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

Most integer operations can result in overflow if the resulting value cannot be represented by the underlying representation of the integer. The following table indicates which operators can result in overflow:

The following sections examine specific operations that are susceptible to integer overflow. The specific tests that are required to guarantee that the operation does not result in an integer overflow depend on the signedness of the integer types. 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 (Unsigned)

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 (Unsigned)

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;

Non-Compliant Code Example (Signed)

This code may result in a signed integer overflow during the addition of the signed operands si1 and si2. If this behavior is unanticipated, it could lead to an exploitable vulnerability.

int si1, si2, sum;

sum = si1 + si2;

Compliant Solution (Two's Complement Signed)

This compliant solution tests the addition operation to ensure no overflow occurs, assuming two's complement representation.

signed int si1, si2, sum;

if ( ((si1^si2) | (((si1^(~(si1^si2) & (1 << (sizeof(int)*CHAR_BIT-1))))+si2)^si2)) >= 0) {
   /* handle error condition */
}

sum = si1 + si2;

Compliant Solution (General Signed)

This compliant solution tests the suspect addition operation to ensure no overflow occurs regardless of representation.

signed int si1, si2, sum;

if (((si1>0) && (si2>0) && (si1 > (INT_MAX-si2))) ||
    ((si1<0) && (si2<0) && (si1 < (INT_MIN-si2)))) {
   /* handle error condition */
}

sum = si1 + si2;

This solution is more readable but contains branches and consequently may be less efficient than the solution that is specific to two's complement representation.

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 (Unsigned)

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 (Unsigned)

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;

Non-Compliant Code Example (Signed)

This code can result in a signed integer overflow during the subtraction of the signed operands si1 and si2. 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.

signed int si1, si2, result;

result = si1 - si2;

Compliant Solution (Two's Complement Signed)

This compliant solution tests the suspect subtraction operation to guarantee there is no possibility of signed overflow, presuming two's complement representation.

signed int si1, si2, result;

if (((si1^si2) & (((si1 ^ ((si1^si2) & (1 << (sizeof(int)*CHAR_BIT-1))))-si2)^si2)) < 0) {
  /* handle error condition */
}
result = si1 - si2;

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 6.5, "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"

      04. Integers (INT)       INT33-C. Ensure that division and modulo operations do not result in divide-by-zero errors