Evaluation

A typical TF-M system diagram from a high-level overview is shown below. TF-M is running in the Secure Processing Environment (SPE) and NS software is running in Non-secure Processing Environment (NSPE). For more details, please refer to Platform Security Architecture Firmware Framework for M (FF-M) [FF-M] and FF-M 1.1 Extensions [FF-M-1.1-Extensions].

The TOE in this general model is the SPE, including TF-M and other components running in SPE.

The TOE can vary in different TF-M models, RoT services and usage scenarios. Refer to dedicated threat models for the specific TOE definitions.

Asset identification

In this threat model, assets include the general items listed below:

• Hardware Root of Trust data, e.g.

  • Hardware Unique Key (HUK)

  • Root authentication key

  • Other embedded root keys

• Software RoT data, e.g.

  • Secure Partition Manager (SPM) code and data

  • Secure partition code and data

  • NSPE data stored in SPE

  • Data generated in SPE as requested by NSPE

• Availability of entire RoT service

• Secure logs, including event logs

Assets may vary in different use cases and implementations. Additional assets can be defined in an actual usage scenario and a dedicated threat model.

For example, in a network camera use case, the following data can be defined as assets too:

• Certificate for connecting to cloud

• Session keys for encryption/decryption in the communication with cloud

• Keys to encrypt/decrypt the videos and photos

Data Flow Diagram

The Trust Boundary isolates SPE from NSPE, according to the TOE definition in Target of Evaluation. The Trust Boundary mapped to block diagram is shown in the figure below. Other modules inside SPE stay in the same TOE as TF-M does.

Valid data flows across the Trust Boundary are also shown in the figure below. This threat model only focuses on the data flows related to TF-M.

More details of data flows are listed below.

Table 2: TF-M Data Flows between NSPE and SPE

Data flow	Description
DF1	TF-M initializes NS entry and activates NSPE. On Armv8-M platforms with TrustZone, TF-M will hand over the control to Non-secure state. On dual-cpu platforms, Secure core starts NS core booting.
DF2	NSPE requests TF-M RoT services. NSPE requests RoT services via PSA Client APIs defined in [FF-M]. In Armv8-M TrustZone scenarios, SG instruction is executed in a Non-secure Callable region to trigger a transition from Non-secure state to Secure state. On dual-cpu platforms, non-secure core sends PSA Client calls to secure core via mailbox.
DF3	Secure Partitions fetch input data from NS and write back output data to NS. As required in [FF-M], Secure Partitions should not directly access NSPE memory. Instead, RoT services relies on TF-M SPM to access NSPE memory.
DF4	TF-M returns RoT service results to NSPE after NS request to RoT service is completed. In Armv8-M TrustZone scenarios, it also triggers a transition from Secure state back to Non-secure state. On dual-cpu platforms, secure core returns the result to non-secure core via mailbox.
DF5	Non-secure interrupts preempt SPE execution in Armv8-M TrustZone scenarios.
DF6	Secure interrupts preempt NSPE execution in Armv8-M TrustZone scenarios.

Note

All the other data flows across the Trust Boundary besides the valid ones mentioned above should be prohibited by default. Proper isolation must be configured to prevent NSPE directly accessing SPE.

Threats irrelevant to data flows in TF-M Data Flows between NSPE and SPE may be specified in Miscellaneous threats.

Data flows inside SPE (informative)

Since all the SPE components stay in the TOE within the same Trust Boundary in this threat model, the data flows between SPE components are not covered in this threat model. Instead, those data flows and corresponding threats will be identified in the dedicated threat model documents of TF-M RoT services and usage scenarios.

Those data flows inside SPE include following examples:

• Data flows between TF-M and BL2

• Data flows between RoT services and SPM

• Data flows between RoT services and corresponding secure hardware and assets, such as secure storage device, crypto hardware accelerator and Hardware Unique Key (HUK).

Threat identification

Threat priority

Threat priority is indicated by the score calculated via Common Vulnerability Scoring System (CVSS) Version 3.1 [CVSS]. The higher the threat scores, the greater severity the threat is with and the higher the priority is.

CVSS scores can be mapped to qualitative severity ratings defined in CVSS 3.1 specification [CVSS_SPEC]. This threat model follows the same mapping between CVSS scores and threat priority rating.

As a generic threat model, this document focuses on Base Score which reflects the constant and general severity of a threat according to its intrinsic characteristics.

The Impacted Component defined in [CVSS_SPEC] refers to the assets listed in Asset identification.

Threats and mitigation list

This section lists generic threats and corresponding mitigation, based on the the analysis of data flows in Data Flow Diagram.

Threats are identified following STRIDE model. Please refer to [STRIDE] for more details.

The field CVSS Score reflects the threat priority defined in Threat priority. The field CVSS Vector String contains the textual representation of the CVSS metric values used to score the threat. Refer to [CVSS_SPEC] for more details of CVSS vector string.

Note

A generic threat may have different behaviors and therefore require different mitigation, in diverse TF-M models and usage scenarios.

This threat model document focuses on general analysis of the following threats. For the details in a specific configuration and usage scenario, please refer to the dedicated threat model document.

NS entry initialization

This section identifies threats on DF1 defined in Data Flow Diagram.

Table 3: TFM-GENERIC-NS-INIT-T-1

Index	TFM-GENERIC-NS-INIT-T-1
Description	The NS image can be tampered by an attacker
Justification	An attack may tamper the NS image to inject malicious code
Category	Tampering
Mitigation	By default TF-M relies on MCUBoot to validate NS image. The validation of NS image integrity and authenticity is completed in secure boot before jumping to NS entry or booting up NS core. Refer to [SECURE-BOOT] for more details. The validation may vary in diverse vendor platforms specific Chain of Trust (CoT) implementation.
CVSS Score	3.5 (Low)
CVSS Vector String	CVSS:3.1/AV:P/AC:L/PR:N/UI:N/S:U/C:L/I:L/A:N

Table 4: TFM-GENERIC-NS-INIT-T-2

Index	TFM-GENERIC-NS-INIT-T-2
Description	An attacker may replace the current NS image with an older version.
Justification	The attacker downgrades the NS image with an older version which has been deprecated due to known security issues. The older version image can pass the image signature validation and its vulnerabilities can be exploited by attackers.
Category	Tampering
Mitigation	TF-M relies on MCUBoot to perform anti-rollback protection. TF-M defines a non-volatile counter API to support anti-rollback. Each platform must implement it using specific trusted hardware non-volatile counters. For more details, refer to [ROLLBACK-PROTECT]. The anti-rollback protection implementation can vary on diverse platforms.
CVSS Score	3.5 (Low)
CVSS Vector String	CVSS:3.1/AV:P/AC:L/PR:N/UI:N/S:U/C:L/I:L/A:N

Table 5: TFM-GENERIC-NS-INIT-T-I-1

Index	TFM-GENERIC-NS-INIT-T-I-1
Description	If SPE doesn’t complete isolation configuration before NSPE starts, NSPE can access secure regions which it is disallowed to.
Justification	Secure data can be tampered or disclosed if NSPE is activated and accesses secure regions before isolation configuration is completed by SPE.
Category	Tampering/Information disclosure
Mitigation	SPE must complete and enable proper isolation to protect secure regions from being accessed by NSPE, before jumping to NS entry or booting up NS core. TF-M executes isolation configuration at early stage of secure initialization before NS initialization starts. On dual-cpu platform, platform specific initialization must halt NS core until isolation is completed, as defined in [DUAL-CPU-BOOT]. TF-M defines isolation configuration HALs for platform implementation. The specific isolation configuration depends on platform specific implementation.
CVSS Score	9.0 (Critical)
CVSS Vector String	CVSS:3.1/AV:L/AC:L/PR:N/UI:N/S:C/C:H/I:H/A:N

Table 6: TFM-GENERIC-NS-INIT-T-I-2

Index	TFM-GENERIC-NS-INIT-T-I-2
Description	If SPE doesn’t complete isolation configuration before NSPE starts, NSPE can control devices or peripherals which it is disallowed to.
Justification	On some platforms, devices and peripherals can be configured as Secure state in runtime. If security status configuration of those device and peripherals are not properly completed before NSPE starts, NSPE can control those device and peripherals and may be able to tamper data or access secure data.
Category	Tampering/Information disclosure
Mitigation	SPE must complete and enable proper configuration and isolation to protect critical devices and peripherals from being accessed by NSPE, before jumping to NS entry or booting up NS core. TF-M executes isolation configuration of devices and peripherals at early stage of secure initialization before NS initialization starts. The specific isolation configuration depends on platform specific implementation.
CVSS Score	9.0 (Critical)
CVSS Vector String	CVSS:3.1/AV:L/AC:L/PR:N/UI:N/S:C/C:H/I:H/A:N

Table 7: TFM-GENERIC-NS-INIT-I-2

Index	TFM-GENERIC-NS-INIT-I-2
Description	If SPE leaves some SPE information in non-secure memory or shared registers when NSPE starts, NSPE may access those SPE information.
Justification	If NSPE can access those SPE information from shared registers or non-secure memory, secure information may be disclosed.
Category	Information disclosure
Mitigation	SPE must clean up the secure information from shared registers before NS starts. TF-M invalidates registers not banked before handing over the system to NSPE on Armv8-M platforms with TrustZone. On dual-cpu platforms, shared registers are implementation defined, such as Inter-Processor Communication registers. Dual-cpu platforms must not store any data which may disclose secure information in the shared registers. SPE must avoid storing SPE information in non-secure memory.
CVSS Score	4.3 (Medium)
CVSS Vector String	CVSS:3.1/AV:L/AC:L/PR:N/UI:N/S:C/C:L/I:N/A:N

Table 8: TFM-GENERIC-NS-INIT-D-1

Index	TFM-GENERIC-NS-INIT-D-1
Description	An attacker may block NS to boot up
Justification	An attacker may block NS to boot up, such as by corrupting NS image, to stop the whole system from performing normal functionalities.
Category	Denial of service
Mitigation	No SPE information will be disclosed and TF-M won’t be directly impacted. It relies on NSPE and platform specific implementation to mitigate this threat. It is out of scope of this threat model.
CVSS Score	4.0 (Medium)
CVSS Vector String	CVSS:3.1/AV:L/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L

NSPE requests TF-M secure service

This section identifies threats on DF2 defined in Data Flow Diagram.

Table 9: TFM-GENERIC-REQUEST-SERVICE-S-1

Index	TFM-GENERIC-REQUEST-SERVICE-S-1
Description	A malicious NS application may pretend as a secure client to access secure data which NSPE must not directly access.
Justification	[FF-M] defines Client ID to distinguish clients which request RoT services. Secure clients are assigned with positive IDs and non-secure clients are assigned with negative ones. A malicious NS application may provide a positive Client ID to pretend as a secure client to access secure data.
Category	Spoofing
Mitigation	TF-M checks the Client ID from NSPE. If the NS Client ID is not a valid one, TF-M will report this as a security error.
CVSS Score	8.4 (High)
CVSS Vector String	CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:C/C:H/I:H/A:N

Table 10: TFM-GENERIC-REQUEST-SERVICE-T-1

Index	TFM-GENERIC-REQUEST-SERVICE-T-1
Description	An attacker in NSPE may tamper the service request input or output vectors between check and use (Time-Of-Check-to-Time-Of-Use (TOCTOU)).
Justification	If SPE validates the content in input/output vectors locally in NSPE memory, an attacker in NSPE can have a chance to tamper the content after the validation successfully passes. Then SPE will provide RoT service according to the corrupted parameters and it may cause further security issues.
Category	Tampering
Mitigation	In TF-M implementation, the validation of NS input/output vectors are only executed after those vectors are copied from NSPE into SPE. It prevents an attack from NSPE to tamper those parameters after validation in TF-M.
CVSS Score	7.8 (High)
CVSS Vector String	CVSS:3.1/AV:L/AC:H/PR:N/UI:N/S:C/C:H/I:H/A:N

Table 11: TFM-GENERIC-REQUEST-SERVICE-T-2

Index	TFM-GENERIC-REQUEST-SERVICE-T-2
Description	A malicious NS application may request to tamper data belonging to SPE.
Justification	A malicious NS application may request SPE RoT services to write malicious value to SPE data. The malicious NS application may try to tamper SPE assets, such as keys, or modify configurations in SPE. The SPE data belongs to components in SPE and must not be accessed by NSPE.
Category	Tampering
Mitigation	TF-M executes memory access check to all the RoT service requests. If a request doesn’t have enough permission to access the target memory region, TF-M will refuse this request and assert a security error.
CVSS Score	7.1 (High)
CVSS Vector String	CVSS:3.1/AV:L/AC:L/PR:N/UI:N/S:C/C:N/I:H/A:N

Table 12: TFM-GENERIC-REQUEST-SERVICE-R-1

Index	TFM-GENERIC-REQUEST-SERVICE-R-1
Description	A NS application may repudiate that it has requested services from a RoT service.
Justification	A malicious NS application may call a RoT service to access critical data in SPE, which it is disallowed to, via a non-public vulnerability. It may refuse to admit that it has accessed that data.
Category	Repudiation
Mitigation	TF-M implements an event logging secure service to record the critical events, such as the access to critical data.
CVSS Score	0.0 (None)
CVSS Vector String	CVSS:3.1/AV:L/AC:L/PR:N/UI:N/S:C/C:N/I:N/A:N

Table 13: TFM-GENERIC-REQUEST-SERVICE-I-1

Index	TFM-GENERIC-REQUEST-SERVICE-I-1
Description	A malicious NS application may request to read data belonging to SPE.
Justification	A malicious NS application may request SPE RoT services to copy SPE data to NS memory. The SPE data belongs to components in SPE and must not be disclosed to NSPE, such as root keys.
Category	Information disclosure
Mitigation	TF-M executes memory access check to all the RoT service requests. If a request doesn’t have enough permission to access the target memory region, TF-M will refuse this request and assert a security error.
CVSS Score	7.1 (High)
CVSS Vector String	CVSS:3.1/AV:L/AC:L/PR:N/UI:N/S:C/C:H/I:N/A:N

Table 14: TFM-GENERIC-REQUEST-SERVICE-T-I-1

Index	TFM-GENERIC-REQUEST-SERVICE-T-I-1
Description	A malicious NS application may request to control secure device and peripherals, on which it doesn’t have the permission.
Justification	A malicious NS application may request RoT services to control secure device and peripherals, on which it doesn’t have the permission.
Category	Tampering/Information disclose
Mitigation	TF-M performs client check to validate whether the client has the permission to access the secure device and peripherals.
CVSS Score	9.0 (Critical)
CVSS Vector String	CVSS:3.1/AV:L/AC:L/PR:N/UI:N/S:C/C:H/I:H/A:N

Table 15: TFM-GENERIC-REQUEST-SERVICE-D-1

Index	TFM-GENERIC-REQUEST-SERVICE-D-1
Description	A Malicious NS applications may frequently call secure services to block secure service requests from other NS applications.
Justification	TF-M runs on IoT devices with constrained resource. Even though multiple outstanding NS PSA Client calls can be supported in system, the number of NS PSA client calls served by TF-M simultaneously are still limited. Therefore, if a malicious NS application or multiple malicious NS applications continue calling TF-M secure services frequently, it may block other NS applications to request secure service from TF-M.
Category	Denial of service
Mitigation	TF-M is unable to manage behavior of NS applications. Assets are not disclosed and TF-M is neither directly impacted in this threat. It relies on NS OS to enhance scheduling policy and prevent a single NS application to occupy entire CPU time. It is beyond the scope of this threat model.
CVSS Score	4.0 (Medium)
CVSS Vector String	CVSS:3.1/AV:L/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L

Table 16: TFM-GENERIC-REQUEST-SERVICE-D-2

Index	TFM-GENERIC-REQUEST-SERVICE-D-2
Description	A malicious NS application may provide invalid NS memory addresses as the addresses of input and output data in RoT service requests.
Justification	SPE may be unable to achieve full knowledge of NS memory mapping. SPE may fail to capture those invalid NS memory addresses during memory access check since those invalid addresses may not be included in isolation configuration. In that case, SPE will access those invalid NS memory addresses later to read or write data. It may trigger a system error to crash the whole system immediately. The malicious NS application may be blocked by NS MPU from directly accessing that invalid NS memory address. But it may manipulate SPE to access that address instead.
Category	Denial of service
Mitigation	TF-M executes memory access check to the memory addresses in all the NS requests. On Armv8-M platforms with TrustZone, TF-M invokes TT instructions to execute memory address check. If a NS memory area is not matched in any valid SAU or MPU region, it will be marked as invalid and any access permission is disallowed. Therefore, SPM will reject any NS request containing invalid NS memory addresses and reports it as as a security error. On dual-core platforms, TF-M implements a default memory access check. If a NS memory area is not found in any memory region configured for isolation, it will be marked as invalid and therefore SPM will reject the corresponding NS request. It will be reported as a security error. Dual-core platforms may implement platform specific memory check to replace the default one. It relies on platform specific implementation to capture invalid memory address. It is out of the scope of this document.
CVSS Score	3.2 (Low)
CVSS Vector String	CVSS:3.1/AV:L/AC:H/PR:N/UI:N/S:C/C:N/I:N/A:L

RoT services read and write NS data

This section identifies threats on DF3 defined in Data Flow Diagram.

RoT services can either directly access NS memory or rely on TF-M SPM to obtain NS input data and send response data back to NS memory.

Table 17: TFM-GENERIC-SECURE-SERVICE-RW-T-1

Index	TFM-GENERIC-SECURE-SERVICE-RW-T-1
Description	An attacker may tamper NS input data while the RoT service is processing those data.
Justification	A RoT service may access NS input data multiple times during its data processing. For example, it may validate or authenticate the NS input data before it performs further processing. If the NS input data remains in NSPE memory during the RoT service execution, an attacker may tamper the NS input data in NSPE memory after the validation passes.
Category	Tampering
Mitigation	If RoT services request SPM to read and write NS data. TF-M SPM follows [FF-M] to copy the NS input data into SPE memory region owned by the RoT service, before the RoT service processes the data. Therefore, the NS input data is protected during the RoT service execution from being tampered. If RoT services can directly access NS memory and read NS input data multiple times during data processing, it is required to review and confirm the implementation of the RoT service copies NS input data into SPE memory area before it processes the data.
CVSS Score	3.2 (Low)
CVSS Vector String	CVSS:3.1/AV:L/AC:H/PR:N/UI:N/S:C/C:N/I:L/A:N

Table 18: TFM-GENERIC-SECURE-SERVICE-RW-T-2

Index	TFM-GENERIC-SECURE-SERVICE-RW-T-2
Description	A malicious NS application may embed secure memory addresses into a structure in RoT service request input vectors, to tamper secure memory which the NS application must not access.
Justification	[FF-M] limits the total number of input/output vectors to 4. If a RoT service requires more input/output vectors, it may define a parameter structure which embeds multiple input/output buffers addresses. However, as a potential security risk, a malicious NS application can put secure memory addresses into a valid parameter structure to bypass TF-M validation on those memory addresses. The parameter structure can pass TF-M memory access check since itself is valid. However, if the RoT service parses the structure and directly write malicious data from NSPE to the secure memory addresses in parameter structure, the secure data will be tampered.
Category	Tampering
Mitigation	It should be avoided to embed memory addresses into a single input/output vector. If more than 4 memory addresses are required in a RoT service request, it is recommended to split this request into two or multiple service calls and therefore each service call requires no more than 4 input/output vectors. If RoT services request SPM to read and write NS data. SPM will validate the target addresses and can detect the invalid addresses to mitigate this threat. If RoT services can directly access NS memory, it is required to review and confirm the implementation of RoT service request doesn’t embed memory addresses.
CVSS Score	7.1 (High)
CVSS Vector String	CVSS:3.1/AV:L/AC:L/PR:N/UI:N/S:C/C:N/I:H/A:N

Table 19: TFM-GENERIC-SECURE-SERVICE-RW-I-1

Index	TFM-GENERIC-SECURE-SERVICE-RW-I-1
Description	Similar to TFM-GENERIC-SECURE-SERVICE-RW-T-2, a malicious NS application can embed secure memory addresses in a parameter structure in RoT service request input vectors, to read secure data which the NS application must not access.
Justification	Similar to the description in TFM-GENERIC-SECURE-SERVICE-RW-T-2, the secure memory addresses hidden in the RoT service input/output vector structure may bypass TF-M validation. Without a proper check, the RoT service may copy secure data to NSPE according to the secure memory addresses in structure, secure information can be disclosed.
Category	Information disclosure
Mitigation	It should be avoided to embed memory addresses into a single input/output vector. If more than 4 memory addresses are required in a RoT service request, it is recommended to split this request into two or multiple service calls and therefore each service call requires no more than 4 input/output vectors. If RoT services request SPM to read and write NS data. SPM will validate the target addresses and can detect the invalid addresses to mitigate this threat. If RoT services can directly access NS memory, it is required to review and confirm the implementation of RoT service request doesn’t embed memory addresses.
CVSS Score	7.1 (High)
CVSS Vector String	CVSS:3.1/AV:L/AC:L/PR:N/UI:N/S:C/C:H/I:N/A:N

TF-M returns secure service result

This section identifies threats on DF4 defined in Data Flow Diagram.

When RoT service completes the request from NSPE, TF-M returns the success or failure error code to NS application.

In Armv8-M TrustZone scenarios, TF-M writes the return code value in the general purpose register and returns to Non-secure state.

On dual-cpu platforms, TF-M writes the return code to NS mailbox message queue via mailbox.

Table 20: TFM-GENERIC-RETURN-CODE-I-1

Index	TFM-GENERIC-RETURN-CODE-I-1
Description	SPE may leave secure data in the registers not banked after the SPE completes PSA Client calls and executes BXNS to switch Armv8-M back to Non-secure state.
Justification	If SPE doesn’t clean up the secure data in registers not banked before switching into NSPE in Armv8-M core, NSPE can read the SPE context from those registers.
Category	Information disclosure
Mitigation	In Armv8-M TrustZone scenarios, TF-M cleans general purpose registers not banked before switching into NSPE to prevent NSPE probing secure context from the registers. When FPU is enabled in TF-M, secure FP context belonging to a secure partition will be saved on this partition’s stack and cleaned by hardware during context switching. Also TF-M cleans secure FP context in FP registers before switching into NSPE to prevent NSPE from probing secure FP context.
CVSS Score	4.3 (Medium)
CVSS Vector String	CVSS:3.1/AV:L/AC:L/PR:N/UI:N/S:C/C:L/I:N/A:N

NS interrupts preempts SPE execution

This section identifies threats on DF5 defined in Data Flow Diagram.

Table 21: TFM-GENERIC-NS-INTERRUPT-I-1

Index	TFM-GENERIC-NS-INTERRUPT-I-1
Description	Shared registers may contain secure data when NS interrupts occur.
Justification	The secure data in shared registers should be cleaned up before NSPE can access shared registers. Otherwise, secure data leakage may occur.
Category	Information disclosure
Mitigation	On Armv8-M processors with TrustZone, Armv8-M architecture automatically cleans up the registers not banked before switching to Non-secure state while taking NS interrupts. When FPU is enabled in TF-M, with setting of FPCCR_S.TS = 1 besides secure FP context in FP caller registers, FP context in FP callee registers will also be cleaned by hardware automatically when NS interrupts occur, to prevent NSPE from probing secure FP context in FP registers. Refer to Armv8-M Architecture Reference Manual [Arm-ARM] for details. On dual-cpu platforms, shared registers are implementation defined, such as Inter-Processor Communication registers. Dual-cpu platforms must not store any data which may disclose secure information in the shared registers.
CVSS Score	4.3 (Medium)
CVSS Vector String	CVSS:3.1/AV:L/AC:L/PR:N/UI:N/S:C/C:L/I:N/A:N

Table 22: TFM-GENERIC-NS-INTERRUPT-D-1

Index	TFM-GENERIC-NS-INTERRUPT-D-1
Description	An attacker may trigger spurious NS interrupts frequently to block SPE execution.
Justification	On Armv8-M processors with TrustZone, an attacker may inject a malicious NS application or hijack a NS hardware to frequently trigger spurious NS interrupts to keep preempting SPE and block SPE to perform normal secure execution.
Category	Denial of service
Mitigation	It is out of scope of TF-M. Assets protected by TF-M won’t be leaked. TF-M won’t be directly impacted.
CVSS Score	4.0 (Medium)
CVSS Vector String	CVSS:3.1/AV:L/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L

Secure interrupts preempts NSPE execution

This section identifies threats on DF6 defined in Data Flow Diagram.

Table 23: TFM-GENERIC-S-INTERRUPT-I-1

Index	TFM-GENERIC-S-INTERRUPT-I-1
Description	Shared registers may contain secure data when Armv8-M core switches back to Non-secure state on Secure interrupt return.
Justification	Armv8-M architecture doesn’t automatically clean up shared registers while returning to Non-secure state during Secure interrupt return. If SPE leaves critical data in the Armv8-M registers not banked, NSPE can read secure context from those registers and secure data leakage may occur.
Category	Information disclosure
Mitigation	TF-M saves NPSE context in general purpose register R4~R11 into secure stack during secure interrupt entry. After secure interrupt handling completes, TF-M unstacks NSPE context from secure stack to overwrite secure context in R4~R11 before secure interrupt return. Armv8-M architecture will automatically unstack NSPE context from non-secure stack to overwrite other registers not banked, such as R0~R3 and R12, during secure interrupt return, before NSPE software can access those registers. When FPU is enabled in TF-M, with setting of FPCCR_S.TS = 1 and FPCCR_S.CLRONRET = 1, besides secure FP context in FP caller registers, FP context in callee registers will also be cleaned by hardware automatically during S exception return, to prevent NSPE from probing secure FP context in FP registers. Refer to Armv8-M Architecture Reference Manual [Arm-ARM] for details.
CVSS Score	4.3 (Medium)
CVSS Vector String	CVSS:3.1/AV:L/AC:L/PR:N/UI:N/S:C/C:L/I:N/A:N

Miscellaneous threats

This section collects threats irrelevant to the valid TF-M data flows shown above.

Table 24: TFM-GENERIC-STACK-SEAL

Index	TFM-GENERIC-STACK_SEAL
Description	Armv8-M processor Secure software Stack Sealing vulnerability.
Justification	On Armv8-M based processors with TrustZone, if Secure software does not properly manage the Secure stacks when the stacks are created, or when performing non-standard transitioning between states or modes, for example, creating a fake exception return stack frame to de-privilege an interrupt, it is possible for Non-secure world software to manipulate the Secure Stacks, and potentially influence Secure control flow. Refer to [STACK-SEAL] for details.
Category	Elevation of privilege
Mitigation	TF-M has implemented common mitigation against stack seal vulnerability. Refer to [ADVISORY-TFMV-1] for details on analysis and mitigation in TF-M.
CVSS Score	5.3 (Medium)
CVSS Vector String	CVSS:3.1/AV:L/AC:H/PR:L/UI:N/S:C/C:L/I:L/A:L

Table 25: TFM-GENERIC-SVC-CALL-SP-FETCH

Index	TFM-GENERIC-SVC-CALL-SP-FETCH
Description	Invoking Secure functions from handler mode may cause TF-M IPC model to behave unexpectedly.
Justification	On Armv8-M based processors with TrustZone, if NSPE calls a secure function via Secure Gateway (SG) from non-secure Handler mode, TF-M selects secure process stack by mistake for SVC handling. It will most likely trigger a crash in secure world or reset the whole system, with a very low likelihood of overwriting some memory contents.
Category	Denial of service/Tampering
Mitigation	TF-M has enhanced implementation to mitigate this vulnerability. Refer to [ADVISORY-TFMV-2] for details on analysis and mitigation in TF-M.
CVSS Score	4.5 (Medium)
CVSS Vector String	CVSS:3.1/AV:L/AC:H/PR:N/UI:N/S:C/C:N/I:L/A:L

Table 26: VLLDM instruction security vulnerability

Index	TFM-GENERIC-FP-VLLDM
Description	Secure data in FP registers may be disclosed to NSPE when VLLDM instruction is abandoned due to an exception mid-way.
Justification	Refer to [VLLDM-Vulnerability] for details.
Category	Tampering/Information disclosure
Mitigation	In current TF-M implementation, when FPU is enabled in SPE, TF-M configures NSACR to disable NSPE to access FPU. Therefore, secure data in FP registers is protected from NSPE. Refer to [VLLDM-Vulnerability], for details on analysis and mitigation.
CVSS Score	3.4 (Low)
CVSS Vector String	CVSS:3.1/AV:L/AC:L/PR:H/UI:N/S:U/C:L/I:L/A:N

Version control

Table 27: Version control

Version	Description	TF-M version
v0.1	Initial draft	TF-M v1.1
v1.0	First version	TF-M v1.2.0
v1.1	Update version	TF-M v1.5.0
v1.2	Update details to align FP support in NSPE.	TF-M v1.5.0
v1.3	Update for validity of dual-cpu model Armv8-M	TF-M v2.1.0

Reference

Security-Incident-Process

Security Incident Handling Process

FF-M(1,2,3,4,5,6)

Arm® Platform Security Architecture Firmware Framework 1.0

FF-M-1.1-Extensions

Arm® Firmware Framework for M 1.1 Extensions

DUAL-CPU-BOOT

Booting a dual core system

CVSS

Common Vulnerability Scoring System Version 3.1 Calculator

CVSS_SPEC(1,2,3)

CVSS v3.1 Specification Document

STRIDE

The STRIDE Threat Model

SECURE-BOOT

Secure boot

ROLLBACK-PROTECT

Rollback protection in TF-M secure boot

Arm-ARM(1,2)

Armv8-M Architecture Reference Manual

STACK-SEAL

Armv8-M processor Secure software Stack Sealing vulnerability

ADVISORY-TFMV-1

Advisory TFMV-1

ADVISORY-TFMV-2

Advisory TFMV-2

VLLDM-Vulnerability(1,2)

VLLDM instruction Security Vulnerability

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