MEM00-C. Allocate and free memory in the same module, at the same level of abstraction

Dynamic memory management is a common source of programming flaws that can lead to security vulnerabilities. Decisions regarding how dynamic memory is allocated, used, and deallocated are the burden of the programmer. Poor memory management can lead to security issues such as heap-buffer overflows, dangling pointers, and double-free issues [Seacord 05a]. From the programmer's perspective, memory management involves allocating memory, reading and writing to memory, and deallocating memory.

Allocating and freeing memory in different modules and levels of abstraction may make it difficult to determine when and if a block of memory has been freed, leading to programming defects such as memory leaks, double-free vulnerabilities, accessing freed memory, or writing to freed or unallocated memory.

To avoid these situations, memory should be allocated and freed at the same level of abstraction, and ideally in the same code module. This includes the use of the following memory allocation and deallocation functions described in C99 Section 7.20.3 [ISO/IEC 9899:1999]:

void *malloc(size_t size);

void *calloc(size_t nmemb, size_t size);

void *realloc(void *ptr, size_t size);

void free(void *ptr);

Failing to follow this recommendation has led to real-world vulnerabilities. For example, freeing memory in different modules resulted in a vulnerability in MIT Kerberos 5 [MIT 04]. The MIT Kerberos 5 code in this case contained error-handling logic, which freed memory allocated by the ASN.1 decoders if pointers to the allocated memory were non-null. However, if a detectable error occurred, the ASN.1 decoders freed the memory that they had allocated. When some library functions received errors from the ASN.1 decoders, they also attempted to free the same memory, resulting in a double-free vulnerability.

Noncompliant Code Example

This noncompliant code example shows a double-free vulnerability resulting from memory being allocated and freed at differing levels of abstraction. In this example, memory for the list array is allocated in the process_list() function. The array is then passed to the verify_size() function that performs error checking on the size of the list. If the size of the list is below a minimum size, the memory allocated to the list is freed and the function returns to the caller. The calling function then frees this same memory again, resulting in a double-free and potentially exploitable vulnerability.

enum { MIN_SIZE_ALLOWED = 32 };

int verify_size(char *list, size_t size) {
  if (size < MIN_SIZE_ALLOWED) {
    /* Handle Error Condition */
    free(list);
    return -1;
  }
  return 0;
}

void process_list(size_t number) {
  char *list = (char *)malloc(number);
  if (list == NULL) {
    /* Handle Allocation Error */
  }

  if (verify_size(list, number) == -1) {
      free(list);
      return;
  }

  /* Continue Processing list */

  free(list);
}

The call to free memory in the verify_size() function takes place in a subroutine of the process_list() function, at a different level of abstraction from the allocation, resulting in a violation of this recommendation. The memory deallocation also occurs in error handling code, which is frequently not as well tested as "green paths" through the code.

Compliant Solution

To correct this problem, the error handling code in verify_size() is modified so that it no longer frees list. This change ensures that list is freed only once, at the same level of abstraction, in the process_list() function.

enum { MIN_SIZE_ALLOWED = 32 };

int verify_size(const char *list, size_t size) {
  if (size < MIN_SIZE_ALLOWED) {
    /* Handle Error Condition */
    return -1;
  }
  return 0;
}

void process_list(size_t number) {
  char *list = (char *)malloc(number);

  if (list == NULL) {
    /* Handle Allocation Error */
  }

  if (verify_size(list, number) == -1) {
      free(list);
      return;
  }

  /* Continue Processing list */

  free(list);
}

Risk Assessment

The mismanagement of memory can lead to freeing memory multiple times or writing to already freed memory. Both of these coding errors can result in an attacker executing arbitrary code with the permissions of the vulnerable process. Memory management errors can also lead to resource depletion and denial-of-service attacks.

Automated Detection

The LDRA tool suite V 7.6.0 can detect violations of this recommendation.

Fortify SCA Version 5.0 with CERT C Rule Pack can detect violations of this recommendation.

Compass/ROSE could detect possible violations by reporting any function that has malloc() or free() but not both. This would catch some false positives, as there would be no way to tell if malloc() and free() are 'at the same level of abstraction' if they are in different functions.

The Coverity Prevent Version 5.0 USE_AFTER_FREE checker can detect the specific instances where Memory is deallocated more than once or Read/Write to target of a freed pointer.

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 MEM40-CPP. Allocate and free memory in the same module, at the same level of abstraction.

References

[ISO/IEC 9899:1999] Section 7.20.3, "Memory Management Functions"
[ISO/IEC PDTR 24772] "XYL Memory Leak"
[MIT 04]
[MITRE 07] CWE ID 416, "Use After Free," and CWE ID 415, "Double Free"
[Plakosh 05]
[Seacord 05a] Chapter 4, "Dynamic Memory Management"

08. Memory Management (MEM)      08. Memory Management (MEM)