Example - 1

Malicious software executes instructions to increase power consumption to the highest possible level while causing the clock frequency to increase to its maximum value.Such a program executing for an extended period of time would likely overheat the device, possibly resulting in permanent damage to the device.A ring, oscillator-based temperature sensor will generally report the sensed value asoscillator frequency rather than degrees centigrade. The temperature sensor will havecalibration values that are used to convert the detected frequency into the corresponding temperature in degrees centigrade.Consider a SoC design where the critical maximum temperature limit is set in fuse values to 100C andis not modifiable by software. If the scaled thermal sensor output equals or exceeds this limit, the system is commanded to shut itself down.The thermal sensor calibration values are programmable through registers that are exposed to system software.These registers allow software to affect the converted temperature output such that the output will never exceed the maximum temperature limit.

<xhtml_p>The sensor frequency value is scaled by applying the function:</xhtml_p>Sensed Temp = a + b * Sensor Freq<xhtml_p>where a and b are the programmable calibration data coefficients. Software sets a and b to zero ensuring the sensedtemperature is always zero.</xhtml_p>

This weakness may be addressed by preventing access to a and b.

<xhtml_p>The sensor frequency value is scaled by applying the function:</xhtml_p>Sensed Temp = a + b * Sensor Freq<xhtml_p>where a and b are the programmable calibration data coefficients. Untrusted software is prevented from changing the values of either a or b, preventing this method of manipulating the temperature.</xhtml_p>