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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 will occur. Thereforewraps, less memory is allocated than was requested. As a result, it is necessary to ensure that these arguments, when multiplied, do not wrap.

Modern implementations of the C standard library should check the product of the arguments to for wrap. If the calloc() for an arithmetic overflow. If an overflow occurs, the program should detect and handle it appropriately.

Non-compliant Code Example 1

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-In this example, the user defined function get_size() (not shown) is used to calculate the size requirements for a dynamic array of unsigned long long int and stored in int that is assigned to the variable num_elements. When calloc() is called to allocate the buffer, num_elements is multiplied by the sizeof(unsigned long long) to compute the overall size requirements. If the number of elements multiplied by the size can not 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

...

To correct In this compliant solution, a test is performed on the product of the two arguments num_elements and sizeof(long) are checked before the call to calloc(). It is assumed in the following example that the call to multsize_t() does not return if the multiplication operation results in an overflow and that, if the function returns succesfully, the multiplication performed by calloc() and can also be performed successfully. to determine if wrapping will occur:

Code Block
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Code Block
long *buffer; size_t num_elements = calc_size(); (void) multsize_t; if (num_elements, > SIZE_MAX/sizeof(long)); { /* Handle 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

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 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.

References

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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