INT31-C. Ensure that integer conversions do not result in lost or misinterpreted data

Integer conversions, including implicit and explicit (using a cast), must be guaranteed not to result in lost or misinterpreted data. This is particularly true for integer values that originate from untrusted sources and are used in any of the following ways:

• 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)
• as an argument to a memory allocation function
• in security critical code

The only integer type conversions that are guaranteed to be safe for all data values and all possible conforming implementations are conversions of an integral value to a wider type of the same signedness. C99 Section 6.3.1.3 says

When a value with integer type is converted to another integer type other than _Bool, if the value can be represented by the new type, it is unchanged.

Otherwise, if the new type is unsigned, the value is converted by repeatedly adding or subtracting one more than the maximum value that can be represented in the new type until the value is in the range of the new type.

Otherwise, the new type is signed and the value cannot be represented in it; either the result is implementation-defined or an implementation-defined signal is raised.

Typically, converting an integer to a smaller type results in truncation of the high-order bits.

Non-Compliant Code Example (Unsigned to Signed)

Type range errors, including loss of data (truncation) and loss of sign (sign errors), can occur when converting from an unsigned type to a signed type. The following non-compliant code example results in a truncation error on most implementations.

unsigned long int ul = ULONG_MAX;
signed char sc;
sc = (signed char)ul; /* cast eliminates warning */

Compliant Solution (Unsigned to Signed)

Validate ranges when converting from an unsigned type to a signed type. The following code, for example, can be used when converting from unsigned long int to a signed char.

unsigned long int ul = ULONG_MAX;
signed char sc;
if (ul <= SCHAR_MAX) {
  sc = (signed char)ul;  /* use cast to eliminate warning */
}
else {
  /* handle error condition */
}

Non-Compliant Code Example (Signed to Unsigned)

Type range errors, including loss of data (truncation) and loss of sign (sign errors), can occur when converting from a signed type to an unsigned type. The following code results in a loss of sign.

signed int si = INT_MIN;
unsigned int ui;
ui = (unsigned int)si;  /* cast eliminates warning */

Compliant Solution (Signed to Unsigned)

Validate ranges when converting from a signed type to an unsigned type. The following code, for example, can be used when converting from signed int to unsigned int.

signed int si = INT_MIN;
unsigned int ui;
if ( (si < 0) || (si > UINT_MAX) ) {
  /* handle error condition */
}
else {
  ui = (unsigned int)si;  /* cast eliminates warning */
}

NOTE: While unsigned types can usually represent all positive values of the corresponding signed type, this relationship is not guaranteed by the C99 standard.

Non-Compliant Code Example (Signed, Loss of Precision)

A loss of data (truncation) can occur when converting from a signed type to a signed type with less precision. The following code results in a truncation error on most implementations.

signed long int sl = LONG_MAX;
signed char sc;
sc = (signed char)sl; /* cast eliminates warning */

Compliant Solution (Signed, Loss of Precision)

Validate ranges when converting from a signed type to a signed type with less precision. The following code can be used, for example, to convert from a signed long int to a signed char.

signed long int sl = LONG_MAX;
signed char sc;
if ( (sl < SCHAR_MIN) || (sl > SCHAR_MAX) ) {
  /* handle error condition */
}
else {
  sc = (signed char)sl; /* use cast to eliminate warning */
}

Conversions from signed types with greater precision to signed types with lesser precision require both the upper and lower bounds to be checked.

Non-Compliant Code Example (Unsigned, Loss of Precision)

A loss of data (truncation) can occur when converting from an unsigned type to an unsigned type with less precision. The following code results in a truncation error on most implementations.

unsigned long int ul = ULONG_MAX;
unsigned char uc;
uc = (unsigned char)ul;  /* cast eliminates warning */

Compliant Solution (Unsigned, Loss of Precision)

Validate ranges when converting from an unsigned type to an unsigned type with lesser precision. The following code can be used, for example, to convert from an unsigned long int to an unsigned char.

unsigned long int ul = ULONG_MAX;
unsigned char uc;
if (ul > UCHAR_MAX) ) {
  /* handle error condition */
}
else {
  uc = (unsigned char)ul; /* use cast to eliminate warning */
}

Conversions from unsigned types with greater precision to signed types with lesser precision only require the upper bounds to be checked.

Exceptions

INT31-EX1: C99 defines minimum ranges for standard integer types. For example, the minimum range for an object of type unsigned short int is 0 to 65,535, while the minimum range for int is -32,767 to +32,767. This means that it is not always possible to represent all possible values of an unsigned short int as an int. However, on the IA-32 architecture, for example, the actual integer range is from -2,147,483,648 to +2,147,483,647, meaning that is quite possible to represent all the values of an unsigned short int as an int on this platform. As a result, it is not necessary to provide a test for this conversion on IA-32. It is not possible to make assumptions about conversions without knowing the precision of the underlying types. If these tests are not provided, assumptions concerning precision must be clearly documented, as the resulting code cannot be safely ported to a system where these assumptions are invalid.

Risk Assessment

Integer truncation errors 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.

The tool Compass Rose is able to detect violations of this rule, but may throw false warnings when limits.h is included. 

The Coverity Prevent NEGATIVE_RETURNS and REVERSE_NEGATIVE checkers both can find violations of this rule. The NEGATIVE_RETURNS checker can find array accesses, loop bounds, and other expressions that may contain dangerous implied integer conversions that would result in unexpected behavior. The REVERSE_NEGATIVE checker can find instances where a negativity check occurs after the negative value has been used for something else. Coverity Prevent cannot discover all violations of this rule, so further verification is necessary.

Related Vulnerabilities

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

References

[[Dowd 06]] Chapter 6, "C Language Issues" (Type Conversions, pp. 223-270)
[[ISO/IEC 9899-1999]] 6.3, "Conversions"
[[ISO/IEC PDTR 24772]] "FLC Numeric Conversion Errors"
[[MISRA 04]] Rules 10.1, 10.3, 10.5, and 12.9
[[MITRE 07]] CWE ID 192, "Integer Coercion Error", CWE ID 197, "Numeric Truncation Error"
[[Seacord 05]] Chapter 5, "Integers"
[[Viega 05]] Section 5.2.9, "Truncation error," Section 5.2.10, "Sign extension error," Section 5.2.11, "Signed to unsigned conversion error," and Section 5.2.12, "Unsigned to signed conversion error"
[[Warren 02]] Chapter 2, "Basics"

INT30-C. Do not perform certain operations on questionably signed results      04. Integers (INT)