MEM30-C. Do not access freed memory

Do not 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.  References to memory that has been deallocated are referred to as dangling pointers. Accessing a dangling pointer can result in exploitable vulnerabilities.

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 behavior 177.

Reading a pointer to deallocated memory is undefined behavior because the pointer value is indeterminate and might be a trap representation. Fetching a trap representation might perform a hardware trap (but is not required to).

When memory is freed, its contents might 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 can appear valid. However, this can change unexpectedly, leading to unexpected program behavior. As a result, it is necessary to guarantee that memory is not written to or read from once it is freed.

Writing to memory after it has been freed may corrupt the data structures used to manage the heap.  Freeing memory multiple times has similar consequences to accessing memory after it is freed.

Noncompliant Code Example

This example from Brian Kernighan and Dennis Ritchie [Kernighan 1988] shows both the incorrect and correct techniques for freeing the memory associated with a linked list. In their (intentionally) incorrect solution, p is freed before p->next is executed, so that p->next reads memory that has already been freed.

#include <stdlib.h>
 
struct node {
  int value;
  struct node *next;
};
 
void free_list(struct node *head) {
  for (struct node *p = head; p != NULL; p = p->next) {
    free(p);
  }
}

Compliant Solution

Kernighan and Ritchie also correct this error by storing a reference to p->next  in q before freeing p.

#include <stdlib.h>
 
struct node {
  int value;
  struct node *next;
};
 
void free_list(struct node *head) {
  struct node *q;
  for (struct node *p = head; p != NULL; p = q) {
    q = p->next;
    free(p);
  }
}

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.

#include <stdlib.h>
#include <string.h>
 
int main(int argc, char *argv[]) {
  if (argc < 2) {
    /* Print usage */
    return EXIT_FAILURE;
  }

  char *return_val = 0;
  const size_t bufsize = strlen(argv[1]) + 1;
  char *buf = (char *)malloc(bufsize);
  if (!buf) {
    return EXIT_FAILURE;
  }
  /* ... */
  free(buf);
  /* ... */
  return_val = strncpy(buf, argv[1], bufsize); // Undefined behavior
  if (return_val) {
    return EXIT_FAILURE;
  }
  return EXIT_SUCCESS;
} 

Compliant Solution

In this compliant solution, the memory is freed after its final use:

#include <stdlib.h>
#include <string.h>
 
int main(int argc, char *argv[]) {
  if (argc < 2) {
    /* Print usage */
    return EXIT_FAILURE;
  }
  char *return_val = 0;
  const size_t bufsize = strlen(argv[1]) + 1;
  char *buf = (char *)malloc(bufsize);
  if (!buf) {
    return EXIT_FAILURE;
  }

  /* ... */

  return_val = strncpy(buf, argv[1], bufsize); 
  if (return_val) {
    free(buf);
    return EXIT_FAILURE;
  }

  /* ... */

  free(buf);
  return EXIT_SUCCESS;
} 

Noncompliant Code Example

In this noncompliant example, a diagnostic is required because realloc() may free c_str1 when it returns a null pointer, resulting in c_str1 being freed twice.  The WG14 committee's proposed response to Defect Report #400 makes it implementation-defined whether the old object is deallocated if size is zero and memory for the new object is not allocated, and the current implementation of realloc() in glibc will free c_str1 and return a null pointer for zero byte allocations.  Freeing a pointer twice can result in a potentially exploitable vulnerability commonly referred to as a "double-free exploit" [Seacord 2013].

#include <stdlib.h>
 
void f(char *c_str1, size_t size) {
  char *c_str2 = (char *)realloc(c_str1, size);
  if (c_str2 == NULL) {
    free(c_str1); // diagnostic required
    return;
  }
}

Compliant Solution

This compliant solution does not pass a size argument of zero to the realloc() function, eliminating the possibility of c_str1 being freed twice. 

#include <stdlib.h>
 
void f(char *c_str1, size_t size) {
  if (size != 0) {
    char *c_str2 = (char *)realloc(c_str1, size); 
    if (c_str2 == NULL) {
      free(c_str1); 
    }
  }
  return;
}

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 that 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 block 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 the realloc fails, then we have not lost the
     * im->clip->list value.
     */
    if (more == 0) return; 
    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. 

Freeing memory multiple times has similar consequences to accessing memory after it is freed.  First, reading a pointer to deallocated memory is undefined behavior because the pointer value is indeterminate and might be a trap representation.  When reading a freed pointer doesn't cause a trap, the underlying data structures that manage the heap can become corrupted in a manner that can be exploited to execute arbitrary code.  These coding errors are referred to as "double-free vulnerabilities".

Programmers should be wary when freeing memory in a loop or conditional statement; if coded incorrectly, these constructs can lead to double-free vulnerabilities. It is also a common error to misuse the realloc() function in a manner that results in double-free vulnerabilities. (See MEM04-C. Beware of zero-length allocations.)

Automated Detection

Related Vulnerabilities

VU#623332 describes a double-free vulnerability in the MIT Kerberos 5 function krb5_recvauth(). 

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

Related Guidelines

Bibliography