Improper Prevention of Lock Bit Modification

Description

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

Example Two

The following example code is a snippet from the register locks inside the buggy OpenPiton SoC of HACK@DAC'21 [REF-1350]. Register locks help prevent SoC peripherals' registers from malicious use of resources. The registers that can potentially leak secret data are locked by register locks.

In the vulnerable code, the reglk_mem is used for locking information. If one of its bits toggle to 1, the corresponding peripheral's registers will be locked. In the context of the HACK@DAC System-on-Chip (SoC), it is pertinent to note the existence of two distinct categories of reset signals.

First, there is a global reset signal denoted as "rst_ni," which possesses the capability to simultaneously reset all peripherals to their respective initial states.

Second, we have peripheral-specific reset signals, such as "rst_9," which exclusively reset individual peripherals back to their initial states. The administration of these reset signals is the responsibility of the reset controller module.

always @(posedge clk_i)

    begin

      if(~(rst_ni && ~jtag_unlock && ~rst_9))

        begin
          for (j=0; j < 6; j=j+1) begin
            reglk_mem[j] <= 'h0;

        end



  end
  ...

In the buggy SoC architecture during HACK@DAC'21, a critical issue arises within the reset controller module. Specifically, the reset controller can inadvertently transmit a peripheral reset signal to the register lock within the user privilege domain.

This unintentional action can result in the reset of the register locks, potentially exposing private data from all other peripherals, rendering them accessible and readable.

To mitigate the issue, remove the extra reset signal rst_9 from the register lock if condition. [REF-1351]

always @(posedge clk_i)

    begin

      if(~(rst_ni && ~jtag_unlock))

        begin
          for (j=0; j < 6; j=j+1) begin
            reglk_mem[j] <= 'h0;

        end



  end
  ...

See Also