According to C99, Section 6.2.5, "Types":

A computation involving unsigned operands can never overflow, because a result that cannot be represented by the resulting unsigned integer type is reduced modulo the number that is one greater than the largest value that can be represented by the resulting type.

This behavior is more informally referred to as unsigned integer wrapping. Unsigned integer operations can wrap if the resulting value cannot be represented by the underlying representation of the integer. The following table indicates which operators can result in wrapping.

The following sections examine specific operations that are susceptible to unsigned integer wrap. When operating on small integer types (smaller than int), integer promotions are applied. The usual arithmetic conversions may also be applied to (implicitly) convert operands to equivalent types before arithmetic operations are performed. Make sure you understand integer conversion rules before trying to implement secure arithmetic operations (see INT02-C. Understand integer conversion rules).

Integer values that originate from untrusted sources must not be allowed to wrap if they 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

Addition

Addition is between two operands of arithmetic type or between a pointer to an object type and an integer type (see ARR37-C. Do not add or subtract an integer to a pointer to a non-array object and ARR38-C. Do not add or subtract an integer to a pointer if the resulting value does not refer to a valid array element for rules about adding a pointer to an integer). Incrementing is equivalent to adding one.

Noncompliant Code Example

This noncompliant code example may result in an unsigned integer wrap during the addition of the unsigned operands ui1 and ui2. If this behavior is unexpected, the resulting value may be used to allocate insufficient memory for a subsequent operation or in some other manner that can lead to an exploitable vulnerability.

unsigned int ui1, ui2, sum;

/* Initialize ui1 and ui2 */

sum = ui1 + ui2;

Compliant Solution

This compliant solution tests the operands of the addition to guarantee there is no possibility of unsigned wrap.

unsigned int ui1, ui2, sum;

/* Initialize ui1 and ui2 */

if (UINT_MAX - ui1 < ui2) {
  /* handle error condition */
}
else {
  sum = ui1 + ui2;
}

Subtraction

Subtraction is between two operands of arithmetic type, two pointers to qualified or unqualified versions of compatible object types, or between a pointer to an object type and an integer type. See ARR36-C. Do not subtract or compare two pointers that do not refer to the same array, ARR37-C. Do not add or subtract an integer to a pointer to a non-array object, and ARR38-C. Do not add or subtract an integer to a pointer if the resulting value does not refer to a valid array element for rules about pointer subtraction. Decrementing is equivalent to subtracting one.

Noncompliant Code Example

This noncompliant code example may result in an unsigned integer wrap during the subtraction of the unsigned operands ui1 and ui2. If this behavior is unanticipated, it may lead to an exploitable vulnerability.

unsigned int ui1, ui2, result;

/* Initialize ui1 and ui2 */

result = ui1 - ui2;

Compliant Solution

This compliant solution tests the unsigned operands of the subtraction to guarantee there is no possibility of unsigned wrap.

unsigned int ui1, ui2, result;

/* Initialize ui1 and ui2 */

if (ui1 < ui2){
   /* handle error condition */
}
else {
  result = ui1 - ui2;
}

Multiplication

Multiplication is between two operands of arithmetic type.

Noncompliant Code Example

The Mozilla Scalable Vector Graphics (SVG) viewer contains a heap buffer wrap vulnerability resulting from an unsigned integer wrap during the multiplication of the signed int value pen->num_vertices and the size_t value sizeof(cairo_pen_vertex_t) [[VU#551436]]. The signed int operand is converted to unsigned int prior to the multiplication operation (see INT02-C. Understand integer conversion rules).

pen->num_vertices = _cairo_pen_vertices_needed(
  gstate->tolerance, radius, &gstate->ctm
);
pen->vertices = malloc(
  pen->num_vertices * sizeof(cairo_pen_vertex_t)
);

The unsigned integer wrap can result in allocating memory of insufficient size.

Compliant Solution

This compliant solution tests the operands of the multiplication to guarantee that there is no unsigned integer wrap.

pen->num_vertices = _cairo_pen_vertices_needed(
  gstate->tolerance, radius, &gstate->ctm
);

if (pen->num_vertices > SIZE_MAX/sizeof(cairo_pen_vertex_t)) {
  /* handle error condition */
}
pen->vertices = malloc(
  pen->num_vertices * sizeof(cairo_pen_vertex_t)
);

Left-Shift Operator

The left-shift operator is between two operands of integer type.

Noncompliant Code Example

This noncompliant code example can result in unsigned wrap left-shifting the unsigned operand ui1 by ui2 bits.

unsigned int ui1, ui2, uresult;

/* Initialize ui1 and ui2 */

uresult = ui1 << ui2;

Compliant Solution

This compliant solution tests the operands of the left shift to guarantee there is no possibility of unsigned wrap. This solution must also be compliant with INT34-C. Do not shift a negative number of bits or more bits than exist in the operand.

unsigned int ui1, ui2, uresult;

/* Initialize ui1 and ui2 */

if ( (ui2 >= sizeof(unsigned int)*CHAR_BIT) 
  || (ui1 > (UINT_MAX  >> ui2))) ) { 
{
  /* handle error condition */
}
else {
  uresult = ui1 << ui2;
}

Exceptions

INT30-EX1. Unsigned integers can exhibit modulo behavior (wrapping) only when this behavior is necessary for the proper execution of the program. It is recommended that the variable declaration be clearly commented as supporting modulo behavior and that each operation on that integer also be clearly commented as supporting modulo behavior.

INT32-EX2. Checks for wraparound can be omitted when it can be determined at compile-time that wraparound will not occur. As such, the following operations on unsigned integers require no validation:

• Operations on two compile-time constants
• Operations on a variable and 0 (except divison by 0, of course)
• Subtracting any variable from its type's maximum. For instance, any unsigned int may safely be subtracted from UINT_MAX.
• Multiplying any variable by 1
• Division, as long as the divisor is nonzero.
• Right-shifting any type maximum by any number smaller than the type size. For instance, UINT_MAX >> x is valid as long as x < sizeof(unsigned int).
• Left-shifting 1 by any number smaller than the type size.

Risk Assessment

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

Automated Detection

Fortify SCA Version 5.0 with the CERT C Rule Pack can detect violations of this rule.

Compass/ROSE can detect violations of this rule by ensuring that operations are checked for overflow before being performed. Be mindful of INT32-EX2, as it excuses many operations from requiring validation; including all the operations that would validate a potentially dangerous operation. For instnace, adding two unsigned int}}s together requires validation involving subtracting one of the numbers from {{UINT_MAX, which itself requires no validation, as it cannot wrap.

Related Vulnerabilities

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

A Linux kernel vmsplice exploit, described at [[Wojtczuk 08]], documents a vulnerability and exploit arising from a buffer overflow (caused by unsigned integer wrapping).

Other Languages

This rule appears in the C++ Secure Coding Standard as INT30-CPP. Ensure that unsigned integer operations do not wrap.

References

[[Dowd 06]] Chapter 6, "C Language Issues" (Arithmetic Boundary Conditions, pp. 211-223)
[[ISO/IEC 9899:1999]] Section 6.2.5, "Types," Section 6.5, "Expressions," and Section 7.10, "Sizes of integer types <limits.h>"
[[ISO/IEC PDTR 24772]] "XYY Wrap-around Error"
[[MITRE 07]] CWE ID 190, "Integer Overflow (Wrap or Wraparound)"
[[Seacord 05]] Chapter 5, "Integers"
[[Viega 05]] Section 5.2.7, "Integer overflow"
[[VU#551436]]
[[Warren 02]] Chapter 2, "Basics"
[[Wojtczuk 08]]

      04. Integers (INT)