MEM31-C. Free dynamically allocated memory exactly once

As noted in undefined behavior 179 of Annex J of the C Standard [ISO/IEC 9899:2011], the behavior a program is undefined when

the pointer argument to the free or realloc function does not match a pointer earlier returned by a memory management function, or the space has been deallocated by a call to free or realloc.

Freeing memory multiple times has similar consequences to accessing memory after it is freed. (See MEM30-C. Do not access freed memory.) First, 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 can cause a hardware trap. When reading a freed pointer doesn't cause a trap, the underlying data structures that manage the heap can become corrupted in a way that can introduce security vulnerabilities into a program. These types of issues are called double-free vulnerabilities. In practice, double-free vulnerabilities can be exploited to execute arbitrary code. One example of this is VU#623332, which describes a double-free vulnerability in the MIT Kerberos 5 function krb5_recvauth().

To eliminate double-free vulnerabilities, it is necessary to guarantee that dynamic memory is freed exactly one time. 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. Do not perform zero-length allocations.)

Noncompliant Code Example (malloc())

In this noncompliant code example, the memory referred to by x may be freed twice: once if error_condition is true and again at the end of the code:

int f(size_t n) {
  int error_condition = 0;

  int *x = (int*)malloc(n * sizeof(int));
  if (x == NULL)
    return -1;

  /* Use x and set error_condition on error. */

  if (error_condition == 1) {
    /* Handle error condition*/
    free(x);
  }

  /* ... */
  free(x);
  return error_condition;
}

Compliant Solution (malloc())

In this compliant solution, the free a referenced by x is only freed once. This is accomplished by eliminating the call to free() when error_condition is set:

int f(size_t n) {
  int error_condition = 0;

  if (n > SIZE_MAX / sizeof(int)) {
    errno = EOVERFLOW;
    return -1;
  }

  int *x = (int*)malloc(n * sizeof(int));
  if (x == NULL) {
    /* Report allocation failure to caller. */
    return -1;
  }

  /* Use x and set error_condition on error. */

  if (error_condition != 0) {
    /* Handle error condition and proceed. */
  }

  free(x);

  return error_condition;
}

Note that this solution checks for numeric overflow. (See INT32-C. Ensure that operations on signed integers do not result in overflow.)

Noncompliant Code Example (realloc())

The memory referenced by p may be freed twice in this noncompliant code example:

/* p is a pointer to dynamically allocated memory */
p2 = realloc(p, size);
if (p2 == NULL) {
  free(p); /* p may be indeterminate when (size == 0) */
  return;
}

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.

And Section 7.22.3.5 states:

If memory for the new object cannot be allocated, the old object is not deallocated and its value is unchanged.

If realloc() is called with size equal to 0, then if a null pointer is returned, the old value should be unchanged. However, there are some common but nonconforming implementations that free the pointer, including the following:

1. Glibc (GNU/Linux)
2. AIX
3. HP-UX
4. Solaris
5. OSF/1

In nonconforming implementations, calling free on the original pointer might result in a double-free vulnerability. However, not calling free on the original pointer might result in a memory leak.

Compliant Code Example (realloc())

In this compliant solution, allocations of zero bytes are prevented, ensuring that p is freed exactly once:

/* p is a pointer to dynamically allocated memory */
if (size) {
  p2 = realloc(p, size);
  if (p2 == NULL) {
    free(p);
    return;
  }
}
else {
  free(p);
  return;
}

Exception

MEM31-EX1: Some library implementations accept and ignore a deallocation of already-free memory. If all libraries used by a project have been validated as having this behavior, then this rule can be ignored.

Risk Assessment

Freeing memory multiple times can result in an attacker executing 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