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When the requested size is 0, the behavior of the memory allocation functions malloc(), calloc(), and realloc() is implementation-defined. Section 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 section 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 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 0 is never passed as a size argument to malloc(), size is checked to confirm 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 (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 vulnurability. 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 7.1 or GCC 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 0 bytes can lead to abnormal program termination.

Recommendation

Severity

Likelihood

Remediation Cost

Priority

Level

MEM04-C

low

likely

medium

P6

L2

Automated Detection

Tool

Version

Checker

Description

Compass/ROSE

 

 

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

Related Vulnerabilities

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

Related Guidelines

 Bibliography

[ISO/IEC 9899:2011]Section 7.22.3, "Memory Management Functions"
[Seacord 2013]Chapter 4, "Dynamic Memory Management"
[Vanegue 2010]"Automated Vulnerability Analysis of Zero-Sized Heap Allocations"

 

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