MEM07-A. Ensure that the arguments to calloc() when multiplied can be represented as a size_t

The calloc() function takes two arguments: the number of elements to allocate and the storage size of those elements. Typically, calloc() implementations multiply these arguments to determine how much memory to allocate. Historically, some implementations failed to check if out-of-bounds results silently wrap [RUS-CERT Advisory 2002-08:02]. If the result of multiplying the number of elements to allocate and the storage size cannot be represented as a size_t, less memory is allocated than was requested. As a result, it is necessary to ensure that these arguments, when multiplied, can be represented as a size_t.

Modern implementations of the C standard library should check for wrap. If the libraries used for a particular implementation properly handle unsigned integer wrapping on the multiplication, that is sufficient to comply with this recommendation.

Non-Compliant Code Example

In this example, the user-defined function get_size() (not shown) is used to calculate the size requirements for a dynamic array of long int that is assigned to the variable num_elements. When calloc() is called to allocate the buffer, num_elements is multiplied by sizeof(long) to compute the overall size requirements. If the number of elements multiplied by the size cannot be represented as a size_t, calloc() may allocate a buffer of insufficient size. When data is copied to that buffer, a buffer overflow may occur.

size_t num_elements;

/* initialize num_elements to the number of elements needed */

long *buffer = (long *)calloc(num_elements, sizeof(long));
if (buffer == NULL) {
  /* handle error condition */
}
/*...*/
free(buffer);
buffer = NULL; 

Compliant Solution

In this compliant solution, the the two arguments num_elements and sizeof(long) are checked before the call to calloc() to determine if wrapping will occur.

long *buffer;
size_t num_elements;

/* initialize num_elements to the number of elements needed */

if (num_elements > SIZE_MAX/sizeof(long)) {
  /* handle error condition */
}
buffer = (long *)calloc(num_elements, sizeof(long));
if (buffer == NULL) {
  /* handle error condition */
}

Note that the maximum amount of allocatable memory is typically limited to a value less than SIZE_MAX (the maximum value of size_t). Always check the return value from a call to any memory allocation function.

Risk Assessment

Unsigned integer wrapping in memory allocation functions can lead to buffer overflows that can be exploited by an attacker to execute arbitrary code with the permissions of the vulnerable process. Most implementations of calloc() now check to make silent wrapping does not occur, but it is not always safe to assume the version of calloc() being used is secure, particularly when using dynamically linked libraries.

Related Vulnerabilities

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

References

[[ISO/IEC 9899:1999]] Section 7.18.3, "Limits of other integer types"
[[Seacord 05]] Chapter 4, "Dynamic Memory Management"
[[RUS-CERT]] Advisory 2002-08:02, "Flaw in calloc and similar routines"
[[Secunia]] Advisory SA10635, "HP-UX calloc Buffer Size Miscalculation Vulnerability"

MEM06-A. Ensure that sensitive data is not written out to disk      08. Memory Management (MEM)       MEM08-A. Use realloc() only to resize dynamically allocated arrays