CWE-1223: Race Condition for Write-Once Attributes

What is Race Condition for Write-Once Attributes?

• Overview: CWE-1223 refers to a vulnerability in hardware design where race conditions occur between trusted and untrusted software components accessing write-once registers, leading to untrusted components potentially setting critical hardware controls.

• Exploitation Methods:

• Attackers can exploit this vulnerability by timing their write operations to occur before the trusted software initializes the register.
• Common attack patterns include manipulating execution order or leveraging faster execution paths in untrusted components to preempt register initialization.

• Security Impact:

• Direct consequences of successful exploitation include unauthorized control over hardware settings, potentially leading to system instability or security bypasses.
• Potential cascading effects include the compromise of system integrity and confidentiality if critical settings are altered.
• Business impact could involve system downtime, data breaches, and loss of customer trust, leading to financial and reputational damage.

• Prevention Guidelines:

• Specific code-level fixes include implementing locking mechanisms or controls to ensure only trusted components can write to critical registers.
• Security best practices involve designing hardware with clear and enforced execution order for trusted components, and auditing access to critical registers.
• Recommended tools and frameworks include static analysis tools that can detect race conditions and hardware verification tools that ensure proper initialization and access control in hardware design.

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Technical Details

Likelihood of Exploit: Not specified

Affected Languages: Verilog, VHDL

Affected Technologies: System on Chip

Vulnerable Code Example

// Vulnerable code demonstrating a race condition for write-once attributes

module write_once_register (
    input logic clk,
    input logic reset,
    input logic [31:0] untrusted_input, // Untrusted source
    input logic [31:0] trusted_input,   // Trusted source
    output logic [31:0] write_once_reg
);

    logic write_once_locked;
    initial begin
        write_once_locked = 0; // Initially unlocked
    end

    always_ff @(posedge clk or posedge reset) begin
        if (reset) begin
            write_once_locked <= 0; // Reset lock state
            write_once_reg <= 32'b0;
        end else if (!write_once_locked) begin
            write_once_reg <= untrusted_input; // Vulnerable: Untrusted input can write before trusted input
            write_once_locked <= 1;            // Lock the register after first write
        end
    end

    always_ff @(posedge clk) begin
        if (!write_once_locked) begin
            write_once_reg <= trusted_input; // Vulnerable: Trusted input might not get a chance to write
            write_once_locked <= 1;
        end
    end

endmodule

How to fix Race Condition for Write-Once Attributes?

To fix this race condition, we should ensure that the write-once register is only initialized by a trusted source before any untrusted source can attempt to write to it. This can be achieved by implementing an initialization phase where only the trusted input is allowed to set the value of the register. After the initialization phase, the register should be locked to prevent any further writes.

The key change is to separate the trusted initialization process from the untrusted writing process and lock the register immediately after the trusted initialization, ensuring that untrusted sources cannot modify the register.

Fixed Code Example

// Fixed code implementing secure initialization and locking of write-once register

module write_once_register (
    input logic clk,
    input logic reset,
    input logic [31:0] untrusted_input, // Untrusted source
    input logic [31:0] trusted_input,   // Trusted source
    output logic [31:0] write_once_reg
);

    logic write_once_locked;
    initial begin
        write_once_locked = 0; // Initially unlocked for trusted input
    end

    always_ff @(posedge clk or posedge reset) begin
        if (reset) begin
            write_once_locked <= 0; // Reset lock state
            write_once_reg <= 32'b0;
        end else if (!write_once_locked) begin
            write_once_reg <= trusted_input; // Only allow trusted input to initialize
            write_once_locked <= 1;          // Lock the register after trusted initialization
        end
    end

    // Untrusted input is ignored, as the register is locked after trusted input
    always_ff @(posedge clk) begin
        // This block is effectively disabled due to the lock
        // Prevents any writes by untrusted sources
    end

endmodule

In the fixed code, the write-once register is securely initialized by the trusted input and locked immediately after, preventing any untrusted writes and effectively eliminating the race condition vulnerability. This ensures the integrity and security of the hardware design by restricting write access to only trusted entities during the initialization phase.