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Insufficient Granularity of Address Regions Protected by Register Locks

Incomplete Variant
Structure: Simple
Description

This vulnerability occurs when a hardware design uses a single lock bit to protect a large, coarse block of memory addresses. This lack of granularity creates a conflict: the system needs to lock critical configuration settings early for security, but software also needs to write to other parts of that same address region during normal operation, which the lock incorrectly prevents.

Extended Description

In many systems, a trusted boot process (like the BIOS or bootloader) sets a hardware lock bit to permanently freeze critical configuration registers, preventing malware or errors from altering them later. This lock typically protects an entire predefined address region, not individual registers. However, if this protected region is too large and not finely segmented, it becomes a security and functionality trade-off. Setting the lock too early can block legitimate software from updating necessary settings within that same region, while not setting it leaves the system vulnerable. Fixing this conflict after manufacturing requires a hardware redesign, as the lock granularity is baked into the silicon.
Common Consequences 1
Scope: Access Control

Impact: Other

System security configuration cannot be defined in a way that does not conflict with functional requirements of device.
Potential Mitigations 1
Phase: Architecture and Design
The defining of protected locked registers should be reviewed or tested early in the design phase with software teams to ensure software flows are not blocked by the security locks. As an alternative to using register lock control bits and fixed access control regions, the hardware design could use programmable security access control configuration so that device trusted firmware can configure and change the protected regions based on software usage and security models.
Demonstrative Examples 1
For example, consider a hardware unit with a 32 kilobyte configuration address space where the first 8 kilobyte address contains security sensitive controls that must only be writable by device bootloader. One way to protect the security configuration could be to define a 32 bit system configuration locking register (SYS_LOCK) where each bit lock locks the corresponding 1 kilobyte region.

Code Example:

Bad
Other
AddressRegister
0x0000SYS_LOCK: 32 bit system configuration lock register, each bit is write-1-once
0x0004SECURITY_FEATURE_ENABLE: 32 bit register controlling enabling of security features
...
0x0310SW_MODE: 32 bit Software Mode indication register
other

| --- | --- | | 0x0000 - 0x03FF | SYS_LOCK[0] | | 0x0400 - 0x07FF | SYS_LOCK[1] | | ... | | | 0x7C00 - 0x7FFF | SYS_LOCK[31] |

If a register exists within the first kilobyte address range (e.g. SW_MODE, address 0x310) and needs to be software writable at runtime, then this register cannot be written in a securely configured system since SYS_LOCK register lock bit 0 must be set to protect other security settings (e.g. SECURITY_FEATURE_ENABLE, address 0x0004). The only fix would be to change the hardware logic or not set the security lock bit.
Applicable Platforms
Languages:
Not Language-Specific : Undetermined
Technologies:
System on Chip : Undetermined
Modes of Introduction
Architecture and Design
Related Attack Patterns
Related Weaknesses
ChildOf: Insufficient Granularity of Access Control (CWE-1220)