MEM30-C. Do not access freed memory

Do no evaluate any pointers into freed memory after an allocated block of dynamic storage has been deallocated by a memory management function, including dereferencing or acting as an operand of an arithmetic operation, type casting, or using the pointer as the right-hand side of an assignment.

According to the C Standard, the behavior of a program that uses the value of a pointer that refers to space deallocated by a call to the free() or realloc() function is undefined. (See undefined behavior 177  of Annex J.)

Reading a pointer to deallocated memory is undefined because the pointer value is indeterminate and can have a trap representation. In the latter case, doing so may cause a hardware trap.

Accessing memory once it is freed may corrupt the data structures used to manage the heap. References to memory that has been deallocated are referred to as dangling pointers. Accessing a dangling pointer can result in exploitable vulnerabilities.

When memory is freed, its contents may remain intact and accessible because it is at the memory manager's discretion when to reallocate or recycle the freed chunk. The data at the freed location may appear valid. However, this can change unexpectedly, leading to unintended program behavior. As a result, it is necessary to guarantee that memory is not written to or read from once it is freed.

Noncompliant Code Example

This example from Brian Kernighan and Dennis Ritchie [Kernighan 1988] shows both the incorrect and correct techniques for deleting items from a linked list. The incorrect solution, clearly marked as wrong in the book, is bad because p is freed before the p->next is executed, so p->next reads memory that has already been freed.

for (p = head; p != NULL; p = p->next)
    free(p);

Compliant Solution

Kernighan and Ritchie also show the correct solution. To correct this error, a reference to p->next is stored in q before freeing p.

for (p = head; p != NULL; p = q) {
  q = p->next;
  free(p);
}
head = NULL;

Noncompliant Code Example

In this noncompliant code example, buff is written to after it has been freed. These vulnerabilities can be easily exploited to run arbitrary code with the permissions of the vulnerable process and are seldom this obvious. Typically, allocations and frees are far removed, making it difficult to recognize and diagnose these problems.

int main(int argc, const char *argv[]) {
  char *buff;

  buff = (char *)malloc(BUFFERSIZE);
  if (!buff) {
     /* Handle error condition */
  }
  /* ... */
  free(buff);
  /* ... */
  strncpy(buff, argv[1], BUFFERSIZE-1);
}

Compliant Solution

In this compliant solution do not free the memory until it is no longer required.

int main(int argc, const char *argv[]) {
  char *buff;

  buff = (char *)malloc(BUFFERSIZE);
  if (!buff) {
     /* Handle error condition */
  }
  /* ... */
  strncpy(buff, argv[1], BUFFERSIZE-1);
  /* ... */
  free(buff);
}

Noncompliant Code Example

In this noncompliant example (CVE-2009-1364) from libwmf version 0.2.8.4, the return value of gdRealloc (a simple wrapper around realloc which reallocates space pointed to by im->clip->list) is set to more. However, the value of im->clip->list is used directly afterwards in the code, and the C Standard specifies that if realloc moves the area pointed to, then the original is freed. An attacker can then execute arbitrary code by forcing a reallocation (with a sufficient im->clip->count) and accessing freed memory [xorl 2009].

void gdClipSetAdd(gdImagePtr im,gdClipRectanglePtr rect) {
  gdClipRectanglePtr more;
  if (im->clip == 0) {
    ...
  }
  if (im->clip->count == im->clip->max) {
    more = gdRealloc (im->clip->list,(im->clip->max + 8) *
                      sizeof (gdClipRectangle));
    if (more == 0) return; //if the realloc fails, then we have not lost the im->clip->list value
    im->clip->max += 8;
  }
  im->clip->list[im->clip->count] = (*rect);
  im->clip->count++;

Compliant Solution

The compliant solution simply reassigns im->clip->list to the value of more after the call to realloc.

void gdClipSetAdd(gdImagePtr im,gdClipRectanglePtr rect) {
  gdClipRectanglePtr more;
  if (im->clip == 0) {
    ...
  }
  if (im->clip->count == im->clip->max) {
    more = gdRealloc (im->clip->list,(im->clip->max + 8) *
                      sizeof (gdClipRectangle));
    if (more == 0) return;
    im->clip->max += 8;
    im->clip->list = more;
  }
  im->clip->list[im->clip->count] = (*rect);
  im->clip->count++;

Risk Assessment

Reading memory that has already been freed can lead to abnormal program termination and denial-of-service attacks. Writing memory that has already been freed can lead to the execution of arbitrary code with the permissions of the vulnerable process.

Automated Detection

Related Vulnerabilities

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

Related Guidelines

Bibliography