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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 == NULL) {
    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 = (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

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.  It is also a common error to misuse the {{realloc()}} function in a manner that results in double-free vulnerabilities (see \[[MEM04-A. Do not make assumptions about the result of allocating 0 bytes]\].

Non-Compliant Code Example

In this 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.

x = (int *)malloc (number * sizeof(int));
if (x == NULL) {
  /* 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.

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

Note that this solution checks for numeric overflow \[[INT32-C. Ensure that integer operations do not result in an overflow]\].

Risk Assessment

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

Automated Detection

The LDRA tool suite V 7.6.0 is able to detect violations of this rule.

The Fortify Source Code Analysis Suite Double Free detects instances of memory being freed more than once.

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Automated Detection

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Related Vulnerabilities

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

References

\[[VU#623332|AA. C References#VU623332]\]
\[[MIT 05|AA. C References#MIT 05]\]
\[[OWASP, Double Free|AA. C References#OWASP Double Free]\]
\[[MITRE 07|AA. C References#MITRE 07]\] [CWE ID 415|http://cwe.mitre.org/data/definitions/415.html], "Double Free"
\[[Viega 05|AA. C References#Viega 05]\] "Doubly freeing memory"

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 AddedImage Removed      08. Memory Management (MEM)       MEM32-C. Detect and handle memory allocation errors