2097
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 AES_KEY_CONTROL_POLICY register defines which agents can write to the
AES_KEY_READ_POLICY or AES_KEY_WRITE_POLICY registers.
• The AES_KEY_READ_POLICY register defines which agents can read the AES-key
registers.
• The AES_KEY_WRITE_POLICY register defines which agents can write the AES key
registers.
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.
Example Language: (Bad)
IThe AES_KEY_CONTROL_POLICY register value is 0x00000018. In binary, the lower 8 bits will
be 0001 1000, meaning that:
• Bits 3 and 4 are set, thus Agents 3 and 4 will have write access to AES_KEY_READ_POLICY
or AES_KEY_WRITE_POLICY.
• All other bits are clear, hence agents other than 3 and 4 will not have access to write to
AES_KEY_READ_POLICY or AES_KEY_WRITE_POLICY.
The AES_KEY_READ_POLICY register value is 0x00000002. In binary, the lower 8 bits will be
0000 0010, meaning that:
• Bit 1 is set, thus Agent 1 will be able to read the AES key registers.
The AES_KEY_WRITE_POLICY register value is 0x00000004. In binary, the lower 8 bits will be
0000 0100, meaning that:
• Bit 2 is set, thus Agent 2 will be able to write the AES Key registers.
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.
CWE Version 4.14
CWE-1269: Product Released in Non-Release Configuration
CWE-1269: Product Released in Non-Release Configuration