MEM04-C. Do not perform zero length allocations

When the requested size is zero the behavior of the memory allocation functions malloc(), calloc(), and realloc() is implementation-defined. According to C99 Section 7.20.3 [[ISO/IEC 9899:1999]]:

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 40 in section J.1 of the 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 will damage the data structures used by the memory.

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 size elements. However, if size is 0, the call to malloc(size) 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 occurs.

size_t size;

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

int *list = (int *)malloc(size);
if (list == NULL) {
  /* Handle allocation error */
}
else {
/* Continue processing list */
}

Compliant Solution

To ensure that zero is never passed as a size argument to malloc(), size is checked to ensure it has a positive value.

size_t size;

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

if (size == 0) {
  /* Handle error */
}
int *list = (int *)malloc(size);
if (list == NULL) {
  /* Handle allocation error */
}
/* Continue processing 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;
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 (e.g., zero-length) object. In cases where the realloc() function frees the memory but returns a null pointer, execution of the code in this example results in a double free. 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 Version 7.1 or GCC version 4.1.0 , realloc(p, 0) returns a null pointer, resulting in a double-free vulnerability.

Compliant Solution

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 zero bytes can lead to abnormal program termination.

Automated Detection

Compass/ROSE can detect some violations of this rule. Is particular, it warns when when the argument to malloc() is a variable that has not been compared against NULL, or is known at compile time to be 0.

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 MEM04-CPP. Do not perform zero length allocations.

References

[[Vanegue 10]] Julien Vanegue. Automated vulnerability analysis of zero sized heap allocations. April 2010.
[[ISO/IEC 9899:1999]] Section 7.20.3, "Memory Management Functions"
[[MITRE 07]] CWE ID 687, "Function Call With Incorrectly Specified Argument Value"
[[Seacord 05]] Chapter 4, "Dynamic Memory Management"

      08. Memory Management (MEM)