Security-Sensitive Hardware Controls with Missing Lock Bit Protection

The product uses a register lock bit protection mechanism, but it does not ensure that the lock bit prevents modification of system registers or controls that perform changes to important hardware system configuration.


Integrated circuits and hardware intellectual properties (IPs) might provide device configuration controls that need to be programmed after device power reset by a trusted firmware or software module, commonly set by BIOS/bootloader. After reset, there can be an expectation that the controls cannot be used to perform any further modification. This behavior is commonly implemented using a trusted lock bit, which can be set to disable 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).

However, if the lock bit does not effectively write-protect all system registers or controls that could modify the protected system configuration, then an adversary may be able to use software to access the registers/controls and modify the protected hardware configuration.


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 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[31:0] 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]

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