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.

What are the consequences of CWE-1232?

Exploiting CWE-1232 can lead to: Modify Memory.

CWE-1232: Improper Lock Behavior After Power State Transition

Code examples

Illustrative examples from MITRE showing how the weakness appears in code.

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.

Vulnerable example

verilog

always @(posedge clk_i)

Safe example

verilog

always @(posedge clk_i)

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.

CWE-1232: Improper Lock Behavior After Power State Transition

Overview

Common consequences

How it happens

When it is introduced

How to prevent it

Code examples

Attack patterns

Frequently asked questions

What is CWE-1232?

What are the consequences of CWE-1232?

References

Stay ahead of CWE-1232

Overview

Common consequences

How it happens

When it is introduced

How to prevent it

Code examples

Related weaknesses

Parent

Attack patterns

Frequently asked questions

What is CWE-1232?

What are the consequences of CWE-1232?

References

Stay ahead of CWE-1232