MEM56-CPP. Do not store an already-owned pointer value in an unrelated smart pointer

Smart pointers such as std::unique_ptr and std::shared_ptr encode pointer ownership semantics as part of the type system. They wrap a pointer value, provide pointer-like semantics through operator *() and operator->() member functions, and control the lifetime of the pointer they manage. When a smart pointer is constructed from a pointer value, that value is said to be owned by the smart pointer.

Calling std::unique_ptr::release() will relinquish ownership of the managed pointer value. Destruction of, move assignment of, or calling std::unique_ptr::reset() on a std::unique_ptr object will also relinquish ownership of the managed pointer value, but results in destruction of the managed pointer value. If a call to std::shared_ptr::unique() returns true, then destruction of or calling std::shared_ptr::reset() on that std::shared_ptr object will relinquish ownership of the managed pointer value but results in destruction of the managed pointer value.

Some smart pointers, such as std::shared_ptr, allow multiple smart pointer objects to manage the same underlying pointer value. In such cases, the initial smart pointer object owns the pointer value, and subsequent smart pointer objects are related to the original smart pointer. Two smart pointers are related when the initial smart pointer is used in the initialization of the subsequent smart pointer objects. For instance, copying a std::shared_ptr object to another std::shared_ptr object via copy assignment creates a relationship between the two smart pointers, whereas creating a std::shared_ptr object from the managed pointer value of another std::shared_ptr object does not.

Do not create an unrelated smart pointer object with a pointer value that is owned by another smart pointer object. This includes resetting a smart pointer's managed pointer to an already-owned pointer value, such as by calling reset().

Noncompliant Code Example

In this noncompliant code example, two unrelated smart pointers are constructed from the same underlying pointer value. When the local, automatic variable p2 is destroyed, it deletes the pointer value it manages. Then, when the local, automatic variable p1 is destroyed, it deletes the same pointer value, resulting in a double-free vulnerability.

#include <memory>

void f() {
  int *i = new int;
  std::shared_ptr<int> p1(i);
  std::shared_ptr<int> p2(i);
}

Compliant Solution

In this compliant solution, the std::shared_ptr objects are related to one another through copy construction. When the local, automatic variable p2 is destroyed, the use count for the shared pointer value is decremented but still nonzero. Then, when the local, automatic variable p1 is destroyed, the use count for the shared pointer value is decremented to zero, and the managed pointer is destroyed. This compliant solution also calls std::make_shared() instead of allocating a raw pointer and storing its value in a local variable.

#include <memory>

void f() {
  std::shared_ptr<int> p1 = std::make_shared<int>();
  std::shared_ptr<int> p2(p1);
}

Noncompliant Code Example

In this noncompliant code example, the poly pointer value owned by a std::shared_ptr object is cast to the D * pointer type with dynamic_cast in an attempt to obtain a std::shared_ptr of the polymorphic derived type. However, this eventually results in undefined behavior as the same pointer is thereby stored in two different std::shared_ptr objects. When g() exits, the pointer stored in derived is freed by the default deleter. Any further use of poly results in accessing freed memory. When f() exits, the same pointer stored in poly is destroyed, resulting in a double-free vulnerability.

#include <memory>

struct B {
  virtual ~B() = default; // Polymorphic object
  // ...
};
struct D : B {};

void g(std::shared_ptr<D> derived);

void f() {
  std::shared_ptr<B> poly(new D);
  // ...
  g(std::shared_ptr<D>(dynamic_cast<D *>(poly.get())));
  // Any use of poly will now result in accessing freed memory.
}

Compliant Solution

In this compliant solution, the dynamic_cast is replaced with a call to std::dynamic_pointer_cast(), which returns a std::shared_ptr of the polymorphic type with the valid shared pointer value. When g() exits, the reference count to the underlying pointer is decremented by the destruction of derived, but because of the reference held by poly (within f()), the stored pointer value is still valid after g() returns.

#include <memory>

struct B {
  virtual ~B() = default; // Polymorphic object
  // ...
};
struct D : B {};

void g(std::shared_ptr<D> derived);

void f() {
  std::shared_ptr<B> poly(new D);
  // ...
  g(std::dynamic_pointer_cast<D, B>(poly));
  // poly is still referring to a valid pointer value.
}

Noncompliant Code Example

In this noncompliant code example, a std::shared_ptr of type S is constructed and stored in s1. Later, S::g() is called to get another shared pointer to the pointer value managed by s1. However, the smart pointer returned by S::g() is not related to the smart pointer stored in s1. When s2 is destroyed, it will free the pointer managed by s1, causing a double-free vulnerability when s1 is destroyed.

#include <memory>

struct S {
  std::shared_ptr<S> g() { return std::shared_ptr<S>(this); }    
};

void f() {
  std::shared_ptr<S> s1 = std::make_shared<S>();
  // ...
  std::shared_ptr<S> s2 = s1->g();
}

Compliant Solution

The compliant solution is to use std::enable_shared_from_this::shared_from_this() to get a shared pointer from S that is related to an existing std::shared_ptr object. A common implementation strategy is for the std::shared_ptr constructors to detect the presence of a pointer that inherits from std::enable_shared_from_this, and automatically update the internal bookkeeping required for std::enable_shared_from_this::shared_from_this() to work. Note that std::enable_shared_from_this::shared_from_this() requires an existing std::shared_ptr instance that manages the pointer value pointed to by this. Failure to meet this requirement results in undefined behavior, as it would result in a smart pointer attempting to manage the lifetime of an object that itself does not have lifetime management semantics.

#include <memory>

struct S : std::enable_shared_from_this<S> {
  std::shared_ptr<S> g() { return shared_from_this(); }    
};

void f() {
  std::shared_ptr<S> s1 = std::make_shared<S>();
  std::shared_ptr<S> s2 = s1->g();
}

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

Related Guidelines

Bibliography