Missing Write Protection for Parametric Data Values

The device does not write-protect the parametric data values for sensors that scale the sensor value, allowing untrusted software to manipulate the apparent result and potentially damage hardware or cause operational failure.


Description

Various sensors are used by hardware to detect any devices operating outside of the design limits. The threshold limit values are set by hardware fuses or trusted software such as the BIOS. These limits may be related to thermal, power, voltage, current, and frequency. Hardware mechanisms may be used to protect against alteration of the threshold limit values by untrusted software.

The limit values are generally programmed in standard units for the type of value being read. However, the hardware-sensor blocks may report the settings in different units depending upon sensor design and operation. The raw sensor output value is converted to the desired units using a scale conversion based on the parametric data programmed into the sensor. The final converted value is then compared with the previously programmed limits.

While the limit values are usually protected, the sensor parametric data values may not be. By changing the parametric data, safe operational limits may be bypassed.

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

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 as oscillator frequency rather than degrees centigrade. The temperature sensor will have calibration 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 and is 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.

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

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

The sensor frequency value is scaled by applying the function:
  Sensed Temp = a + b * Sensor Freq
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.

See Also

Power, Clock, and Reset Concerns

Weaknesses in this category are related to system power, voltage, current, temperature, clocks, system state saving/restoring, and resets at the platform and SoC level.

Privilege Separation and Access Control Issues

Weaknesses in this category are related to features and mechanisms providing hardware-based isolation and access control (e.g., identity, policy, locking control) of s...

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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