INT31-C. Ensure that integer conversions do not result in lost or misinterpreted data

Integer conversions, both implicit and explicit (using a cast), must be guaranteed not to result in lost or misinterpreted data. This is particularly true for integer values that originate from untrusted sources and are used in any of the following ways:

• As an array index
• In any pointer arithmetic
• As a length or size of an object
• As the bound of an array (for example, a loop counter)
• As an argument to a memory allocation function
• In security-critical code

The only integer type conversions that are guaranteed to be safe for all data values and all possible conforming implementations are conversions of an integral value to a wider type of the same signedness. Subclause 6.3.1.3 of the C Standard [ISO/IEC 9899:2011] says,

When a value with integer type is converted to another integer type other than _Bool, if the value can be represented by the new type, it is unchanged.

Otherwise, if the new type is unsigned, the value is converted by repeatedly adding or subtracting one more than the maximum value that can be represented in the new type until the value is in the range of the new type.

Otherwise, the new type is signed and the value cannot be represented in it; either the result is implementation-defined or an implementation-defined signal is raised.

Typically, converting an integer to a smaller type results in truncation of the high-order bits.

Noncompliant Code Example (Unsigned to Signed)

Type range errors, including loss of data (truncation) and loss of sign (sign errors), can occur when converting from an unsigned type to a signed type. The following noncompliant code example results in a truncation error on most implementations:

#include <limits.h>
 
void func(void) {
  unsigned long int u_a = ULONG_MAX;
  signed char sc;
  sc = (signed char)u_a; /* Cast eliminates warning */
  /* ... */
}

Compliant Solution (Unsigned to Signed)

Validate ranges when converting from an unsigned type to a signed type. The following code, for example, can be used when converting from unsigned long int to a signed char:

#include <limits.h>
 
void func(void) {
  unsigned long int u_a = ULONG_MAX;
  signed char sc;
  if (u_a <= SCHAR_MAX) {
    sc = (signed char)u_a;  /* Use cast to eliminate warning */
  } else {
    /* Handle error condition */
  }
}

Noncompliant Code Example (Signed to Unsigned)

Type range errors, including loss of data (truncation) and loss of sign (sign errors), can occur when converting from a signed type to an unsigned type. The following code results in a loss of sign:

#include <limits.h>

void func(void) {
  signed int si = INT_MIN;
  unsigned int ui = (unsigned int)si;  /* Cast eliminates warning */

  /* ... */
}

Compliant Solution (Signed to Unsigned)

Validate ranges when converting from a signed type to an unsigned type. The following code, for example, can be used when converting from signed int to unsigned int:

#include <limits.h>

void func(void) {
  signed int si = INT_MIN;
  unsigned int ui;
  if (si < 0) {
    /* Handle error condition */
  } else {
    ui = (unsigned int)si;  /* Cast eliminates warning */
  }
  /* ... */
}

NOTE: Although unsigned types can usually represent all positive values of the corresponding signed type, this relationship is not guaranteed by the C Standard.

Noncompliant Code Example (Signed, Loss of Precision)

A loss of data (truncation) can occur when converting from a signed type to a signed type with less precision. The following code can result in truncation:

#include <limits.h>

void func(void) {
  signed long int s_a = LONG_MAX;
  signed char sc = (signed char)s_a; /* Cast eliminates warning */
  /* ... */
}

Compliant Solution (Signed, Loss of Precision)

Validate ranges when converting from a signed type to a signed type with less precision. The following code can be used, for example, to convert from a signed long int to a signed char:

#include <limits.h>

void func(void) {
  signed long int s_a = LONG_MAX;
  signed char sc;
  if ((s_a < SCHAR_MIN) || (s_a > SCHAR_MAX)) {
    /* Handle error condition */
  } else {
    sc = (signed char)s_a; /* Use cast to eliminate warning */
  }
  /* ... */
}

Conversions from signed types with greater precision to signed types with lesser precision require both the upper and lower bounds to be checked.

Noncompliant Code Example (Unsigned, Loss of Precision)

A loss of data (truncation) can occur when converting from an unsigned type to an unsigned type with less precision. The following code results in a truncation error on most implementations:

#include <limits.h>

void func(void) {
  unsigned long int u_a = ULONG_MAX;
  unsigned char uc = (unsigned char)u_a;  /* Cast eliminates warning */
  /* ... */
}

Compliant Solution (Unsigned, Loss of Precision)

Validate ranges when converting from an unsigned type to an unsigned type with lesser precision. The following code can be used, for example, to convert from an unsigned long int to an unsigned char:

#include <limits.h>

void func(void) {
  unsigned long int u_a = ULONG_MAX;
  unsigned char uc;
  if (u_a > UCHAR_MAX) ) {
    /* Handle error condition */
  } else {
    uc = (unsigned char)u_a; /* Use cast to eliminate warning */
  }
  /* ... */
}

Conversions from unsigned types with greater precision to unsigned types with lesser precision require only the upper bounds to be checked.

Noncompliant Code Example (time_t Return Value)

The time() function returns a (time_t)(-1) to indicate that the calendar time is not available. The C Standard requires only that the time_t type is an arithmetic type capable of representing time. It is left to the implementor to decide the best arithmetic type to use to represent time. If time_t is implemented as an unsigned integer type smaller than a signed int, the return value of time() will never compare equal to the integer literal -1.

#include <time.h>
 
void func(void) {
  time_t now = time(NULL);
  if (now != -1) {
    /* Continue processing */
  }
}

Compliant Solution (time_t Return Value)

To ensure the comparison is properly performed, the return value of time() should be compared against -1 cast to type time_t:

#include <time.h>
 
void func(void) {
  time_t now = time(NULL);
  if (now != (time_t)-1) {
    /* Continue processing */
  }
}

This solution is in accordance with INT18-C. Evaluate integer expressions in a larger size before comparing or assigning to that size.

Exceptions

INT31-EX0: The C Standard defines minimum ranges for standard integer types. For example, the minimum range for an object of type unsigned short int is 0 to 65,535, whereas the minimum range for int is −32,767 to +32,767. Consequently, it is not always possible to represent all possible values of an unsigned short int as an int. However, on the IA-32 architecture, for example, the actual integer range is from −2,147,483,648 to +2,147,483,647, meaning that it is quite possible to represent all the values of an unsigned short int as an int for this architecture. As a result, it is not necessary to provide a test for this conversion on IA-32. It is not possible to make assumptions about conversions without knowing the precision of the underlying types. If these tests are not provided, assumptions concerning precision must be clearly documented, as the resulting code cannot be safely ported to a system where these assumptions are invalid. A good way to document these assumptions is to use static assertions. (See DCL03-C. Use a static assertion to test the value of a constant expression.)

Risk Assessment

Integer truncation errors can lead to buffer overflows and the execution of arbitrary code by an attacker.

Automated Detection

* Coverity Prevent cannot discover all violations of this rule, so further verification is necessary.

Related Vulnerabilities

CVE-2009-1376 results from a violation of this rule. In version 2.5.5 of Pidgin, a size_t offset is set to the value of a 64-bit unsigned integer, which can lead to truncation [xorl 2009] on platforms where a size_t is interpreted as a 32-bit unsigned integer. An attacker can execute arbitrary code by carefully choosing this value and causing a buffer overflow.

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

Related Guidelines

Bibliography