MEM34-C. Only free memory allocated dynamically

Freeing memory that is not allocated dynamically can lead to serious errors. The specific consequences of this error depend on the compiler, but they range from nothing to abnormal program termination. Regardless of the compiler, avoid calling free() on anything other than a pointer returned by a dynamic-memory allocation function, such as malloc(), calloc(), or realloc().

A similar situation arises when realloc() is supplied a pointer to non-dynamically allocated memory. The realloc() function is used to resize a block of dynamic memory. If realloc() is supplied a pointer to memory not allocated by a memory allocation function, such as malloc(), the program may terminate abnormally.

Non-Compliant Code Example

This non-compliant code example sets str to reference either dynamically allocated memory or a statically allocated string literal depending on the value of argc. In either case, str is passed as an argument to free(). If anything other than dynamically allocated memory is referenced by str, the call to free(str) is erroneous.

enum { MAX_ALLOCATION = 1000 };

int main(int argc, const char *argv[]) {
  char *str = NULL;
  size_t len;

  if (argc == 2) {
    len = strlen(argv[1])+1;
    if (len > MAX_ALLOCATION) {
      /* Handle Error */
    }
    str = (char *)malloc(len);
    if (str == NULL) {
      /* Handle Allocation Error */
    }
    strcpy(str, argv[1]);
  }
  else {
    str = "usage: $>a.exe [string]";
    printf("%s\n", str);
  }
  /* ... */
  free(str);
  return 0;
}

Compliant Solution

In the compliant solution, the program has been modified to eliminate the possibility of str referencing non-dynamic memory when it is supplied to free().

enum { MAX_ALLOCATION = 1000 };

int main(int argc, const char *argv[]) {
  char *str = NULL;
  size_t len;

  if (argc == 2) {
    len = strlen(argv[1])+1;
    if (len > MAX_ALLOCATION) {
      /* Handle Error */
    }
    str = (char *)malloc(len);
    if (str == NULL) {
      /* Handle Allocation Error */
    }
    strcpy(str, argv[1]);
  }
  else {
    printf("%s\n", "usage: $>a.exe [string]");
    return -1;
  }
  /* ... */
  free(str);
  return 0;
}

Risk Assessment

Freeing or reallocating memory that was not dynamically allocated could lead to abnormal termination and denial-of-service attacks.

Automated Detection

The Coverity Prevent BAD_FREE checker identifies calls to free() where the argument is a pointer to a function or an array. Coverity Prevent cannot discover all violations of this rule, so further verification is necessary.

Related Vulnerabilities

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

References

[[ISO/IEC 9899-1999]] Section 7.20.3, "Memory management functions"
[[Seacord 05]] Chapter 4, "Dynamic Memory Management"

MEM33-C. Use the correct syntax for flexible array members      08. Memory Management (MEM)