MEM30-C. Do not access freed memory

According [[ISO/IEC 9899-1999]], 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 168 of Annex J).

Reading a pointer to deallocated memory is undefined because the pointer value is indeterminate and may 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 Kernighan and Ritchie [[Kernighan 88]] 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 [ISO/IEC 9899:1999] 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

The LDRA tool suite Version 7.6.0 can detect violations of this rule.

Fortify SCA Version 5.0 can detect violations of this rule.

Splint Version 3.1.1 can detect violations of this rule.

Compass/ROSE can detect violations of the rule.

Klocwork can detect violations of this rule with the UFM.DEREF.MIGHT, UFM.DEREF.MUST, UFM.RETURN.MIGHT, UFM.RETURN.MUST, UFM.USE.MIGHT, and UFM.USE.MUST checkers.  See Klocwork Cross Reference

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 MEM30-CPP. Do not access freed memory.

References

[[ISO/IEC 9899:1999]] Section 7.20.3.2, "The free function"
[[ISO/IEC PDTR 24772]] "DCM Dangling references to stack frames" and "XYK Dangling Reference to Heap"
[[Kernighan 88]] Section 7.8.5, "Storage Management"
[[MISRA 04]] Rule 17.6
[[MITRE 07]] CWE ID 416, "Use After Free"
[[OWASP Freed Memory]]
[[Seacord 05a]] Chapter 4, "Dynamic Memory Management"
[[Viega 05]] Section 5.2.19, "Using freed memory"
[[xorl 2009]] "CVE-2009-1364: LibWMF Pointer Use after free()"

MEM12-C. Consider using a Goto-Chain when leaving a function on error when using and releasing resources      08. Memory Management (MEM)      MEM31-C. Free dynamically allocated memory exactly once