CertC-INT30¶

Ensure that unsigned integer operations do not wrap

Required inputs: IR, StaticSemanticAnalysis

The C Standard, 6.2.5, paragraph 9 [ ISO/IEC 9899:2011], states

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 called 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:

Operator	Wrap	Operator	Wrap	Operator	Wrap	Operator	Wrap
`+`	Yes	`-=`	Yes	`<<`	Yes	`<`	No
`-`	Yes	`*=`	Yes	`>>`	No	`>`	No
`*`	Yes	`/=`	No	`&`	No	`>=`	No
`/`	No	`%=`	No	`\|`	No	`<=`	No
`%`	No	`<<=`	Yes	`^`	No	`==`	No
`++`	Yes	`>>=`	No	`~`	No	`!=`	No
`--`	Yes	`&=`	No	`!`	No	`&&`	No
`=`	No	`\|=`	No	`un +`	No	`\|\|`	No
`+=`	Yes	`^=`	No	`un -`	Yes	`?:`	No

The following sections examine specific operations that are susceptible to unsigned integer wrap. When operating on integer types with less precision 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. Programmers should understand integer conversion rules before trying to implement secure arithmetic operations. (See INT02-C. Understand integer conversion rules.)

Integer values must not be allowed to wrap, especially if they are used in any of the following ways:

Integer operands of any pointer arithmetic, including array indexing
The assignment expression for the declaration of a variable length array
The postfix expression preceding square brackets [] or the expression in square brackets [] of a subscripted designation of an element of an array object
Function arguments of type size_t or rsize_t (for example, an argument to a memory allocation function)
In security-critical code

The C Standard defines arithmetic on atomic integer types as read-modify-write operations with the same representation as regular integer types. As a result, wrapping of atomic unsigned integers is identical to regular unsigned integers and should also be prevented or detected.

Addition

Addition is between two operands of arithmetic type or between a pointer to an object type and an integer type. This rule applies only to addition between two operands of arithmetic type. (See ARR37-C. Do not add or subtract an integer to a pointer to a non-array object and ARR30-C. Do not form or use out-of-bounds pointers or array subscripts.)

Incrementing is equivalent to adding 1.

Noncompliant Code Example

This noncompliant code example can result in an unsigned integer wrap during the addition of the unsigned operands ui_a and ui_b. 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.

void func(unsigned int ui_a, unsigned int ui_b) {
  unsigned int usum = ui_a + ui_b;
  /* ... */
}

Compliant Solution (Precondition Test)

This compliant solution performs a precondition test of the operands of the addition to guarantee there is no possibility of unsigned wrap:

#include <limits.h>
 
void func(unsigned int ui_a, unsigned int ui_b) {
  unsigned int usum;
  if (UINT_MAX - ui_a < ui_b) {
    /* Handle error */
  } else {
    usum = ui_a + ui_b;
  }
  /* ... */
}

Compliant Solution (Postcondition Test)

This compliant solution performs a postcondition test to ensure that the result of the unsigned addition operation usum is not less than the first operand:

void func(unsigned int ui_a, unsigned int ui_b) {
  unsigned int usum = ui_a + ui_b;
  if (usum < ui_a) {
    /* Handle error */
  }
  /* ... */
}

Subtraction

Subtraction is between two operands of arithmetic type, two pointers to qualified or unqualified versions of compatible object types, or a pointer to an object type and an integer type. This rule applies only to subtraction between two operands of arithmetic 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 ARR30-C. Do not form or use out-of-bounds pointers or array subscripts for information about pointer subtraction.)

Decrementing is equivalent to subtracting 1.

Noncompliant Code Example

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

void func(unsigned int ui_a, unsigned int ui_b) {
  unsigned int udiff = ui_a - ui_b;
  /* ... */
}

Compliant Solution (Precondition Test)

This compliant solution performs a precondition test of the unsigned operands of the subtraction operation to guarantee there is no possibility of unsigned wrap:

void func(unsigned int ui_a, unsigned int ui_b) {
  unsigned int udiff;
  if (ui_a < ui_b){
    /* Handle error */
  } else {
    udiff = ui_a - ui_b;
  }
  /* ... */
}

Compliant Solution (Postcondition Test)

This compliant solution performs a postcondition test that the result of the unsigned subtraction operation udiff is not greater than the minuend:

void func(unsigned int ui_a, unsigned int ui_b) {
  unsigned int udiff = ui_a - ui_b;
  if (udiff > ui_a) {
    /* Handle error */
  }
  /* ... */
}

Multiplication

Multiplication is between two operands of arithmetic type.

Noncompliant Code Example

The Mozilla Foundation Security Advisory 2007-01 describes a heap buffer overflow vulnerability in the Mozilla Scalable Vector Graphics (SVG) viewer 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 size_t prior to the multiplication operation so that the multiplication takes place between two size_t integers, which are unsigned. (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 */
}
pen->vertices = malloc(
  pen->num_vertices * sizeof(cairo_pen_vertex_t)
);

Exceptions

INT30-C-EX1: Unsigned integers can exhibit modulo behavior (wrapping) when 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.

INT30-C-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 division or remainder by 0)
Subtracting any variable from its type's maximum; for example, any unsigned int may safely be subtracted from UINT_MAX
Multiplying any variable by 1
Division or remainder, as long as the divisor is nonzero
Right-shifting any type maximum by any number no larger than the type precision; for example, UINT_MAX >> x is valid as long as 0 <= x < 32 (assuming that the precision of unsigned int is 32 bits)

INT30-C-EX3. The left-shift operator takes two operands of integer type. Unsigned left shift << can exhibit modulo behavior (wrapping). This exception is provided because of common usage, because this behavior is usually expected by the programmer, and because the behavior is well defined. For examples of usage of the left-shift operator, see INT34-C. Do not shift an expression by a negative number of bits or by greater than or equal to the number of bits that exist in the operand.

Risk Assessment

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

Rule	Severity	Likelihood	Remediation Cost	Priority	Level
INT30-C	High	Likely	High	P9	L2

Related Guidelines

Taxonomy	Taxonomy item	Relationship
CERT C	INT02-C. Understand integer conversion rules	Prior to 2018-01-12: CERT: Unspecified Relationship
CERT C	ARR30-C. Do not form or use out-of-bounds pointers or array subscripts	Prior to 2018-01-12: CERT: Unspecified Relationship
CERT C	ARR36-C. Do not subtract or compare two pointers that do not refer to the same array	Prior to 2018-01-12: CERT: Unspecified Relationship
CERT C	ARR37-C. Do not add or subtract an integer to a pointer to a non-array object	Prior to 2018-01-12: CERT: Unspecified Relationship
CERT C	CON08-C. Do not assume that a group of calls to independently atomic methods is atomic	Prior to 2018-01-12: CERT: Unspecified Relationship
ISO/IEC TR 24772:2013	Arithmetic Wrap-Around Error [FIF]	Prior to 2018-01-12: CERT: Unspecified Relationship
CWE 2.11	CWE-190, Integer Overflow or Wraparound	2016-12-02: CERT: Rule subset of CWE
CWE 2.11	CWE-131	2017-05-16: CERT: Partial overlap
CWE 2.11	CWE-191	2017-05-18: CERT: Partial overlap
CWE 2.11	CWE-680	2017-05-18: CERT: Partial overlap

Bibliography

[ Bailey 2014]	Raising Lazarus - The 20 Year Old Bug that Went to Mars
[ Dowd 2006]	Chapter 6, "C Language Issues" ("Arithmetic Boundary Conditions," pp. 211-223)
[ ISO/IEC 9899:2011]	Subclause 6.2.5, "Types"
[ Seacord 2013b]	Chapter 5, "Integer Security"
[ Viega 2005]	Section 5.2.7, "Integer Overflow"
[ VU#551436]
[ Warren 2002]	Chapter 2, "Basics"
[ Wojtczuk 2008]
[ xorl 2009]	"CVE-2009-1385: Linux Kernel E1000 Integer Underflow"

Excerpt from SEI CERT C Coding Standard: Rules for Developing Safe, Reliable, and Secure Systems (2016 Edition) and SEI CERT C Coding Standard [https://cmu-sei.github.io/secure-coding-standards/sei-cert-c-coding-standard/rules/integers-int/int30-c], Copyright (C) 1995-2026 Carnegie Mellon University. See section 9.4. "3rd-Party Licenses" in the documentation for full details.

Possible Messages

Key	Text	Severity	Disabled
unsigned_overflow	Arithmetic computation may cause wrap-around	None	False
unsigned_underflow	Arithmetic computation may cause wrap-around	None	False

Options¶

This rule shares the following common options: exclude_in_macros, exclude_messages_in_system_headers, excludes, extend_exclude_to_macro_invocations, includes, justification_checker, languages, post_processing, provider, report_at, severity
The following places define options that affect this rule: Stylechecks, Analysis-GlobalOptions

abstract_interpretation_maximal_tracked_array_index¶

abstract_interpretation_maximal_tracked_array_index : int = 10

The number of explicit indices in array expressions per routine tracked by the "symbolic expression analysis". For example, consider the following program.

extern signed char a[6];
int main()
{
    if (a[2] < 0)
    {
        a[2]++;
    }
    if (a[3] < 0)
    {
        a[3]++;
    }
    if (a[4] < 0)
    {
        a[4]++;
    }
    return 0;
}

If the value of this option is set to 2, the first two array index expressions encountered in the routine are tracked. Hence, the analysis can use the facts a[2] < 0 and a[3] < 0 to infer that a[2]++ and a[3]++ do not overflow, but it will not track the third array access in this routine.

A higher value of the option can cause more consumption of memory and time for the analysis.

abstract_interpretation_overflow¶

abstract_interpretation_overflow : bool = True

Use abstract-interpretation-based "symbolic expression analysis" as additional postprocessing step.

abstract_interpretation_overflow_unrolling_level¶

abstract_interpretation_overflow_unrolling_level : int = 0

How many levels of conditions are traversed to compute additional constraints for the "symbolic expression analysis".

check_signed¶

check_signed : bool = False

Whether issues for signed integer operations should be reported. For casts including implicit conversions, the target type of the cast is used.

check_unsigned¶

check_unsigned : bool = True

Whether wrap-around for unsigned integer operations should be reported. For casts including implicit conversions, the target type of the cast is used.

suppress_well_defined_findings¶

suppress_well_defined_findings : SuppressionMode = 'NONE'

Some overflows have well-defined semantics in all C/C++ standard versions. The typical example is UINT_MAX+1 which is well-defined as 0 via wraparound. This differs from INT_MAX+1 which is either undefined or implementation-defined depending on the considered standard version. Most CPUs will compute INT_MIN but this wraparound is not guaranteed by any C/C++ standard.

Both cases are overflows and are reported by this rule. However, one might want to suppress messages for the well-defined cases. To suppress these activate this option.

Different C and C++ standard versions differ in what is well-defined, implementation-defined, or undefined. Luckily, if we only consider well-defined and do not discern between implementation-defined and undefined, we end up with only two groups: pre-C++20 and since-C++20.

Option Types¶

These types are used by options listed above:

SuppressionMode¶

An enumeration.

NONE

Suppress nothing.

PRE_CPP2020

Suppress findings that are well-defined before C++20. These are:

Over- and underflows of unsigned integers during addition, subtraction, and multiplication
Conversions from unsigned to unsigned integers
Wrap-around caused by left-shifting of unsigned integer

CPP2020

Suppress findings that are well-defined since C++20. These are:

Over- and underflows of unsigned integers during addition, subtraction, and multiplication
Conversions between signed and unsigned integers
Wrap-around caused by left-shifting
Shifting negative integers

Surprising mechanics of C++20 signed narrow integers

Since C++20, casts between signed and unsigned are defined as two-complement wrap-around. Overflows of signed integers are still undefined behavior and are reported by this rule. But, due to integer promotion rules, certain expressions are computed using wider integer types, which can lead to the false impression that this is no longer the case, because no overflow findings are reported there.

Suppose, that the code is compiled on a platform where short is smaller than or equal to half the size of an int. Very commonly the sizes are 2 and 4. This assumption is thus true for many platforms.

In this case, narrow signed integer types such as short or signed char are first implicitly promoted to int before the arithmetic operation is executed. Because of this promotion, the actual operation does not overflow and is thus well-defined. After the operation, an implicit cast is performed to the narrower type. This cast is well-defined in C++20 as wrapping around.

Consider the following snippet:

    static_assert(sizeof(short) == 2);
    static_assert(sizeof(int) == 4);
    short a = 0x1000;
    short b = 0x1001;
    short c = a*b;

C++20 defines c as 0x1000. The reason is that a*b is implicitly promoted to

static_cast<int>
(a)*static_cast<int>(b)

. After the promotion, the multiplication does not overflow and yields a well-defined 0x1001000. This number is then implicitly cast to 0x1000 which is also a well-defined operation.

An analogous effect can be observed for signed short addition and multiplication. Another effect is that it is well-defined to shift by up to as many bits as int has even if the shifted integer has fewer bits.

DERIVE_FROM_IR

Derive the language version from the IR compilation flags and suppress findings accordingly.

Axivion Suite 7.12.0

Navigation

CertC-INT30¶

Addition

Noncompliant Code Example

Compliant Solution (Precondition Test)

Compliant Solution (Postcondition Test)

Subtraction

Noncompliant Code Example

Compliant Solution (Precondition Test)

Compliant Solution (Postcondition Test)

Multiplication

Noncompliant Code Example

Compliant Solution

Exceptions

Risk Assessment

Related Guidelines

Bibliography

Options¶

abstract_interpretation_maximal_tracked_array_index¶

abstract_interpretation_overflow¶

abstract_interpretation_overflow_unrolling_level¶

check_signed¶

check_unsigned¶

suppress_well_defined_findings¶

Option Types¶

SuppressionMode¶

Surprising mechanics of C++20 signed narrow integers