Improper Prevention of Lock Bit Modification

The product uses a trusted lock bit for restricting access to registers, address regions, or other resources, but the product does not prevent the value of the lock bit from being modified after it has been set.


Description

In integrated circuits and hardware intellectual property (IP) cores, device configuration controls are commonly programmed after a device power reset by a trusted firmware or software module (e.g., BIOS/bootloader) and then locked from any further modification.

This behavior is commonly implemented using a trusted lock bit. When set, the lock bit disables writes to a protected set of registers or address regions. Design or coding errors in the implementation of the lock bit protection feature may allow the lock bit to be modified or cleared by software after it has been set. Attackers might be able to unlock the system and features that the bit is intended to protect.

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

Consider the example design below for a digital thermal sensor that detects overheating of the silicon and triggers system shutdown. The system critical temperature limit (CRITICAL_TEMP_LIMIT) and thermal sensor calibration (TEMP_SENSOR_CALIB) data have to be programmed by firmware, and then the register needs to be locked (TEMP_SENSOR_LOCK).

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. Slope value used to map sensor reading to degrees Centigrade.TEMP_SENSOR_LOCK[31:1] Reserved field; Read only; Default 0[0] Lock bit, locks CRITICAL_TEMP_LIMIT and TEMP_SENSOR_CALIB registers; Write-1-once; Default 0TEMP_HW_SHUTDOWN[31:2] Reserved field; Read only; Default 0[1] Enable hardware shutdown on 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 if the system heats to critical temperature, the response of the system is controlled by the TEMP_HW_SHUTDOWN bit [1], which is not lockable. Thus, the intended security property of the critical temperature sensor cannot be fully protected, since software can misconfigure the TEMP_HW_SHUTDOWN register even after the lock bit is set to disable the shutdown response.

To fix this weakness, one could change the TEMP_HW_SHUTDOWN field to be locked by TEMP_SENSOR_LOCK.TEMP_HW_SHUTDOWN[31:2] Reserved field; Read only; Default 0[1] Enable hardware shutdown on critical temperature detection; Read-write-Lock; Default 0[0] Locked by TEMP_SENSOR_LOCK

See Also

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Weaknesses in the 2021 CWE Most Important Hardware Weaknesses List

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