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Dynamic memory management is a common source of programming flaws that can lead to security vulnerabilities. Poor memory management can lead to security issues, such as heap-buffer overflows, dangling pointers, and double-free issues [Seacord 2013]. 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 burdens the programmer with tracking the lifetime of that block of memory. This may cause confusion regarding may make it difficult to determine when and if a block of memory has been allocated or 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, it is recommended that memory should be allocated and freed at the same level of abstraction and, and ideally, in the same code module. This includes the use of the following memory allocation and deallocation functions described in subclause 7.23.3 of the C Standard [ISO/IEC 9899:2011]:

void *malloc(size_t size);

void *calloc(size_t nmemb, size_t size);

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

void *aligned_alloc(size_t alignment, size_t size);
 
void free(void *ptr);

Failing to follow this recommendation has led to real-world vulnerabilities. For example, freeing The affects of not following this recommendation are best demonstrated by an actual vulnerability. Freeing memory in different modules resulted in a vulnerability in MIT Kerberos 5 MITKRB5-SA-2004-002 [MIT 2004]. The problem was that 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 occuredoccurred, 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, causing resulting in a double-free vulnerability.

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Noncompliant Code Example

This example demonstrates an error that can occur when memory is freed in different functions. The array, which is referred to by list and its size, number, are 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_listsize() function that performs error checking on the size of the list. If the number size of elements in the array is less than the value MIN_SIZE_ALLOWED, list is processed. Otherwise, it is assumed an error has occurred, list is below a minimum size, the memory allocated to the list is freed, and the function returns . If the error occurs in verify_list(), the dynamic memory referred to by list will be freed twice: once in verify_list() and again at the end of process_list()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 list_size) {
  if (size < MIN_SIZE_ALLOWED) {
    /* Handle Errorerror Conditioncondition */
    free(list);
    return -1;
  }

  /* Process list */

  return 0;
}

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

  if (verify_size(list, number) == -1) {
      free(list);
   /* Handle Error */return;
  }

  /* Continue Processingprocessing 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 logic in the error-handling code should be changed in verify_size() is modified so that it no longer frees list. This change ensures that list is freed only once, in func1at the same level of abstraction, in the process_list() function.

enum {
#define MIN_SIZE_ALLOWED 10 = 32 };

int  func2(intverify_size(const char *list, size_t list_size) {
  if (size < MIN_SIZE_ALLOWED) {
     /* Handle Errorerror Conditioncondition */
    return  return-1;
  }
  /* Process list */return 0;
}

void func1 process_list(size_t number) {
  char *list = (char *)malloc(sizeof(intnumber));

  if (list == NULL) {
    /* Handle Allocationallocation Errorerror */
  }

  func2 if (verify_size(list, number) == -1) {
      free(list);
      return;
  }

  /* Continue Processingprocessing 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 problems 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

Related Vulnerabilities

Search for Examples of vulnerabilities resulting from the violation of this recommendation can be found rule on the CERTwebsite CERT website.

References

Related Guidelines

Bibliography

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Image Added Image Added Image Added Wiki Markup\[[ISO/IEC 9899-1999|AA. C References#ISO/IEC 9899-1999]\] Section 7.20.3, "Memory Management Functions" \[[Seacord 05|AA. C References#Seacord 05]\] Chapter 4, "Dynamic Memory Management" \[[Plakosh 05|AA. C References#Plakosh 05]\] \[[MIT Kerberos 5 Security Advisory 2004-002 | http://web.mit.edu/kerberos/advisories/MITKRB5-SA-2004-002-dblfree.txt]\]