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This guideline has not been reviewed recently and may be outdated. Please review it and comment to reflect any newly available information.

The size_t type is the unsigned integer type of the result of the sizeof operator. Variables of type size_t are guaranteed to be of sufficient precision to represent the size of an object. The limit of size_t is specified by the SIZE_MAX macro.

The type size_t generally covers the entire address space. ISO/IEC TR 24731-1-2007 introduces a new type rsize_t, defined to be size_t but explicitly used to hold the size of a single object. In code that documents this purpose by using the type rsize_t, the size of an object can be checked to verify that it is no larger than RSIZE_MAX, the maximum size of a normal single object, which provides additional input validation for library functions. See VOID Use TR 24731 for remediation of existing string manipulation code for additional discussion of TR 24731-1.

Any variable that is used to represent the size of an object, including integer values used as sizes, indices, loop counters, and lengths, should be declared as rsize_t if available, or otherwise as size_t.

Noncompliant Code Example

In this noncompliant code example, the dynamically allocated buffer referenced by p overflows for values of n > INT_MAX.

Signed integer overflow causes undefined behavior. The following are two possible conditions under which this code constitutes a serious vulnerability:

sizeof(size_t) == sizeof(int)

The unsigned n may contain a value greater than INT_MAX. Assuming quiet wraparound on signed overflow, the loop executes n times because the comparison i < n is an unsigned comparison. Once i is incremented beyond INT_MAX, i takes on negative values starting with (INT_MIN). Consequently, the memory locations referenced by p[i] precede the memory referenced by p and a write-outside-array bounds occurs.

sizeof(size_t) > sizeof(int)

For values of n where 0 < n <= INT_MAX, the loop executes n times, as expected.

For values of n where INT_MAX < n <= (size_t)INT_MIN, the loop executes INT_MAX times. Once i becomes negative the loop stops, and i remains in the range 0 through INT_MAX.

For values of n where (size_t)INT_MIN < n <= SIZE_MAX, i wraps and takes the values INT_MIN to INT_MIN + (n - (size_t)INT_MIN - 1). Execution of the loop overwrites memory from p[INT_MIN] through p[INT_MIN + (n - (size_t)INT_MIN - 1)].

Compliant Solution (TR 24731-1)

Declaring i to be of type rsize_t eliminates the possible integer overflow condition (in this example).  Also, the argument n is changed to be of type rsize_t to document additional validation in the form of a check against RSIZE_MAX.

Noncompliant Code Example

In this noncompliant code example, the value of length is read from a network connection and passed as an argument to a wrapper to malloc() to allocate the appropriate data block. Provided that the size of an unsigned long is equal to the size of an unsigned int, and both sizes are equal to or smaller than the size of size_t, this code runs as expected. However, if the size of an unsigned long is greater than the size of an unsigned int, length is truncated when passed as an argument to alloc().

Compliant Solution (TR 24731-1)

Declaring both length and the blocksize argument to alloc() as rsize_t eliminates the possibility of truncation. This compliant solution assumes that read_integer_from_network() and read_network_data() can also be modified to accept a length argument of type pointer to rsize_t and rsize_t, respectively. If these functions are part of an external library that cannot be updated, care must be taken when casting length into an unsigned long to ensure that integer truncation does not occur.

Risk Assessment

The improper calculation or manipulation of an object's size can result in exploitable vulnerabilities.

Recommendation

Severity

Likelihood

Remediation Cost

Priority

Level

INT01-CPP

medium

probable

medium

P8

L2

Automated Detection

Tool

Version

Checker

Description

CodeSonar4.4

LANG.TYPE.BASIC

Basic numerical type used
Fortify SCA5.0 with CERT C Rule Pack will detect integer operations that cause overflow, but not all cases where size_t is not used.
LDRA tool suite9.5.8

 

458 S

Partially implemented

Splint3.1.1 can detect violations of this rule.
Compass/ROSE  can detect violations of this recommendation. In particular, it catches comparisons and operations where one operand if of type size_t or rsize_t and the other is not.

Related Vulnerabilities

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 INT01-C. Use rsize_t or size_t for all integer values representing the size of an object.

Bibliography

[ISO/IEC 9899:1999] Section 7.17, "Common definitions <stddef.h>", Section 7.20.3, "Memory management functions"
[ISO/IEC TR 24731-1:2007]


            

2 Comments

  1. How receptive is C++ towards TR24731-1? Is rsize_t acceptable to C++?

  2. Probably you will be interested by other similar patterns of errors described in "Lessons on development of 64-bit C/C ++ applications".