Improper Hardware Lock Protection for Security Sensitive Controls

The product implements a register lock bit protection feature that permits security sensitive controls to modify the protected configuration.


Description

Integrated circuits and hardware IPs can expose the device configuration controls that need to be programmed after device power reset by a trusted firmware or software module (commonly set by BIOS/bootloader) and then locked from any further modification. This is commonly implemented using a trusted lock bit, which when set disables writes to a protected set of registers or address regions. The lock protection is intended to prevent modification of certain system configuration (e.g., memory/memory protection unit configuration). If any system registers/controls that can modify the protected configuration are not write-protected by the lock, they can then be leveraged by software to modify the protected configuration.

Demonstrations

The following examples help to illustrate the nature of this weakness and describe methods or techniques which can be used to mitigate the risk.

Note that the examples here are by no means exhaustive and any given weakness may have many subtle varieties, each of which may require different detection methods or runtime controls.

Example One

For example, consider the example design below or a digital thermal sensor used in the design to detect overheating of the silicon to trigger a system shutdown. The system critical temperature limit (CRITICAL_TEMP_LIMIT) and thermal sensor calibration (TEMP_SENSOR_CALIB) data have to be programmed by the firmware.

RegisterField descriptionCRITICAL_TEMP_LIMIT[31:8]
Reserved field;
Read only;
Default 0
[7:0]
Critical temp 0-255 Centigrade;
Read-write-lock; Default 125TEMP_SENSOR_CALIB[31:0]
Thermal sensor calibration data. A slope value used to map sensor reading to a degree Centigrade.
Read-write;
Default 25TEMP_SENSOR_LOCK[31:1]
Reserved field;
Read only;
Default 0 [0]   Lock bit, locks CRITICAL_TEMP_LIMIT register;
Write-1-once;
Default 0TEMP_HW_SHUTDOWN[31:2]
Reserved field;
Read only;
Default 0 [1] Enable hardware shutdown on a critical temperature detection;
Read-write;
Default 0CURRENT_TEMP[31:8]
Reserved field;
Read only;
Default 0
[7:0]
Current Temp 0-255 Centigrade;
Read-only;
Default 0

In this example note that only the CRITICAL_TEMP_LIMIT register is protected by the TEMP_SENSOR_LOCK bit, while the security design intent is to protect any modification of the critical temperature detection and response.

The response of the system, if the system heats to a critical temperature, is controlled by TEMP_HW_SHUTDOWN bit [1], which is not lockable. Also, the TEMP_SENSOR_CALIB register is not protected by the lock bit.

By modifying the temperature sensor calibration, the conversion of the sensor data to a degree centigrade can be changed, such that the current temperature will never be detected to exceed critical temperature value programmed by the protected lock.

Similarly, by modifying the TEMP_HW_SHUTDOWN.Enable bit, the system response detection of the current temperature exceeding critical temperature can be disabled.

Change TEMP_HW_SHUTDOWN and TEMP_SENSOR_CALIB controls to be locked by TEMP_SENSOR_LOCK.TEMP_SENSOR_CALIB[31:8]
Thermal sensor calibration data. A slope value used to map sensor reading to a degree Centigrade.
Read-write-Lock;
Default 25
Locked by TEMP_SENSOR_LOCK bit[0]TEMP_HW_SHUTDOWN[31:2]
Reserved field;
Read only;
Default 0
[1] Enable hardware shutdown on critical temperature detection;
Read-write-Lock;
Default 0
Locked by TEMP_SENSOR_LOCK bit[0]

See Also

General Circuit and Logic Design Concerns

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Comprehensive CWE Dictionary

This view (slice) covers all the elements in CWE.

Entries with Maintenance Notes

CWE entries in this view have maintenance notes. Maintenance notes are an indicator that an entry might change significantly in future versions. This view was created...

Weaknesses without Software Fault Patterns

CWE identifiers in this view are weaknesses that do not have associated Software Fault Patterns (SFPs), as covered by the CWE-888 view. As such, they represent gaps in...


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