Hardware Child Block Incorrectly Connected to Parent System

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

Many SoCs use hardware to partition system resources between trusted and un-trusted entities. One example of this concept is the Arm TrustZone, in which the processor and all security-aware IP attempt to isolate resources based on the status of a privilege bit. This privilege bit is part of the input interface in all TrustZone-aware IP. If this privilege bit is accidentally grounded or left unconnected when the IP is instantiated, privilege escalation of all input data may occur.

// IP definition
module tz_peripheral(clk, reset, data_in, data_in_security_level, ...);

  input clk, reset;
  input [31:0] data_in;
  input data_in_security_level;
  ...

endmodule
// Instantiation of IP in a parent system
module soc(...)

  ...
  tz_peripheral u_tz_peripheral(

    .clk(clk),
    .rst(rst),
    .data_in(rdata),
    //Copy-and-paste error or typo grounds data_in_security_level (in this example 0=secure, 1=non-secure) effectively promoting all data to "secure")
    .data_in_security_level(1'b0),

  );
  ...

endmodule

In the Verilog code below, the security level input to the TrustZone aware peripheral is correctly driven by an appropriate signal instead of being grounded.

// Instantiation of IP in a parent system
module soc(...)

  ...
  tz_peripheral u_tz_peripheral(

    .clk(clk),
    .rst(rst),
    .data_in(rdata),
    // This port is no longer grounded, but instead driven by the appropriate signal
    .data_in_security_level(rdata_security_level),

  );
  ...

endmodule

Example Two

Here is a code snippet from the Ariane core module in the HACK@DAC'21 Openpiton SoC [REF-1362]. To ensure full functional correctness, developers connect the ports with names. However, in some cases developers forget to connect some of these ports to the desired signals in the parent module. These mistakes by developers can lead to incorrect functional behavior or, in some cases, introduce security vulnerabilities.

...
csr_regfile #(

  ...

) csr_regfile_i (

  .flush_o             ( flush_csr_ctrl ),
  .halt_csr_o          ( halt_csr_ctrl ),
  ...
  .irq_i(),
  .time_irq_i(),
  .*

);
...

In the above example from HACK@DAC'21, since interrupt signals are not properly connected, the CSR module will fail to send notifications in the event of interrupts. Consequently, critical information in CSR registers that should be flushed or modified in response to an interrupt won't be updated. These vulnerabilities can potentially result in information leakage across various privilege levels.

To address the aforementioned vulnerability, developers must follow a two-step approach. First, they should ensure that all module signals are properly connected. This can often be facilitated using automated tools, and many simulators and sanitizer tools issue warnings when a signal remains unconnected or floats. Second, it is imperative to validate that the signals connected to a module align with the specifications. In the provided example, the developer should establish the correct connection of interrupt signals from the parent module (Ariane core) to the child module (csr_regfile) [REF-1363].

...
csr_regfile #(

  ...

) csr_regfile_i (

  .flush_o             ( flush_csr_ctrl ),
  .halt_csr_o          ( halt_csr_ctrl ),
  ...
  .irq_i			(irq_i),
  .time_irq_i	(time_irq_i),
  .*

);
...

See Also