Integer values used in any of the the following ways must be guaranteed correct:

• 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)
• in security-critical code

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_. Most operations on 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 types (smaller than int), integer conversion rules apply. The usual arithmetic conversions may also be applied to (implicitly) convert operands to equivalent types before arithmetic operations are performed. Make sure you understand implicit conversion rules before trying to implement secure arithmetic operations (see INT02-A. Understand integer conversion rules).

Addition

Addition is between two operands of arithmetic type or between a pointer to an object type and an integer type. Incrementing is equivalent to adding one.

Non-Compliant Code Example

This code may result in an unsigned integer overflow 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 could lead to an exploitable vulnerability.

unsigned int ui1, ui2, sum;

sum = ui1 + ui2;

Compliant Solution

This compliant solution tests the suspect addition operation to guarantee there is no possibility of unsigned overflow.

unsigned int ui1, ui2, sum;

if (UINT_MAX - ui1 < ui2) {
  /* handle error condition */
}
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. Decrementing is equivalent to subtracting one.

Non-Compliant Code Example

This code may result in an unsigned integer overflow 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;

result = ui1 - ui2;

Compliant Solution

This compliant solution tests the suspect unsigned subtraction operation to guarantee there is no possibility of unsigned overflow.

unsigned int ui1, ui2, result;

if (ui1 < ui2){
   /* handle error condition */
}

result = ui1 - ui2;

Multiplication

Multiplication is between two operands of arithmetic type.

Non-Compliant Code Example

The Mozilla Scalable Vector Graphics (SVG) viewer contains a heap buffer overflow vulnerability resulting from an unsigned integer overflow 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-A. 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 overflow can result in allocating memory of insufficient size.

Compliant Solution

This compliant solution tests the suspect multiplication operation to guarantee that there is no unsigned integer overflow.

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.

Non-Compliant Code Example

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

unsigned int ui1, ui2, uresult;

uresult = ui1 << ui2;

Compliant Solution

This compliant solution tests the suspect shift operation to guarantee there is no possibility of unsigned overflow. This solution must also be compliant with INT36-C. Do not shift a negative number of bits or more bits than exist in the operand.

unsigned int ui1, ui2, uresult;

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

Exceptions

INT32-EX1. Unsigned integers can exhibit modulo behavior 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.

Risk Assessment

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

Automated Detection

Fortify SCA Version 5.0 with CERT C Rule Pack is able to detect violations of this rule.

Related Vulnerabilities

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

A Linux kernel vmsplice exploit, described at http://www.avertlabs.com/research/blog/index.php/2008/02/13/analyzing-the-linux-kernel-vmsplice-exploit/,
documents a vulnerability and exploit arising directly out of integer overflow.

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"
[[Seacord 05]] Chapter 5, "Integers"
[[Viega 05]] Section 5.2.7, "Integer overflow"
[[VU#551436]]
[[Warren 02]] Chapter 2, "Basics"

INT15-A. Take care when converting from pointer to integer or integer to pointer      04. Integers (INT)