Policy Privileges are not Assigned Consistently Between Control and Data Agents

Integrated circuits and hardware engines may provide access to resources (device-configuration, encryption keys, etc.) belonging to trusted firmware or software modules (commonly set by a BIOS or a bootloader). These accesses are typically controlled and limited by the hardware. Hardware design access control is sometimes implemented using a policy. A policy defines which entity or agent may or may not be allowed to perform an action. When a system implements multiple levels of policies, a control policy may allow direct access to a resource as well as changes to the policies themselves.

Resources that include agents in their control policy but not in their write policy could unintentionally allow an untrusted agent to insert itself in the write policy register. Inclusion in the write policy register could allow a malicious or misbehaving agent write access to resources. This action could result in security compromises including leaked information, leaked encryption keys, or modification of device configuration.

Consider a system of seven registers for storing and configuring an AES key for encryption or decryption.

Four 32-bit registers are used to store a 128-bit AES key. The names of those registers are AES_ENC_DEC_KEY_0, AES_ENC_DEC_KEY_1, AES_ENC_DEC_KEY_2, and AES_ENC_DEC_KEY_3. Collectively these are referred to as the AES Key registers.

Three 32-bit registers are used to define access control for the AES-key registers. The names of those registers are AES_KEY_CONTROL_POLICY, AES_KEY_READ_POLICY, and AES_KEY_WRITE_POLICY. Collectively these registers are referred to as the Policy registers, and their functions are explained next.

The preceding three policy registers encode access control at the bit level. Therefore a maximum of 32 agents can be defined (1 bit per agent). The value of the bit when set (i.e., "1") allows the respective action from an agent whose identity corresponds to the number of the bit. If clear (i.e., "0"), it disallows the respective action to that corresponding agent. For example, if bit 0 is set to "1" in the AES_KEY_READ_POLICY register, then agent 0 has permission to read the AES-key registers.

Consider that there are 4 agents named Agent 1, Agent 2, Agent 3, and Agent 4. For access control purposes Agent 1 is assigned to bit 1, Agent 2 to bit 2, Agent 3 to bit 3, and Agent 4 to bit 4. All agents are trusted except for Agent 3 who is untrusted. Also consider the register values in the below table.

IThe AES_KEY_CONTROL_POLICY register value is 0x00000018. In binary, the lower 8 bits will be 0001 1000, meaning that:

The AES_KEY_READ_POLICY register value is 0x00000002. In binary, the lower 8 bits will be 0000 0010, meaning that:

The AES_KEY_WRITE_POLICY register value is 0x00000004. In binary, the lower 8 bits will be 0000 0100, meaning that:

The configured access control policy for Agents 1,2,3,4 is summarized in table below.

At this point Agents 3 and 4 can only configure which agents can read AES keys and which agents can write AES keys. Agents 3 and 4 cannot read or write AES keys - just configure access control.

Now, recall Agent 3 is untrusted. As explained above, the value of the AES_KEY_CONTROL_POLICY register gives agent 3 access to write to the AES_KEY_WRITE_POLICY register. Agent 3 can use this write access to add themselves to the AES_KEY_WRITE_POLICY register. This is accomplished by Agent 3 writing the value 0x00000006. In binary, the lower 8 bits are 0000 0110, meaning that bit 3 will be set. Thus, giving Agent 3 having the ability to write to the AES Key registers.

If the AES_KEY_CONTROL_POLICY register value is 0x00000010, the lower 8 bits will be 0001 0000. This will give Agent 4, a trusted agent, write access to AES_KEY_WRITE_POLICY, but Agent 3, who is untrusted, will not have write access. The Policy register values should therefore be as follows: