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The result of calling malloc(0) or calloc() - calloc(1,0When the requested size is 0, the behavior of the memory allocation functions malloc(), calloc(0,0), or calloc(0,1) - to allocate 0 bytes is implementation defined. From a practical standpoint, allocating 0 bytes with calloc() and malloc() can lead to programming errors with critical security implications, such as buffer overflows. This occurs because the result of allocating 0 bytes with calloc() and malloc() may not be considered an error, thus the pointer returned may not be NULL. Instead, the pointer may reference a block of memory on the heap of size zero. If memory is fetched from or stored in that location, a serious error could occur.

Non-Compliant Code Example 1

and realloc() is implementation-defined. Subclause 7.22.3 of the C Standard [ISO/IEC 9899:2011] states:

If the size of the space requested is zero, the behavior is implementation-defined: either a null pointer is returned, or the behavior is as if the size were some nonzero value, except that the returned pointer shall not be used to access an object.

In addition, the amount of storage allocated by a successful call to the allocation function when 0 bytes was requested is unspecified. See unspecified behavior 41 in subclause J.1 of the C Standard.

In cases where the memory allocation functions return a non-null pointer, reading from or writing to the allocated memory area results in undefined behavior. Typically, the pointer refers to a zero-length block of memory consisting entirely of control structures. Overwriting these control structures damages the data structures used by the memory manager.

Noncompliant Code Example (malloc())

The result of calling malloc(0) to allocate 0 bytes is implementation-defined. In this example, a dynamic array of integers is allocated to store s size elements. However, if s size is zero0, the call to malloc(ssize) will may return a reference to a block of memory of size 0 instead of a null pointer. When (nonempty) data is copied to this location, a heap-buffer overflow will occuroccurs.

...
size_t size;

/* Initialize size, possibly by user-controlled input */

int *list = malloc(sizeof(int) * s)malloc(size);
if (list == NULL) {
  /* Handle Allocationallocation Errorerror */
}
else {
/* Continue Processingprocessing list */
}

...

...

Compliant Solution (malloc())

To ensure that zero 0 is never passed as a size argument to malloc(), a check must be made on s to ensure it is not zero. size is checked to confirm it has a positive value:

size_t size;

/* Initialize size, possibly by user-controlled input */

...
if (ssize == 0) {
  /* Handle Errorerror */
}
int *list = malloc(sizeof(int) * s)malloc(size);
if (list == NULL) {
  /* Handle Allocationallocation Errorerror */
}
/* Continue Processingprocessing list */

Noncompliant Code Example (realloc())

The realloc() function deallocates the old object and returns a pointer to a new object of a specified size. If memory for the new object cannot be allocated, the realloc() function does not deallocate the old object, and its value is unchanged. If the realloc() function returns NULL, failing to free the original memory will result in a memory leak. As a result, the following idiom is often recommended for reallocating memory:

size_t nsize = /* Some value, possibly user supplied */;
char *p2;

Risk Assessment

Assuming that allocating zero bytes results in an error can lead to buffer overflows when zero bytes are allocated. Buffer overflows can be exploited by an attacker to run arbitrary code with the permissions of the vulnerable process.

char *p = (char *)malloc(100);
if (p == NULL) {
  /* Handle error */
}

/* ... */

if ((p2 = (char *)realloc(p, nsize)) == NULL) {
  free(p);
  p = NULL;
  return NULL;
}
p = p2;

However, this commonly recommended idiom has problems with zero-length allocations. If the value of nsize in this example is 0, the standard allows the option of either returning a null pointer or returning a pointer to an invalid (for example, zero-length) object. In cases where the realloc() function frees the memory but returns a null pointer, execution of the code results in a double-free vulnerability. If the realloc() function returns a non-null value, but the size was 0, the returned memory will be of size 0, and a heap overflow will occur if nonempty data is copied there.

Implementation Details

If this noncompliant code is compiled with GCC 3.4.6 and linked with libc 2.3.4, invoking realloc(p, 0) returns a non-null pointer to a zero-sized object (the same as malloc(0)). However, if the same code is compiled with either Microsoft Visual Studio or GCC 4.1.0 , realloc(p, 0) returns a null pointer, resulting in a double-free vulnerability.

Compliant Solution (realloc())

This compliant solution does not pass a size argument of zero to the realloc() function:

size_t nsize;
/* Initialize nsize */
char *p2;
char *p = (char *)malloc(100);
if (p == NULL) {
  /* Handle error */
}

/* ... */

p2 = NULL;
if (nsize != 0) {
  p2 = (char *)realloc(p, nsize);
}
if (p2 == NULL) {
  free(p);
  p = NULL;
  return NULL;
}
p = p2;

Risk Assessment

Allocating 0 bytes can lead to abnormal program termination.

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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