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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:

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 perform zero length allocations).

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.

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

int f(void) {
  char *text_buffer = (char *)malloc(BUFFER_SIZE); 
  if (text_buffer
size_t number;
int error_condition = 0;

/* initialize number */

int *x = (int *)malloc(number * sizeof(int));
if (x == 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/* Handle Allocation Error */
}
/* ... */
if (error_condition == 1) {
  /* Handle Error Condition*/  return -1;
  }
 
  free(xtext_buffer);
}
/* ... */
free(x);

Compliant Solution

  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: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.

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

int f(void) {
  static char *text_buffer = NULL;
  if (text_buffer == NULL) {
    text_buffer = (char *)malloc(BUFFER_SIZE); 
    if (text_buffer
size_t number;
int error_condition = 0;

/* initialize number */

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

Note that this solution checks for numeric overflow (see INT32-C. Ensure that operations on signed integers do not result in 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.

...

Tool	Version	Checker	Description
Astrée	Include Page Astrée_V Astrée_V		Supported, but no explicit checker
Axivion Bauhaus Suite	Include Page Axivion Bauhaus Suite_V Axivion Bauhaus Suite_V	CertC-MEM31	Can detect dynamically allocated resources that are not freed
CodeSonar	Include Page CodeSonar_V CodeSonar_V	ALLOC.LEAK	Leak
Compass/ROSE
Coverity	Include Page Coverity_V Coverity_V	RESOURCE_LEAK ALLOC_FREE_MISMATCH	Finds resource leaks from variables that go out of scope while owning a resource

...

Cppcheck	Include Page Cppcheck_V Cppcheck_V	memleak leakReturnValNotUsed leakUnsafeArgAlloc memleakOnRealloc	Doesn't use return value of memory allocation function
Cppcheck Premium	Include Page Cppcheck Premium_V Cppcheck Premium_V	memleak leakReturnValNotUsed leakUnsafeArgAlloc memleakOnRealloc	Doesn't use return value of memory allocation function
Helix QAC	Include Page Helix QAC_V Helix QAC_V	DF2706, DF2707, DF2708 C++3337, C++3338
Klocwork	Include Page Klocwork_V Klocwork_V	CL.FFM.ASSIGN CL.FFM.COPY CL.SHALLOW.ASSIGN CL.SHALLOW.COPY FMM.MIGHT FMM.MUST
LDRA tool suite	Include Page LDRA_V LDRA_V	50 D	Partially implemented
Parasoft C/C++test	Include Page Parasoft_V Parasoft_V	CERT_C-MEM31-a	Ensure resources are freed
Parasoft Insure++			Runtime analysis
PC-lint Plus	Include Page PC-lint Plus_V PC-lint Plus_V	429	Fully supported
Polyspace Bug Finder	Include Page Polyspace Bug Finder_V Polyspace Bug Finder_V	CERT C: Rule MEM31-C	Checks for memory leak (rule fully covered)
PVS-Studio	Include Page PVS-Studio_V PVS-Studio_V	V773
SonarQube C/C++ Plugin	Include Page SonarQube C/C++ Plugin_V SonarQube C/C++ Plugin_V	S3584
Splint	Include Page Splint_V Splint_V
TrustInSoft Analyzer	Include Page TrustInSoft Analyzer_V TrustInSoft Analyzer_V	malloc	Exhaustively verified.

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