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Before the lifetime of the last pointer that stores the return value of a call to a standard memory allocation function has ended, it must be matched by a call to free() with that pointer value.

Noncompliant Code Example

In this noncompliant example, the object allocated by the call to malloc() is not freed before the end of the lifetime of the last pointer text_buffer referring to the object:

#include <stdlib.h>
 
enum { BUFFER_SIZE = 32 };

int f(void) {
  char *text_buffer = (char *)malloc(BUFFER_SIZE); 
  if (text_buffer

Freeing memory multiple times has similar consequences to accessing memory after it is freed. The underlying data structures that manage the heap can become corrupted in a way that could introduce security vulnerabilities into a program. These types of issues are referred to as double-free vulnerabilities. In practice, double-free vulnerabilities can be exploited to execute arbitrary code. For instance, VU#62332, 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 only once. Programmers should be wary when freeing memory in a loop or conditional statement, if coded incorrectly, these constructs can lead to double-free vulnerabilities.

Non-compliant Code Example 1

In this example, the memory referred to by x may be freed multiple times. if error_condition is true, then x is freed, and then freed again further along in the code.

*x = malloc (number * sizeof(int));
if (x == NULL) {
   /* Handle Allocation Error */
}
if (error_conditon == 1 return -1;
  }
  return 0;
}

Compliant Solution

In this compliant solution, the pointer is deallocated with a call to free():

#include <stdlib.h>

enum { BUFFER_SIZE = 32 };

int f(void) {
  char /*text_buffer Handle= Error(char Condition*/*)malloc(BUFFER_SIZE); 
  free(x);
}
/* ... */
free(x);

Compliant Solution 1

Only free a pointer to dynamic memory referred to by x once. This can be accomplished in this example by removing the call to free() in the section of code executed when error_condition is true.

if (text_buffer == NULL) {
    return -1;
  }
 
  free(text_buffer);
  return 0;
}

Exceptions

MEM31-C-EX1: Allocated memory does not need to be freed if it is assigned to a pointer whose lifetime includes program termination. The following code example illustrates a pointer that stores the return value from malloc() in a static variable:

#include <stdlib.h>
 
enum { BUFFER_SIZE = 32 };

int f(void) {
  static char *text_buffer = NULL;
  if (text_buffer

*x = malloc (number * sizeof(int));
if (x == NULL) {
  /* Handle Allocation Error */
}
if (error_conditon  text_buffer = (char *)malloc(BUFFER_SIZE); 
    if (text_buffer == 1NULL) {
      return -1;
    }
  /*}
 Handle Error Condition*/
}
/* ... */
free(x);

References

return 0;
}

Risk Assessment

Failing to free memory can result in the exhaustion of system memory resources, which can lead to a denial-of-service attack.

Automated Detection

Related Vulnerabilities

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

Related Guidelines

Key here (explains table format and definitions)

CERT-CWE Mapping Notes

Key here for mapping notes

CWE-404/CWE-459/CWE-771/CWE-772 and FIO42-C/MEM31-C

Intersection( FIO42-C, MEM31-C) = Ø

CWE-404 = CWE-459 = CWE-771 = CWE-772

CWE-404 = Union( FIO42-C, MEM31-C list) where list =

• Failure to free resources besides files or memory chunks, such as mutexes)

Bibliography

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Image Added Image Added Image AddedVU#623332, http://www.kb.cert.org/vuls/id/623332