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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.

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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;

...

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;
}

...

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;

...

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;
}

...

The Mozilla Scalable Vector Graphics (SVG) viewer contains a heap buffer overflow 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)}} \[[VU#551436|AA. C References#VU551436]\].  The {{signed int}} operand is converted to {{unsigned int}} prior to the multiplication operation (see [INT02-C. Understand integer conversion rules|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)
);

...

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)
);

...

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;

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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;
}

...

Compass/ROSE can detect violations of this rule by ensuring that operations are checked for overflow before being performed. Be mindful of INT30-EX2, as it excuses many operations from requiring validation; including all the operations that would validate a potentially dangerous operation. For instance, 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

[CVE-2009-1385|http://web.nvd.nist.gov/view/vuln/detail?vulnId=CVE-2009-1385] results from a violation of this rule. The value performs an unchecked subtraction on the {{length}} of a buffer, and then adds that many bytes of data to another buffer \[[xorl 2009|http://xorl.wordpress.com/2009/06/10/cve-2009-1385-linux-kernel-e1000-integer-underflow/]\]. This can cause a buffer overflow, which allows an attacker to execute arbitrary code

.

A Linux kernel vmsplice [exploit|BB. Definitions#exploit], described at \[[Wojtczuk 08|AA. C References#Wojtczuk 08]\], documents a vulnerability and exploit arising from a buffer overflow (caused by unsigned integer wrapping).

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

Other Languages

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

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\[[Dowd 06|AA. C References#Dowd 06]\] Chapter 6, "C Language Issues" (Arithmetic Boundary Conditions, pp. 211-223)
\[[ISO/IEC 9899:1999|AA. C References#ISO/IEC 9899-1999]\] Section 6.2.5, "Types," Section 6.5, "Expressions," and Section 7.10, "Sizes of integer types {{<limits.h>}}"
\[[ISO/IEC PDTR 24772|AA. C References#ISO/IEC PDTR 24772]\] "XYY Wrap-around Error"
\[[MITRE 07|AA. C References#MITRE 07]\] [CWE ID 190|http://cwe.mitre.org/data/definitions/190.html], "Integer Overflow (Wrap or Wraparound)"
\[[Seacord 05|AA. C References#Seacord 05]\] Chapter 5, "Integers"
\[[Viega 05|AA. C References#Viega 05]\] Section 5.2.7, "Integer overflow"
\[[VU#551436|AA. C References#VU551436]\]
\[[Warren 02|AA. C References#Warren 02]\] Chapter 2, "Basics"
\[[Wojtczuk 08|AA. C References#Wojtczuk 08]\]
\[[xorl 2009|AA. C References#xorl 2009]\] ["CVE-2009-1385: Linux kernel E1000 Integer Underflow"|http://xorl.wordpress.com/2009/06/10/cve-2009-1385-linux-kernel-e1000-integer-underflow/]

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      04. Integers (INT)