FLP04-C. Check floating-point inputs for exceptional values

Floating-point numbers can take on two classes of exceptional values; infinity and NaN (not-a-number). These values are returned as the result of exceptional or otherwise unresolvable floating-point operations. (See also FLP32-C. Prevent or detect domain and range errors in math functions.) Additionally, they can be directly input by a user by scanf or similar functions. Failure to detect and handle such values can result in undefined behavior.

NaN values are particularly problematic because the expression NaN == NaN (for every possible value of NaN) returns false. Any comparisons made with NaN as one of the arguments returns false, and all arithmetic functions on NaNs simply propagate them through the code. Hence, a NaN entered in one location in the code and not properly handled could potentially cause problems in other, more distant sections.

Formatted-input functions such as scanf will accept the values INF, INFINITY, or NAN (case insensitive) as valid inputs for the %f format specification, allowing malicious users to feed them directly to a program. Programs should therefore check to ensure that all input floating-point values (especially those controlled by the user) have neither of these values if doing so would be inappropriate. The <math.h> library provides two macros for this purpose: isinf and isnan.

isinf and isnan

The isinf macro tests an input floating-point value for infinity. isinf(val) is nonzero if val is an infinity (positive or negative), and 0 otherwise.

isnan tests if an input is NaN. isnan(val) is nonzero if val is a NaN, and 0 otherwise.

If infinity or NaN values are not acceptable inputs in a program, these macros should be used to ensure they are not passed to vulnerable functions.

Noncompliant Code Example

This noncompliant code example accepts user data without first validating it:

float currentBalance; /* User's cash balance */
void doDeposit() {
  float val;

  scanf("%f", &val);

  if(val >= MAX_VALUE - currentBalance) {
    /* Handle range error */
  }

  currentBalance += val;
}

This can be a problem if an invalid value is entered for val and subsequently used for calculations or as control values. The user could, for example, input the strings "INF", "INFINITY", or "NAN" (case insensitive) on the command line, which would be parsed by scanf into the floating-point representations of infinity and NaN. All subsequent calculations using these values would be invalid, possibly crashing the program and enabling a denial-of-service attack.

Here, for example, entering "nan" for val would force currentBalance to also equal "nan", corrupting its value. If this value is used elsewhere for calculations, every resulting value would also be a NaN, possibly destroying important data.

Implementation Details

The following code was run on 32-bit GNU Linux using the GCC 3.4.6 compiler. On this platform, FLT_MAX has the value 340282346638528859811704183484516925440.000000.

#include <stdio.h>

int main(int argc, char *argv[]) {
  float val, currentBalance=0;
  scanf("%f", &val);
  currentBalance+=val;
  printf("%f\n", currentBalance);
  return 0;
}

The following table shows the value of currentBalance returned for various arguments:

As this example demonstrates, the user can enter the exceptional values infinity and NaN, as well as force a float's value to be infinite, by entering out-of-range floats. These entries subsequently corrupt the value of currentBalance. So by entering exceptional floats, an attacker can corrupt the program data, possibly leading to a crash.

Compliant Solution

This compliant solution first validates the input float before using it. The value is tested to ensure that it is neither an infinity nor a NaN.

float currentBalance; /* User's cash balance */

void doDeposit() {
  float val;

  scanf("%f", &val);
  if (isinf(val)) {
    /* Handle infinity error */
  }
  if (isnan(val)) {
    /* Handle NaN error */
  }
  if (val >= MAX_VALUE - currentBalance) {
    /* Handle range error */
  }

  currentBalance += val;
}

Exceptions

Occasionally, NaN or infinity may be acceptable or expected inputs to a program. If this is the case, then explicit checks may not be necessary. Such programs must, however, be prepared to handle these inputs gracefully and not blindly use them in mathematical expressions where they are not appropriate.

Risk Assessment

Inappropriate floating-point inputs can result in invalid calculations and unexpected results, possibly leading to crashing and providing a denial-of-service opportunity.

Automated Detection

 Related Vulnerabilities

Search for vulnerabilities resulting from the violation of this recommendation on the CERT website.

Related Guidelines

Bibliography