Improper Prevention of Lock Bit Modification

The product uses a trusted lock bit for restricting access to registers, address regions, or other resources, but the product does not prevent the value of the lock bit from being modified after it has been set.

Abstraction

Base

Structure

Simple

Likelihood of Exploit

Medium

Understand the problem

What is the problem?

Extended description

In integrated circuits and hardware intellectual property (IP) cores, device configuration controls are commonly programmed after a device power reset by a trusted firmware or software module (e.g., BIOS/bootloader) and then locked from any further modification. This behavior is commonly implemented using a trusted lock bit. When set, the lock bit disables writes to a protected set of registers or address regions. Design or coding errors in the implementation of the lock bit protection feature may allow the lock bit to be modified or cleared by software after it has been set. Attackers might be able to unlock the system and features that the bit is intended to protect.

Root cause

How and when does the problem arise?

Lifecycle phase

Architecture and Design

Such issues could be introduced during hardware architecture and design and identified later during Testing or System Configuration phases.

Lifecycle phase

Implementation

Such issues could be introduced during implementation and identified later during Testing or System Configuration phases.

Risk

What does successful exploitation lead to?

Modify Memory

Affected security scopes: Access Control

Registers protected by lock bit can be modified even when lock is set.

Defence

How is it prevented and fixed?

Architecture and Design · Implementation · Testing

Defence to apply across several lifecycle phases

Detail

- Security lock bit protections must be reviewed for design inconsistency and common weaknesses. - Security lock programming flow and lock properties must be tested in pre-silicon and post-silicon testing.

Effectiveness: High

Verification

How is it detected?

Manual Analysis

High

Set the lock bit. Power cycle the device. Attempt to clear the lock bit. If the information is changed, implement a design fix. Retest. Also, attempt to indirectly clear the lock bit or bypass it.