MEM51-CPP. Properly deallocate dynamically allocated resources

The C programming language provides several ways to allocate memory, such as std::malloc(), std::calloc(), and std::realloc(), which can be used by a C++ program. However, the C programming language defines only a single way to free the allocated memory: std::free(). See MEM31-C. Free dynamically allocated memory when no longer needed and MEM34-C. Only free memory allocated dynamically for rules specifically regarding C allocation and deallocation requirements.

The C++ programming language adds additional ways to allocate memory, such as the operators new, new[], and placement new, and allocator objects. Unlike C, C++ provides multiple ways to free dynamically allocated memory, such as the operators delete, delete[](), and deallocation functions on allocator objects.

Do not call a deallocation function on anything other than  nullptr , or a pointer returned by the corresponding allocation function described by the following.

Passing a pointer value to an inappropriate deallocation function can result in undefined behavior.

The C++ Standard, [expr.delete], paragraph 2 [ISO/IEC 14882-2014], in part, states the following:

In the first alternative (delete object), the value of the operand of delete may be a null pointer value, a pointer to a non-array object created by a previous new-expression, or a pointer to a subobject (1.8) representing a base class of such an object (Clause 10). If not, the behavior is undefined. In the second alternative (delete array), the value of the operand of delete may be a null pointer value or a pointer value that resulted from a previous array new-expression. If not, the behavior is undefined.

Deallocating a pointer that is not allocated dynamically (including non-dynamic pointers returned from calls to placement new()) is undefined behavior because the pointer value was not obtained by an allocation function. Deallocating a pointer that has already been passed to a deallocation function is undefined behavior because the pointer value no longer points to memory that has been dynamically allocated.

When an operator such as new is called, it results in a call to an overloadable operator of the same name, such as operator new(). These overloadable functions can be called directly but carry the same restrictions as their operator counterparts. That is, calling operator delete() and passing a pointer parameter has the same constraints as calling the delete operator on that pointer. Further, the overloads are subject to scope resolution, so it is possible (but not permissible) to call a class-specific operator to allocate an object but a global operator to deallocate the object.

See MEM53-CPP. Explicitly construct and destruct objects when manually managing object lifetime for information on lifetime management of objects when using memory management functions other than the new and delete operators.

Noncompliant Code Example (placement new())

In this noncompliant code example, the local variable space is passed as the expression to the placement new operator. The resulting pointer of that call is then passed to ::operator delete(), resulting in undefined behavior due to ::operator delete() attempting to free memory that was not returned by ::operator new().

#include <iostream>
 
struct S {
  S() { std::cout << "S::S()" << std::endl; }
  ~S() { std::cout << "S::~S()" << std::endl; }
};

void f() {
  alignas(struct S) char space[sizeof(struct S)];
  S *s1 = new (&space) S;

  // ...

  delete s1;
}

Compliant Solution (placement new())

This compliant solution removes the call to ::operator delete(), instead explicitly calling s1's destructor. This is one of the few times when explicitly invoking a destructor is warranted.

#include <iostream>
 
struct S {
  S() { std::cout << "S::S()" << std::endl; }
  ~S() { std::cout << "S::~S()" << std::endl; }
};
 
void f() {
  alignas(struct S) char space[sizeof(struct S)];
  S *s1 = new (&space) S;
 
  // ...

  s1->~S();
}

Noncompliant Code Example (Uninitialized delete)

In this noncompliant code example, two allocations are attempted within the same try block, and if either fails, the catch handler attempts to free resources that have been allocated. However, because the pointer variables have not been initialized to a known value, a failure to allocate memory for i1 may result in passing ::operator delete() a value (in i2) that was not previously returned by a call to ::operator new(), resulting in undefined behavior.

#include <new>
 
void f() {
  int *i1, *i2;
  try {
    i1 = new int;
    i2 = new int;
  } catch (std::bad_alloc &) {
    delete i1;
    delete i2;
  }
}

Compliant Solution (Uninitialized delete)

This compliant solution initializes both pointer values to nullptr, which is a valid value to pass to ::operator delete().

#include <new>
 
void f() {
  int *i1 = nullptr, *i2 = nullptr;
  try {
    i1 = new int;
    i2 = new int;
  } catch (std::bad_alloc &) {
    delete i1;
    delete i2;
  }
}

Noncompliant Code Example (Double-Free)

Once a pointer is passed to the proper deallocation function, that pointer value is invalidated. Passing the pointer to a deallocation function a second time when the pointer value has not been returned by a subsequent call to an allocation function results in an attempt to free memory that has not been allocated dynamically. The underlying data structures that manage the heap can become corrupted in a way that can introduce security vulnerabilities into a program. These types of issues are called double-free vulnerabilities. In practice, double-free vulnerabilities can be exploited to execute arbitrary code.

In this noncompliant code example, the class C is given ownership of a P *, which is subsequently deleted by the class destructor. The C++ Standard, [class.copy], paragraph 7 [ISO/IEC 14882-2014], states the following: 

If the class definition does not explicitly declare a copy constructor, one is declared implicitly. If the class definition declares a move constructor or move assignment operator, the implicitly declared copy constructor is defined as deleted; otherwise, it is defined as defaulted (8.4). The latter case is deprecated if the class has a user-declared copy assignment operator or a user-declared destructor.

Despite the presence of a user-declared destructor, C will have an implicitly defaulted copy constructor defined for it, and this defaulted copy constructor will copy the pointer value stored in p, resulting in a double-free: the first free happens when g() exits and the second free happens when h() exits.

struct P {};

class C {
  P *p;
  
public:
  C(P *p) : p(p) {}
  ~C() { delete p; }  
  
  void f() {}
};

void g(C c) {
  c.f();
}

void h() {
  P *p = new P;
  C c(p);
  g(c);
}

Compliant Solution (Double-Free)

In this compliant solution, the copy constructor and copy assignment operator for C are explicitly deleted. This deletion would result in an ill-formed program with the definition of g() from the preceding noncompliant code example due to use of the deleted copy constructor. Consequently, g() was modified to accept its parameter by reference, removing the double-free.

struct P {};

class C {
  P *p;
  
public:
  C(P *p) : p(p) {}
  C(const C&) = delete;
  ~C() { delete p; }
 
  void operator=(const C&) = delete;
  
  void f() {}
};

void g(C &c) {
  c.f();
}

void h() {
  P *p = new P;
  C c(p);
  g(c);
}

Noncompliant Code Example (array new[])

In the following noncompliant code example, an array is allocated with array new[] but is deallocated with a scalar delete call instead of an array delete[] call, resulting in undefined behavior.

void f() {
  int *array = new int[10];
  // ...
  delete array;
}

Compliant Solution (array new[])

In the compliant solution, the code is fixed by replacing the call to delete with a call to delete [] to adhere to the correct pairing of memory allocation and deallocation functions.

void f() {
  int *array = new int[10];
  // ...
  delete[] array;
}

Noncompliant Code Example (malloc())

In this noncompliant code example, the call to malloc() is mixed with a call to delete.

#include <cstdlib>
void f() {
  int *i = static_cast<int *>(std::malloc(sizeof(int)));
  // ...
  delete i;
}

This code does not violate MEM53-CPP. Explicitly construct and destruct objects when manually managing object lifetime because it complies with the MEM53-CPP-EX1 exception.

Implementation Details

Some implementations of ::operator new() result in calling std::malloc(). On such implementations, the ::operator delete() function is required to call std::free() to deallocate the pointer, and the noncompliant code example would behave in a well-defined manner. However, this is an implementation detail and should not be relied on—implementations are under no obligation to use underlying C memory management functions to implement C++ memory management operators.

Compliant Solution (malloc())

In this compliant solution, the pointer allocated by std::malloc() is deallocated by a call to std::free() instead of delete.

#include <cstdlib>

void f() {
  int *i = static_cast<int *>(std::malloc(sizeof(int)));
  // ...
  std::free(i);
}

Noncompliant Code Example ( new )

In this noncompliant code example, std::free() is called to deallocate memory that was allocated by new. A common side effect of the undefined behavior caused by using the incorrect deallocation function is that destructors will not be called for the object being deallocated by std::free().

#include <cstdlib>
 
struct S {
  ~S();
};

void f() {
  S *s = new S();
  // ...
  std::free(s);
}

Additionally, this code violates MEM53-CPP. Explicitly construct and destruct objects when manually managing object lifetime.

Compliant Solution (new)

In this compliant solution, the pointer allocated by new is deallocated by calling delete instead of std::free().

struct S {
  ~S();
};

void f() {
  S *s = new S();
  // ...
  delete s;
}

Noncompliant Code Example (Class new)

In this noncompliant code example, the global new operator is overridden by a class-specific implementation of operator new(). When new is called, the class-specific override is selected, so S::operator new() is called. However, because the object is destroyed with a scoped ::delete operator, the global operator delete() function is called instead of the class-specific implementation S::operator delete(), resulting in undefined behavior.

#include <cstdlib>
#include <new>
 
struct S {
  static void *operator new(std::size_t size) noexcept(true) {
    return std::malloc(size);
  }
  
  static void operator delete(void *ptr) noexcept(true) {
    std::free(ptr);
  }
};

void f() {
  S *s = new S;
  ::delete s;
}

Compliant Solution (class new)

In this compliant solution, the scoped ::delete call is replaced by a nonscoped delete call, resulting in S::operator delete() being called.

#include <cstdlib>
#include <new>
 
struct S {
  static void *operator new(std::size_t size) noexcept(true) {
    return std::malloc(size);
  }
  
  static void operator delete(void *ptr) noexcept(true) {
    std::free(ptr);
  }
};

void f() {
  S *s = new S;
  delete s;
}

Noncompliant Code Example (std::unique_ptr)

In this noncompliant code example, a std::unique_ptr is declared to hold a pointer to an object, but is direct-initialized with an array of objects. When the std::unique_ptr is destroyed, its default deleter calls delete instead of delete[], resulting in undefined behavior.

#include <memory>

struct S {};

void f() {
  std::unique_ptr<S> s{new S[10]};
}

Compliant Solution (std::unique_ptr)

In this compliant solution, the std::unique_ptr is declared to hold an array of objects instead of a pointer to an object. Additionally, std::make_unique() is used to initialize the smart pointer.

#include <memory>

struct S {};

void f() {
  std::unique_ptr<S[]> s = std::make_unique<S[]>(10);
}

Use of std::make_unique() instead of direct initialization will emit a diagnostic if the resulting std::unique_ptr is not of the correct type. Had it been used in the noncompliant code example, the result would have been an ill-formed program instead of undefined behavior. It is best to use std::make_unique() instead of manual initialization by other means.

Noncompliant Code Example (std::shared_ptr)

In this noncompliant code example, a std::shared_ptr is declared to hold a pointer to an object, but is direct-initialized with an array of objects. As with std::unique_ptr, when the std::shared_ptr is destroyed, its default deleter calls delete instead of delete[], resulting in undefined behavior.

#include <memory>

struct S {};

void f() {
  std::shared_ptr<S> s{new S[10]};
}

Compliant Solution (std::shared_ptr)

Unlike the compliant solution for std::unique_ptr, where std::make_unique() is called to create a unique pointer to an array, it is ill-formed to call std::make_shared() with an array type. Instead, this compliant solution manually specifies a custom deleter for the shared pointer type, ensuring that the underlying array is properly deleted.

#include <memory>

struct S {};

void f() {
  std::shared_ptr<S> s{new S[10], [](const S *ptr) { delete [] ptr; }};
}

Risk Assessment

Passing a pointer value to a deallocation function that was not previously obtained by the matching allocation function results in undefined behavior, which can lead to exploitable vulnerabilities.

Automated Detection

Related Vulnerabilities

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

Related Guidelines

Bibliography