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The calloc() function takes two arguments: the number of elements to allocate and the storage size of those elements. The Typically, calloc() function multiples these arguments together and allocates the resulting quantity of memory. However, if implementations multiply these arguments to determine how much memory to allocate. Historically, some implementations failed to check whether out-of-bounds results silently wrapped [RUS-CERT Advisory 2002-08:02]. If the result of multiplying the number of elements to allocate and the storage size cannot be represented properly as a size_t, an arithmetic overflow might occur. Thereforewraps, less memory is allocated than was requested. As a result, it is necessary to check the product of the arguments to calloc() for an arithmetic overflow. If an overflow occurs, the program should detect and handle it appropriately.

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ensure that these arguments, when multiplied, do not wrap.

Modern implementations of the C standard library should check for wrap. If the calloc() function implemented by the libraries used for a particular implementation properly handles unsigned integer wrapping (in conformance with INT30-C. Ensure that unsigned integer operations do not wrap) when multiplying the number of elements to allocate and the storage size, that is sufficient to comply with this recommendation and no further action is required.

Noncompliant Code Example

In this noncompliant 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, then calloc() may allocate a buffer of insufficient size. When data is copied to that buffer, a buffer an overflow may occur.

size_t num_elements = get_size();

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

Compliant Solution

In this compliant solution, the multiplication of the two arguments num_elements and sizeof(long) is evaluated are checked before the call to calloc() to determine if an overflow will occur. The multsize_t() function sets errno to a non-zero value if the multiplication operation overflows.wrapping will occur:

long *buffer;
size_t num_elements = calc_size();
(void) multsize_t
if (num_elements,  > SIZE_MAX/sizeof(long));
if (errno) {
  /* handleHandle error condition */
}
buffer = (long *)calloc(num_elements, sizeof(long));
if (buffer == NULL) {
  /* handleHandle 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 in compliance with ERR33-C. Detect and handle standard library errors.

Risk Assessment

Integer overflow 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 sure integer overflow 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 linnked linked libraries.

References

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Automated Detection

Related Vulnerabilities

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

Related Guidelines

Bibliography

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