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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

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. VU#623332, which describes a double-free vulnerability in the MIT Kerberos 5 function krb5_recvauth(), is one example. To eliminate double-free vulnerabilities, it is necessary to guarantee that dynamic memory is freed exactly one time. Programmers should be wary when freeing memory in a loop or conditional statement; if coded incorrectly, these constructs can lead to double-free vulnerabilities.

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Code Example

In this noncompliant example, the memory referred to by x may be freed twice: once if error_condition is true and again at the end of the code.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
x = malloc (number * sizeof(int));
if (x 32 };

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

Compliant Solution

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

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 = (char *)malloc(BUFFER_SIZE); 
  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) {
   /* Handletext_buffer Allocation= Error(char */
}
/* ... */
)malloc(BUFFER_SIZE); 
    if (errortext_conditonbuffer == 1NULL) {
  /*   Handle Error Condition*/
}
/* ... */
free(x);

Priority: P6 Level: L2

Freeing memory multiple times can result in an attacker executing arbitrary code with the permissions of the vulnerable process.

References

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return -1;
    }
  }
  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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