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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 un-allocated 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, ideally, and preferably 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 http://web.mit.edu/kerberos/advisories/MITKRB5-SA-2004-002-dblfree.txt. The problem is the [MIT 2004]. The MIT Kerberos 5 code contains in this case contained error-handling logic, which frees freed memory allocated by the ASN.1 decoders if pointers to the allocated memory are were non-null. However, if a detectable error occursoccurred, the ASN.1 decoders themselves free freed the memory which that they have had allocated. When some library functions receive received errors from the ASN.1 decoders, they also attempt attempted to free the same memory, causing resulting in a double-free vulnerability.

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Noncompliant Code Example

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This example attempts to resize the string referenced by buf to make enough room to append the string line. However, once in the function append(), there is no way to determine how buf was allocated. When realloc() is called on buf, since buf does not point to dynamic memory, an error may occur.

Code Block
void append(char *buf, size_t count, size_t size) { char *line = " <- THIS IS A LINE"; int line_len = strlen(line); if ((count + line_len) > size) buf = realloc(buf,count+line_len); strcat(buf,line); }

Compliant Solution 2

/\* NOTE: buf must point to dynamically allocated memory \*/
void append(char \*buf, size_t count, size_t size) {   
  char *line = " <- THIS IS A LINE";   
  int line_len = strlen(line);    
  if ((count + line_len) > size)      
  buf = realloc(buf,count+line_len);   
  strncat(buf,line); 
}

References

Seacord 05 Chapter 4 Dynamic Memory Management

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

Related Vulnerabilities

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

Related Guidelines

Bibliography

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Image Added Image Added Image Added MIT Kerberos 5 [http://web.mit.edu/kerberos/advisories/MITKRB5-SA-2004-002-dblfree.txt