Improper Lock Behavior After Power State Transition
Register lock bit protection disables changes to system configuration once the bit is set. Some of the protected registers or lock bits become programmable after power state transitions (e.g., Entry and wake from low power sleep modes) causing the system configuration to be changeable.
Devices may allow device configuration controls which need to be programmed after device power reset via a trusted firmware or software module (commonly set by BIOS/bootloader) and then locked from any further modification. This action is commonly implemented using a programmable lock bit, which, when set, disables writes to a protected set of registers or address regions.
After a power state transition, the lock bit is set to unlocked. Some common weaknesses that can exist in such a protection scheme are that the lock gets cleared, the values of the protected registers get reset, or the lock become programmable.
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.
Consider the memory configuration settings of a system that uses DDR3 DRAM memory. Protecting the DRAM memory configuration from modification by software is required to ensure that system memory access control protections cannot be bypassed. This can be done by using lock bit protection that locks all of the memory configuration registers. The memory configuration lock can be set by the BIOS during the boot process.
If such a system also supports a rapid power on mode like hibernate, the DRAM data must be saved to a disk before power is removed and restored back to the DRAM once the system powers back up and before the OS resumes operation after returning from hibernate.
To support the hibernate transition back to the operating state, the DRAM memory configuration must be reprogrammed even though it was locked previously. As the hibernate resume does a partial reboot, the memory configuration could be altered before the memory lock is set. Functionally the hibernate resume flow requires a bypass of the lock-based protection. The memory configuration must be securely stored and restored by trusted system firmware. Lock settings and system configuration must be restored to the same state it was in before the device entered into the hibernate mode.
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.
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...
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This view (slice) lists weaknesses that can be introduced during implementation.
This view (slice) lists weaknesses that can be introduced during design.