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diff --git a/docs/threat_model/threat_model.rst b/docs/threat_model/threat_model.rst new file mode 100644 index 0000000..0da2558 --- /dev/null +++ b/docs/threat_model/threat_model.rst @@ -0,0 +1,1103 @@ +Generic Threat Model +******************** + +************ +Introduction +************ + +This document provides a generic threat model for TF-A firmware. + +.. _Target of Evaluation: + +******************** +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. + +- 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. + + +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_dfd.puml + :caption: Figure 1: TF-A Data Flow Diagram + +.. table:: 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 or | + | | informative messages over a UART interface. | + | | | + | | | Also, characters can be read from 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: + +*************** +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:: 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:: 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 occurring 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:: Table 4: Rating and score as applied to impact and likelihood + + +-----------------------+-------------------------+---------------------------+ + | **Rating (Score)** | **Impact** | **Likelihood** | + +=======================+=========================+===========================+ + | Critical (5) | | Extreme impact to | | Threat is almost | + | | entire organization | certain to be exploited.| + | | if exploited. | | + | | | | Knowledge of the threat | + | | | and how to exploit it | + | | | are in the public | + | | | domain. | + +-----------------------+-------------------------+---------------------------+ + | High (4) | | Major impact to entire| | Threat is relatively | + | | organization or single| easy to detect and | + | | line of business if | exploit by an attacker | + | | exploited | with little skill. | + +-----------------------+-------------------------+---------------------------+ + | Medium (3) | | Noticeable impact to | | A knowledgeable insider | + | | line of business if | or expert attacker could| + | | exploited. | exploit the threat | + | | | without much difficulty.| + +-----------------------+-------------------------+---------------------------+ + | Low (2) | | Minor damage if | | Exploiting the threat | + | | exploited or could | would require | + | | be used in conjunction| considerable expertise | + | | with other | and resources | + | | vulnerabilities to | | + | | perform a more serious| | + | | attack | | + +-----------------------+-------------------------+---------------------------+ + | Informational (1) | | Poor programming | | Threat is not likely | + | | practice or poor | to be exploited on its | + | | design decision that | own, but may be used to | + | | may not represent an | gain information for | + | | immediate risk on its | launching another | + | | own, but may have | attack | + | | security implications | | + | | if multiplied and/or | | + | | combined with other | | + | | threats. | | + +-----------------------+-------------------------+---------------------------+ + +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:: 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. + +When such conditions must be met, these are highlighted in the ``Mitigations +implemented?`` box. + +As our :ref:`Target of Evaluation` is made of several, distinct firmware images, +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 +--------------------------------------- + ++------------------------+---------------------------------------------------+ +| ID | 05 | ++========================+===================================================+ +| Threat | | **Information leak via UART logs** | +| | | +| | | During the development stages of software it is | +| | common to print all sorts of information on the | +| | console, including sensitive or confidential | +| | information such as crash reports with detailed | +| | information of the CPU state, current registers | +| | values, privilege level or 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 enabled in | +| | the production version. | +| | | +| | | This happens when directly logging sensitive | +| | information and more subtly when logging | +| | side-channel information that can be used by an | +| | attacker to learn about sensitive information. | ++------------------------+---------------------------------------------------+ +| Diagram Elements | DF2 | ++------------------------+---------------------------------------------------+ +| Affected TF-A | BL1, BL2, BL31 | +| Components | | ++------------------------+---------------------------------------------------+ +| 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 sensitive information logging in | +| | production releases. | +| | | +| | | Do not conditionally log information depending | +| | on potentially sensitive data. | +| | | +| | | Do not log high precision timing information. | ++------------------------+---------------------------------------------------+ +| Mitigations | | Yes / Platform Specific. | +| implemented? | 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 | BL1, BL2, BL31 | +| Components | | ++------------------------+----------------------------------------------------+ +| 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 | | Platform specific. | +| implemented? | | +| | | Configuration of debug and trace capabilities is | +| | entirely platform specific. | ++------------------------+----------------------------------------------------+ + ++------------------------+------------------------------------------------------+ +| 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 | BL1, BL2, BL31 | +| Components | | ++------------------------+------------------------------------------------------+ +| 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 | | 1) Yes. | +| implemented? | 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 | 11 | ++========================+====================================================+ +| Threat | | **Misconfiguration of the Memory Management Unit | +| | (MMU) may allow a normal world software to | +| | access sensitive data, execute arbitrary | +| | code or access otherwise restricted HW | +| | interface** | +| | | +| | | 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 | BL1, BL2, BL31 | +| Components | | ++------------------------+----------------------------------------------------+ +| 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 | | Platform specific. | +| implemented? | | +| | | 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 | 13 | ++========================+=====================================================+ +| Threat | | **Leaving sensitive information in the memory, | +| | can allow an attacker to retrieve them.** | +| | | +| | | Accidentally leaving not-needed sensitive data in | +| | internal buffers can leak them if an attacker | +| | gains access to memory due to a vulnerability. | ++------------------------+-----------------------------------------------------+ +| Diagram Elements | DF4, DF5 | ++------------------------+-----------------------------------------------------+ +| Affected TF-A | BL1, BL2, BL31 | +| Components | | ++------------------------+-----------------------------------------------------+ +| Assets | Sensitive Data | ++------------------------+-----------------------------------------------------+ +| Threat Agent | NSCode, SecCode | ++------------------------+-----------------------------------------------------+ +| Threat Type | Information Disclosure | ++------------------------+-------------------+----------------+----------------+ +| 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 | Clear the sensitive data from internal buffers as | +| | soon as they are not needed anymore. | ++------------------------+-----------------------------------------------------+ +| Mitigations | | Yes / Platform specific | +| implemented? | | ++------------------------+-----------------------------------------------------+ + + ++------------------------+-----------------------------------------------------+ +| ID | 15 | ++========================+=====================================================+ +| Threat | | **Improper handling of input data received over | +| | a UART interface may allow an attacker to tamper | +| | with TF-A execution environment.** | +| | | +| | | The consequences of the attack depend on the | +| | the exact usage of input data received over UART. | +| | Examples are injection of arbitrary data, | +| | sensitive data tampering, influencing the | +| | execution path, denial of service (if using | +| | blocking I/O). This list may not be exhaustive. | ++------------------------+-----------------------------------------------------+ +| Diagram Elements | DF2, DF4, DF5 | ++------------------------+-----------------------------------------------------+ +| Affected TF-A | BL1, BL2, BL31 | +| Components | | ++------------------------+-----------------------------------------------------+ +| Assets | Sensitive Data, Code Execution, Availability | ++------------------------+-----------------------------------------------------+ +| Threat Agent | NSCode, SecCode | ++------------------------+-----------------------------------------------------+ +| Threat Type | Tampering, Information Disclosure, Denial of | +| | service, 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 | | By default, the code to read input data from UART | +| | interfaces is disabled (see `ENABLE_CONSOLE_GETC` | +| | build option). It should only be enabled on a | +| | need basis. | +| | | +| | | Data received over UART interfaces should be | +| | treated as untrusted data. As such, it should be | +| | properly sanitized and handled with caution. | ++------------------------+-----------------------------------------------------+ +| Mitigations | | Platform specific. | +| implemented? | | +| | | Generic code does not read any input data from | +| | UART interface(s). | ++------------------------+-----------------------------------------------------+ + + +.. _Boot Firmware Threats: + +Threats to be Mitigated by the Boot Firmware +-------------------------------------------- + +The boot firmware here refers to the boot ROM (BL1) and the trusted boot +firmware (BL2). Typically it does not stay resident in memory and it is +dismissed once execution has reached the runtime EL3 firmware (BL31). Thus, past +that point in time, the threats below can no longer be exploited. + +Note, however, that this is not necessarily true on all platforms. Platform +vendors should review these threats to make sure they cannot be exploited +nonetheless once execution has reached the runtime EL3 firmware. + ++------------------------+----------------------------------------------------+ +| 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 | BL2, BL31 | +| Components | | ++------------------------+----------------------------------------------------+ +| 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 | | 1) Yes, provided that the ``TRUSTED_BOARD_BOOT`` | +| implemented? | 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 | BL2, BL31 | +| Components | | ++------------------------+----------------------------------------------------+ +| 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 | | Yes / Platform specific. | +| implemented? | | +| | | 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 | BL1, BL2 | +| Components | | ++------------------------+-------------------------------------------------------+ +| 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 | | Platform specific. | +| implemented? | | +| | | 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 | BL1, BL2 | +| Components | | ++------------------------+-------------------------------------------------------+ +| 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 | | No. | +| implemented? | | +| | | 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.** | ++------------------------+-------------------------------------------------------+ + +.. topic:: Measured Boot Threats (or lack of) + + In the current Measured Boot design, BL1, BL2, and BL31, as well as the + secure world components, form the |SRTM|. Measurement data is currently + considered an asset to be protected against attack, and this is achieved + by storing them in the Secure Memory. + Beyond the measurements stored inside the TCG-compliant Event Log buffer, + there are no other assets to protect or threats to defend against that + could compromise |TF-A| execution environment's security. + + There are general security assets and threats associated with remote/delegated + attestation. However, these are outside the |TF-A| security boundary and + should be dealt with by the appropriate agent in the platform/system. + Since current Measured Boot design does not use local attestation, there would + be no further assets to protect(like unsealed keys). + + A limitation of the current Measured Boot design is that it is dependent upon + Secure Boot as implementation of Measured Boot does not extend measurements + into a discrete |TPM|, where they would be securely stored and protected + against tampering. This implies that if Secure-Boot is compromised, Measured + Boot may also be compromised. + + Platforms must carefully evaluate the security of the default implementation + since the |SRTM| includes all secure world components. + + +.. _Runtime Firmware Threats: + +Threats to be Mitigated by the Runtime EL3 Firmware +--------------------------------------------------- + ++------------------------+------------------------------------------------------+ +| 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 | BL31 | +| Components | | ++------------------------+------------------------------------------------------+ +| 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 | | Yes / Platform specific. | +| implemented? | | +| | | For standard services, all input is validated. | +| | | +| | | Platforms that implement SiP services must also | +| | validate SMC call arguments. | ++------------------------+------------------------------------------------------+ + + ++------------------------+------------------------------------------------------+ +| 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 | BL31 | +| Components | | ++------------------------+------------------------------------------------------+ +| 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 | | Yes. | +| implemented? | | +| | | 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 | BL31 | +| Components | | ++------------------------+-----------------------------------------------------+ +| 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 | | Yes / Platform specific. | +| implemented? | | +| | | 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 | 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 | BL31 | +| Components | | ++------------------------+-----------------------------------------------------+ +| Assets | Sensitive Data | ++------------------------+-----------------------------------------------------+ +| Threat Agent | NSCode | ++------------------------+-----------------------------------------------------+ +| Threat Type | Information Disclosure | ++------------------------+-------------------+----------------+----------------+ +| Application | Server | IoT | Mobile | ++------------------------+-------------------+----------------+----------------+ +| 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 | | Yes / platform specific. | +| implemented? | | +| | | 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. | ++------------------------+-----------------------------------------------------+ + + +Threats to be Mitigated by an External Agent Outside of TF-A +------------------------------------------------------------ + ++------------------------+-----------------------------------------------------+ +| ID | 14 | ++========================+=====================================================+ +| Threat | | **Attacker wants to execute an arbitrary or | +| | untrusted binary as the secure OS.** | +| | | +| | | When the option OPTEE_ALLOW_SMC_LOAD is enabled, | +| | this trusts the non-secure world up until the | +| | point it issues the SMC call to load the Secure | +| | BL32 payload. If a compromise occurs before the | +| | SMC call is invoked, then arbitrary code execution| +| | in S-EL1 can occur or arbitrary memory in EL3 can | +| | be overwritten. | ++------------------------+-----------------------------------------------------+ +| Diagram Elements | DF5 | ++------------------------+-----------------------------------------------------+ +| Affected TF-A | BL31, BL32 | +| Components | | ++------------------------+-----------------------------------------------------+ +| Assets | Code Execution, Sensitive Data | ++------------------------+-----------------------------------------------------+ +| Threat Agent | NSCode | ++------------------------+-----------------------------------------------------+ +| Threat Type | Tampering, 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 enabling the option OPTEE_ALLOW_SMC_LOAD, | +| | the non-secure OS must be considered a closed | +| | platform up until the point the SMC can be invoked | +| | to load OP-TEE. | ++------------------------+-----------------------------------------------------+ +| Mitigations | | None in TF-A itself. This option is only used by | +| implemented? | ChromeOS currently which has other mechanisms to | +| | to mitigate this threat which are described in | +| | `OP-TEE Dispatcher`_. | ++------------------------+-----------------------------------------------------+ + +-------------- + +*Copyright (c) 2021-2023, Arm Limited. All rights reserved.* + + +.. _STRIDE threat analysis technique: https://docs.microsoft.com/en-us/azure/security/develop/threat-modeling-tool-threats#stride-model +.. _DEN0034: https://developer.arm.com/documentation/den0034/latest +.. _Cache Speculation Side-channels: https://developer.arm.com/support/arm-security-updates/speculative-processor-vulnerability +.. _Spectre: https://developer.arm.com/support/arm-security-updates/speculative-processor-vulnerability +.. _TBBR-Client specification: https://developer.arm.com/documentation/den0006/d/ +.. _Trusted Board Boot (TBB): https://trustedfirmware-a.readthedocs.io/en/latest/design/trusted-board-boot.html +.. _TF-A error handling policy: https://trustedfirmware-a.readthedocs.io/en/latest/process/coding-guidelines.html#error-handling-and-robustness +.. _Secure Development Guidelines: https://trustedfirmware-a.readthedocs.io/en/latest/process/security-hardening.html#secure-development-guidelines +.. _Trusted Firmware-A Tests: https://git.trustedfirmware.org/TF-A/tf-a-tests.git/about/ +.. _OP-TEE Dispatcher: https://github.com/ARM-software/arm-trusted-firmware/blob/master/docs/components/spd/optee-dispatcher.rst |