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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.)

...

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.

Anchor
Addition
Addition

Addition

Addition is between two operands of arithmetic type or between a pointer to an object type and an integer type. This rule only applies to only to addition between two operands of arithmetic type. (see 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.

Code Block
bgColor#FFcccc
langc
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:

Code Block
bgColor#ccccff
langc
#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:

...

Anchor
Subtraction
Subtraction

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 only applies to only to subtraction between two operands of arithmetic type. (see 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.

Code Block
bgColor#FFcccc
langc
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:

Code Block
bgColor#ccccff
langc
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:

...

Anchor
Multiplication
Multiplication

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 See INT02-C. Understand integer conversion rules.).

Code Block
bgColor#FFcccc
langc
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:

Code Block
bgColor#ccccff
langc
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:

...

Anchor
Left Shift Operator
Left Shift Operator

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.

...

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

Automated Detection

Tool

Version

Checker

Description

Astrée
Include Page
Astrée_V
Astrée_V
integer-overflowFully checked
CodeSonar
Include Page
CodeSonar_V
CodeSonar_V

ALLOC.SIZE.ADDOFLOW
ALLOC.SIZE.IOFLOW
ALLOC.SIZE.MULOFLOW
ALLOC.SIZE.SUBUFLOW
MISC.MEM.SIZE.ADDOFLOW
MISC.MEM.SIZE.BAD
MISC.MEM.SIZE.MULOFLOW
MISC.MEM.SIZE.SUBUFLOW

Addition

Overflow

overflow of

Allocation Size

allocation size
Integer

Overflow

overflow of

Allocation Size

allocation size
Multiplication

Overflow

overflow of

Allocation Size

allocation size
Subtraction

Underflow

underflow of

Allocation Size

allocation size
Addition

Overflow

overflow of

Size

size
Unreasonable

Size Argument

size argument
Multiplication

Overflow

overflow of

Size

size
Subtraction

Underflow

underflow of

Size

size

Compass/ROSE

 

 



Can detect violations of this rule by ensuring that operations are checked for overflow before being performed

.

(Be mindful of exception INT30-EX2 because it excuses many operations from requiring validation, including all the operations that would validate a potentially dangerous operation. For instance, adding two unsigned ints together requires validation involving subtracting one of the numbers from UINT_MAX, which itself requires no validation because it cannot wrap.)

Coverity
6.5
Include Page
Coverity_V
Coverity_V
INTEGER_OVERFLOWImplemented

Fortify SCA

5.0

 

Can detect violations of this rule with the CERT C Rule Pack

Klocwork
Include Page
Klocwork_V
Klocwork_V
NUM.OVERFLOW
CWARN.NOEFFECT.OUTOFRANGE

LDRA tool suite
Include Page
LDRA_V
LDRA_V
493 S, 494 SPartially implemented
Parasoft C/C++test

Include Page
Parasoft_V
Parasoft_V

CERT_C-INT30-a
CERT_C-INT30-b
CERT_C-INT30-c

Avoid integer overflows
Integer overflow or underflow in constant expression in '+', '-', '*' operator
Integer overflow or underflow in constant expression in '<<' operator

Polyspace Bug Finder

Include Page
Polyspace Bug Finder_V
Polyspace Bug Finder_V

CERT C: Rule INT30-C

Checks for:

  • Unsigned integer overflow
  • Unsigned integer constant overflow

Rule fully covered.

PRQA QA-C
Include Page
PRQA QA-C_v
PRQA QA-C_v

2910

(

[C

)

], 2911

(

[D

)

], 2912

(

[A

)

],

2913

(S)

[S], 3383, 3384, 3385, 3386

Partially implemented
PRQA QA-C++
Include Page
cplusplus:PRQA QA-C++_V
cplusplus:PRQA QA-C++_V
2910, 2911, 2912, 2913
PVS-Studio

Include Page
PVS-Studio_V
PVS-Studio_V

V658
TrustInSoft Analyzer

Include Page
TrustInSoft Analyzer_V
TrustInSoft Analyzer_V

unsigned overflowExhaustively verified.

Related Vulnerabilities

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 those many bytes of data to another buffer [xorl 2009]. This can cause a buffer overflow, which allows an attacker to execute arbitrary code.

A Linux kernel Kernel vmsplice exploit, described by Rafal Wojtczuk [Wojtczuk 2008], documents a vulnerability and exploit arising from a buffer overflow (caused by unsigned integer wrapping).

Don Bailey [Bailey 2014] describes an unsigned integer wrap vulnerability in the LZO compression algorithm, which can be exploited in some implementations.

CVE-2014-4377 describes a vulnerability in iOS 7.1 resulting from a multiplication operation that wraps, producing an insufficiently small value to pass to a memory allocation routine, which is subsequently overflowed.

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

Related Guidelines

Key here (explains table format and definitions)

Taxonomy

Taxonomy item

Relationship

CERT C
Secure Coding Standard
INT02-C. Understand integer conversion rulesPrior to 2018-01-12: CERT: Unspecified Relationship
CERT CARR30-C. Do not form or use out-of-bounds pointers or array subscriptsPrior to 2018-01-12: CERT: Unspecified Relationship
CERT CARR36-C. Do not subtract or compare two pointers that do not refer to the same arrayPrior to 2018-01-12: CERT: Unspecified Relationship
CERT CARR37-C. Do not add or subtract an integer to a pointer to a non-array objectPrior to 2018-01-12: CERT: Unspecified Relationship
CERT CCON08-C. Do not assume that a group of calls to independently atomic methods is atomic
CERT C++ Secure Coding StandardVOID INT30-CPP. Ensure that unsigned integer operations do not wrap
Prior to 2018-01-12: CERT: Unspecified Relationship
ISO/IEC TR 24772:2013Arithmetic Wrap-
around
Around Error [FIF]
MITRE CWE
Prior to 2018-01-12: CERT: Unspecified Relationship
CWE 2.11CWE-190, Integer Overflow or Wraparound2016-12-02: CERT: Rule subset of CWE
CWE 2.11CWE-1312017-05-16: CERT: Partial overlap
CWE 2.11CWE-1912017-05-18: CERT: Partial overlap
CWE 2.11CWE-6802017-05-18: CERT: Partial overlap

CERT-CWE Mapping Notes

Key here for mapping notes

CWE-131 and INT30-C


  • Intersection( INT30-C, MEM35-C) = Ø



  • Intersection( CWE-131, INT30-C) =



  • Calculating a buffer size such that the calculation wraps. This can happen, for example, when using malloc() or operator new[] to allocate an array, multiplying the array item size with the array dimension. An untrusted dimension could cause wrapping, resulting in a too-small buffer being allocated, and subsequently overflowed when the array is initialized.



  • CWE-131 – INT30-C =



  • Incorrect calculation of a buffer size that does not involve wrapping. This includes off-by-one errors, for example.


INT30-C – CWE-131 =


  • Integer wrapping where the result is not used to allocate memory.




CWE-680 and INT30-C

Intersection( CWE-680, INT30-C) =


  • Unsigned integer overflows that lead to buffer overflows


CWE-680 - INT30-C =


  • Signed integer overflows that lead to buffer overflows


INT30-C – CWE-680 =


  • Unsigned integer overflows that do not lead to buffer overflows


CWE-191 and INT30-C

Union( CWE-190, CWE-191) = Union( INT30-C, INT32-C) Intersection( INT30-C, INT32-C) == Ø

Intersection(CWE-191, INT30-C) =


  • Underflow of unsigned integer operation


CWE-191 – INT30-C =


  • Underflow of signed integer operation


INT30-C – CWE-191 =


  • Overflow of unsigned integer operation


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"

 


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