Missing Security Identifier

In a System-On-Chip (SoC), various integrated circuits and hardware engines generate transactions such as to access (reads/writes) assets or perform certain actions (e.g., reset, fetch, compute). A typical transaction is comprised of source identity (to identify the originator of the transaction) and a destination identity (to route the transaction to the respective entity) in addition to much more information in the message. Sometimes the transactions are qualified with a Security Identifier. This Security Identifier helps the destination agent decide on the set of allowed or disallowed actions.

A common weakness that can exist in such transaction schemes is that the source agent fails to include the necessary, security identifier with the transaction. Because of the missing security identifier, the destination agent might drop the message, thus resulting in Denial-of-Service (DoS), or get confused in its attempt to execute the given action, which confusion could result in privilege escalation or a gain of unintended access.

Consider a system with a register for storing AES key for encryption or decryption. The key is of 128 bits implemented as a set of four 32-bit registers. The key registers are assets, and the register AES_KEY_ACCESS_POLICY is defined to provide the necessary access controls.

The access-policy register defines which agents with a security identifier in the transaction can access the AES-key registers. Each bit in this 32-bit register defines a security identifier. There could be a maximum of 32 security identifiers that are allowed accesses to the AES-key registers. The number of the bit when set (i.e., "1") allows for a respective action from an agent whose identity matches the number of the bit; if set to "0" (i.e., Clear), it disallows the respective action to that corresponding agent.