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Evaluating a pointer—including dereferencing the pointer, using it as an operand of an arithmetic operation, type casting it,

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and using

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it as the right-hand side of an

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assignment—into memory that has been deallocated by a memory management function is undefined behavior. Pointers to memory that has been deallocated are called dangling pointers. Accessing a dangling pointer can result in exploitable vulnerabilities.

According to the C Standard, using 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. (See undefined behavior 177 of Annex J.)  

Reading a pointer to deallocated memory is undefined behavior because the pointer value is indeterminate and can have  and might be a trap representation. In the latter case, doing so may cause Fetching a trap representation might perform a hardware trap (but is not required to).

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 It is at the memory manager's discretion when to reallocate or recycle the freed memory. When memory is freed, all pointers into it become invalid, and its contents might either be returned to the operating system, making the freed space inaccessible, or remain intact and accessible. As a result, the data at the freed location may can appear valid. However, this can change unexpectedly, leading to unintended program behavior. As a result, it is necessary to guarantee that memory is not to be valid but change unexpectedly. Consequently, memory must not be written to or read from once it is freed.

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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 incorrect solutiontheir (intentionally) incorrect example, p is freed before the p->next is executed, so that p->next reads memory that has already been freed.

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Compliant Solution

Kernighan and Ritchie also show the correct solution. To correct this error , by storing a reference to p->next is stored   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);
  }
}

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In this noncompliant code example, buff buf is written to after it has been freed. These Write-after-free 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[10]) + 1;
  char *buf = (char *)malloc(bufsize);
  if (!buf) {
    return EXIT_FAILURE;
  }
  /* ... */
  free(buf);
  /* ... */
  return_val = strncpystrcpy(buf, argv[10], 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, and the pointer is zeroed in compliance with MEM01-C. Store a new value in pointers immediately after free():

#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[10]) + 1;
  char *buf = (char *)malloc(bufsize);
  if (!buf) {
    return EXIT_FAILURE;
  }

  /* ... */

  return_val = strncpystrcpy(buf, argv[1], bufsize); 
  if (return_val) {
    free(buf)0]);
    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 committee C Standards Committee's proposed response to Defect Report #400 makes it implementation-defined whether or not the old object is deallocated if when size is zero and memory for the new object is not allocated, and the . The current implementation of realloc() in glibc the GNU C Library and Microsoft Visual Studio's Runtime Library 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" vulnerability [Seacord 20132013b].

#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); 
    }
  }
  else {
    return;
}free(c_str1);
  }
 
}

If the intent of calling f() is to reduce the size of the object, then doing nothing when the size is zero would be unexpected; instead, this compliant solution frees the object.

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 This 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 additionally 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. Reading a pointer to deallocated memory is undefined behavior because the pointer value is indeterminate and might be a trap representation. When reading from or writing to freed memory does not cause a trap, it may corrupt the underlying data structures that manage the heap in a manner that can be exploited to execute arbitrary code. Alternatively, writing to memory after it has been freed might modify memory that has been reallocated.

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

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Related Vulnerabilities

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

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

Related Guidelines

Key here (explains table format and definitions)

CERT-CWE Mapping Notes

Key here for mapping notes

CWE-672 and MEM30-C

Intersection( MEM30-C, FIO46-C) = Ø CWE-672 = Union( MEM30-C, list) where list =

• Use of a resource, other than memory after it has been released (eg: reusing a closed file, or expired mutex)

CWE-666 and MEM30-C

Intersection( MEM30-C, FIO46-C) = Ø

CWE-672 = Subset( CWE-666)

CWE-758 and MEM30-C

CWE-758 = Union( MEM30-C, list) where list =

• Undefined behavior that is not covered by use-after-free errors

CWE-415 and MEM30-C

MEM30-C = Union( CWE-456, list) where list =

• Dereference of a pointer after freeing it (besides passing it to free() a second time)

Bibliography

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