Improper Implementation of Lock Protection Registers

The product incorrectly implements register lock bit protection features such that protected controls can be programmed even after the lock has been set.


Description

In integrated circuits and hardware IPs, 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 is commonly implemented using a trusted lock bit, which when set 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 being set to unlock the system.

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 or a digital thermal sensor used in the design to detect overheating of the silicon and trigger 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 degree 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 the response of the system if the system heats to critical temperature is controlled by 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.

Change 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

General Circuit and Logic Design Concerns

Weaknesses in this category are related to hardware-circuit design and logic (e.g., CMOS transistors, finite state machines, and registers) as well as issues related t...

Comprehensive CWE Dictionary

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

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...

Weaknesses Introduced During Implementation

This view (slice) lists weaknesses that can be introduced during implementation.


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