Reliance on Component That is Not Updateable

Description

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

A refrigerator has an Internet interface for the official purpose of alerting the manufacturer when that refrigerator detects a fault. Because the device is attached to the Internet, the refrigerator is a target for hackers who may wish to use the device other potentially more nefarious purposes.

The refrigerator has no means of patching and is hacked becoming a spewer of email spam.

The device automatically patches itself and provides considerable more protection against being hacked.

Example Two

ROM does not have built-in application-programming interfaces (APIs) to patch if the code is vulnerable. Implement mechanisms to patch the vulnerable ROM code.

Example Three

The example code is taken from the JTAG module of the buggy OpenPiton SoC of HACK@DAC'21. JTAG is protected with a password checker. Access to JTAG operations will be denied unless the correct password is provided by the user. This user-provided password is first sent to the HMAC module where it is hashed with a secret crypto key. This user password hash (pass_hash) is then compared with the hash of the correct password (exp_hash). If they match, JTAG will then be unlocked.

module dmi_jtag(...)(...);
...



      PassChkValid: begin
      if(hashValid) begin


          if(exp_hash == pass_hash) begin

            pass_check = 1'b1;

          end else begin

            pass_check = 1'b0;

          end
          state_d = Idle;

        end else begin
        state_d = PassChkValid;
        end

      end



...

  hmac hmac(

...


    .key_i(256'h24e6fa2254c2ff632a41b...),


...

  );

...
endmodule

However, the SoC's crypto key is hardcoded into the design and cannot be updated [REF-1387]. Therefore, if the key is leaked somehow, there is no way to reprovision the key without having the device replaced.

To fix this issue, a local register should be used (hmac_key_reg) to store the crypto key. If designers need to update the key, they can upload the new key through an input port (hmac_key_i) to the local register by enabling the patching signal (hmac_patch_en) [REF-1388].

module dmi_jtag(...
) (

  input logic [255:0] hmac_key_i,
  input logic         hmac_patch_en,
  ...
  reg [255:0] hmac_key_reg;
  ...

);
...

  always_ff @(posedge tck_i or negedge trst_ni) begin
  ...
  if (hmac_patch_en)

    hmac_key_reg <= hmac_key_i;

  ...
  end

...

  hmac hmac(
  ...
  .key_i(hmac_key_reg),
  ...
  );

...
endmodule

See Also