Insufficient Granularity of Address Regions Protected by Register Locks
The product defines a large address region protected from modification by the same register lock control bit. This results in a conflict between the functional requirement that some addresses need to be writable by software during operation and the security requirement that the system configuration lock bit must be set during the boot process.
Description
Integrated circuits and hardware IPs can expose the device configuration controls that need to be programmed after device power reset by a trusted firmware or software module (commonly set by BIOS/bootloader) and then locked from any further modification. In hardware design, this is commonly implemented using a programmable lock bit which enables/disables writing to a protected set of registers or address regions. When the programmable lock bit is set, the relevant address region can be implemented as a hardcoded value in hardware logic that cannot be changed later.
A problem can arise wherein the protected region definition is not granular enough. After the programmable lock bit has been set, then this new functionality cannot be implemented without change to the hardware design.
Demonstrations
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.
Example One
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.
AddressRegister0x0000SYS_LOCK: 32 bit system configuration lock register, each bit is write-1-once0x0004SECURITY_FEATURE_ENABLE: 32 bit register controlling enabling of security features...0x0310SW_MODE: 32 bit Software Mode indication registerAddress regionLock bit0x0000 - 0x03FFSYS_LOCK[0]0x0400 - 0x07FFSYS_LOCK[1]...0x7C00 - 0x7FFFSYS_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.
See Also
Weaknesses in this category are related to access control.
Weaknesses in this category are related to features and mechanisms providing hardware-based isolation and access control (e.g., identity, policy, locking control) of s...
This view (slice) covers all the elements in CWE.
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