Improper Protection for Out of Bounds Signal Level Alerts

Untrusted agents can disable alerts about signal conditions exceeding limits or the response mechanism that handles such alerts.


Description

Hardware sensors are used to detect whether a device is operating within design limits. The threshold values for these limits are set by hardware fuses or trusted software such as a BIOS. Modification of these limits may be protected by hardware mechanisms.

When device sensors detect out of bound conditions, alert signals may be generated for remedial action, which may take the form of device shutdown or throttling.

Warning signals that are not properly secured may be disabled or used to generate spurious alerts, causing degraded performance or denial-of-service (DoS). These alerts may be masked by untrusted software. Examples of these alerts involve thermal and power sensor alerts.

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 a platform design where a Digital-Thermal Sensor (DTS) is used to monitor temperature and compare that output against a threshold value. If the temperature output equals or exceeds the threshold value, the DTS unit sends an alert signal to the processor.

The processor, upon getting the alert, input triggers system shutdown. The alert signal is handled as a General-Purpose-I/O (GPIO) pin in input mode.

The processor-GPIO controller exposes software-programmable controls that allow untrusted software to reprogram the state of the GPIO pin.

Reprogramming the state of the GPIO pin allows malicious software to trigger spurious alerts or to set the alert pin to a zero value so that thermal sensor alerts are not received by the processor.

The GPIO alert-signal pin is blocked from untrusted software access and is controlled only by trusted software, such as the System BIOS.

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.

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Weaknesses without Software Fault Patterns

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Weaknesses Introduced During Implementation

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