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Improper Lock Behavior After Power State Transition

Incomplete Base
Structure: Simple
Description

This vulnerability occurs when a hardware lock bit, designed to protect critical system configuration registers, is improperly reset or becomes programmable after a device transitions between power states, such as entering or waking from a low-power sleep mode. This allows the protected configuration to be altered after it should be permanently locked.

Extended Description

Many hardware systems use a programmable lock bit to secure configuration settings. A trusted agent like the BIOS or bootloader sets this bit after initial configuration, which should permanently prevent further writes to sensitive registers. However, if this lock is cleared or the protected registers are reset when the device wakes from a sleep state or undergoes another power transition, the intended security mechanism fails. This flaw exposes the system to post-boot attacks where malicious software can alter low-level device settings that control memory, peripherals, or security features. Developers must verify that lock bit states and protected register values are preserved consistently across all designed power state transitions, including sleep, hibernation, and warm resets, to ensure runtime integrity.
Common Consequences 1
Scope: Access Control

Impact: Modify Memory
Potential Mitigations 1
Phase: Architecture and DesignImplementationTesting
- Security Lock bit protections should be reviewed for behavior across supported power state transitions. - Security lock programming flow and lock properties should be tested in pre-silicon and post-silicon testing including testing across power transitions.

Effectiveness: High
Demonstrative Examples 2
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.
The example code below is taken from the register lock module (reglk_wrapper) of the Hack@DAC'21 buggy OpenPiton System-on-Chip (SoC). Upon powering on, most of the silicon registers are initially unlocked. However, critical resources must be configured and locked by setting the lock bit in a register. In this module, a set of six memory-mapped I/O registers (reglk_mem) is defined and maintained to control the access control of registers inside different peripherals in the SoC [REF-1432]. Each bit represents a register's read/write ability or sets of registers inside a peripheral. Setting improper lock values after system power transition or system rest would make a temporary window for the attackers to read unauthorized data, e.g., secret keys from the crypto engine, and write illegitimate data to critical registers, e.g., framework data. Furthermore, improper register lock values can also result in DoS attacks. In this faulty implementation, the locks are disabled, i.e., initialized to zero, at reset instead of setting them to their appropriate values [REF-1433]. Improperly initialized locks might allow unauthorized access to sensitive registers, compromising the system's security.

Code Example:

Bad
Verilog

module reglk_wrapper #( ...

verilog

reglk_mem[j] <= 'h0;** end end ...

To resolve this issue, it is crucial to ensure that register locks are correctly initialized during the reset phase of the SoC. Correct initialization values should be established to maintain the system's integrity, security, and predictable behavior and allow for proper control of peripherals. The specifics of initializing register locks and their values depend on the SoC's design and the system's requirements; for example, access to all registers through the user privilege level should be locked at reset. To address the problem depicted in the bad code example [REF-1433], the default value for "reglk_mem" should be set to 32'hFFFFFFFF. This ensures that access to protected data is restricted during power state transition or after reset until the system state transition is complete and security procedures have properly configured the register locks.

Code Example:

Good
Verilog

module reglk_wrapper #( ...

verilog

reglk_mem[j] <= 'hffffffff;** end end ...
References 3
reglk_wrapper.sv
2021
https://github.com/HACK-EVENT/hackatdac21/blob/65d0ffdab7426da4509c98d62e163bcce642f651/piton/design/chip/tile/ariane/src/reglk/reglk_wrapper.sv#L39C1-L39C1
ID: REF-1432
Bad Code reglk_wrapper.sv
2021
https://github.com/HACK-EVENT/hackatdac21/blob/65d0ffdab7426da4509c98d62e163bcce642f651/piton/design/chip/tile/ariane/src/reglk/reglk_wrapper.sv#L78C1-L85C16
ID: REF-1433
Good Code reglk_wrapper.sv
2021
https://github.com/HACK-EVENT/hackatdac21/blob/5e2031fd3854bcc0b2ca11d13442542dd5ea98e0/piton/design/chip/tile/ariane/src/reglk/reglk_wrapper.sv#L83
ID: REF-1434
Applicable Platforms
Languages:
Not Language-Specific : Undetermined
Technologies:
Not Technology-Specific : Undetermined
Modes of Introduction
Architecture and Design
Implementation
Related Attack Patterns
Related Weaknesses
ChildOf: Improper Locking (CWE-667)