Use of Incorrect Operator

Description

These types of errors are generally the result of a typo by the programmer.

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

The following C/C++ and C# examples attempt to validate an int input parameter against the integer value 100.

int isValid(int value) {
  if (value=100) {
    printf("Value is valid\n");
    return(1);
  }
  printf("Value is not valid\n");
  return(0);
}

bool isValid(int value) {
  if (value=100) {
    Console.WriteLine("Value is valid.");
    return true;
  }
  Console.WriteLine("Value is not valid.");
  return false;
}

However, the expression to be evaluated in the if statement uses the assignment operator "=" rather than the comparison operator "==". The result of using the assignment operator instead of the comparison operator causes the int variable to be reassigned locally and the expression in the if statement will always evaluate to the value on the right hand side of the expression. This will result in the input value not being properly validated, which can cause unexpected results.

Example Two

The following C/C++ example shows a simple implementation of a stack that includes methods for adding and removing integer values from the stack. The example uses pointers to add and remove integer values to the stack array variable.

#define SIZE 50
int *tos, *p1, stack[SIZE];

void push(int i) {

  p1++;
  if(p1==(tos+SIZE)) {


    // Print stack overflow error message and exit


  }
  *p1 == i;

}

int pop(void) {

  if(p1==tos) {


    // Print stack underflow error message and exit


  }
  p1--;
  return *(p1+1);

}

int main(int argc, char *argv[]) {


  // initialize tos and p1 to point to the top of stack
  tos = stack;
  p1 = stack;
  // code to add and remove items from stack
  ...
  return 0;

}

The push method includes an expression to assign the integer value to the location in the stack pointed to by the pointer variable.

However, this expression uses the comparison operator "==" rather than the assignment operator "=". The result of using the comparison operator instead of the assignment operator causes erroneous values to be entered into the stack and can cause unexpected results.

Example Three

The example code below is taken from the CVA6 processor core of the HACK@DAC'21 buggy OpenPiton SoC. Debug access allows users to access internal hardware registers that are otherwise not exposed for user access or restricted access through access control protocols. Hence, requests to enter debug mode are checked and authorized only if the processor has sufficient privileges. In addition, debug accesses are also locked behind password checkers. Thus, the processor enters debug mode only when the privilege level requirement is met, and the correct debug password is provided.

The following code [REF-1377] illustrates an instance of a vulnerable implementation of debug mode. The core correctly checks if the debug requests have sufficient privileges and enables the debug_mode_d and debug_mode_q signals. It also correctly checks for debug password and enables umode_i signal.

module csr_regfile #(
...

  // check that we actually want to enter debug depending on the privilege level we are currently in
  unique case (priv_lvl_o)

    riscv::PRIV_LVL_M: begin

      debug_mode_d = dcsr_q.ebreakm;



...


    riscv::PRIV_LVL_U: begin

      debug_mode_d = dcsr_q.ebreaku;



...

  assign priv_lvl_o = (debug_mode_q || umode_i) ?  riscv::PRIV_LVL_M : priv_lvl_q;

...

  debug_mode_q  <= debug_mode_d;

...

However, it grants debug access and changes the privilege level, priv_lvl_o, even when one of the two checks is satisfied and the other is not. Because of this, debug access can be granted by simply requesting with sufficient privileges (i.e., debug_mode_q is enabled) and failing the password check (i.e., umode_i is disabled). This allows an attacker to bypass the debug password checking and gain debug access to the core, compromising the security of the processor.

A fix to this issue is to only change the privilege level of the processor when both checks are satisfied, i.e., the request has enough privileges (i.e., debug_mode_q is enabled) and the password checking is successful (i.e., umode_i is enabled) [REF-1378].

module csr_regfile #(
...

  // check that we actually want to enter debug depending on the privilege level we are currently in
  unique case (priv_lvl_o)

    riscv::PRIV_LVL_M: begin

      debug_mode_d = dcsr_q.ebreakm;



...


    riscv::PRIV_LVL_U: begin

      debug_mode_d = dcsr_q.ebreaku;



...

  assign priv_lvl_o =  (debug_mode_q && umode_i) ? riscv::PRIV_LVL_M : priv_lvl_q;

...

  debug_mode_q  <= debug_mode_d;

...

See Also