Update access.c with potential fix for Type Confusion alert no. 1893

[HaplessIdiot]

Certain casts in C and C++ place no restrictions on the target type. For example, C style casts such as (MyClass*)p allows the programmer to cast any pointer p to an expression of type MyClass*. If the runtime type of p turns out to be a type that's incompatible with MyClass, this results in undefined behavior.

Recommendation
If possible, use dynamic_cast to safely cast between polymorphic types. If dynamic_cast is not an option, use static_cast to restrict the kinds of conversions that the compiler is allowed to perform. If C++ style casts is not an option, carefully check that all casts are safe.

Example
Consider the following class hierachy where we define a base class Shape and two derived classes Circle and Square that are mutually incompatible:

struct Shape {
virtual ~Shape();

virtual void draw() = 0;
};

struct Circle : public Shape {
Circle();

void draw() override {
/* ... */
}

int getRadius();
};

struct Square : public Shape {
Square();

void draw() override {
/* ... */
}

int getLength();
};
The following code demonstrates a type confusion vulnerability where the programmer assumes that the runtime type of p is always a Square. However, if p is a Circle, the cast will result in undefined behavior.

void allocate_and_draw_bad() {
Shape* shape = new Circle;
// ...
// BAD: Assumes that shape is always a Square
Square* square = static_cast<Square*>(shape);
int length = square->getLength();
}
The following code fixes the vulnerability by using dynamic_cast to safely cast between polymorphic types. If the cast fails, dynamic_cast returns a null pointer, which can be checked for and handled appropriately.

void allocate_and_draw_good() {
Shape* shape = new Circle;
// ...
// GOOD: Dynamically checks if shape is a Square
Square* square = dynamic_cast<Square*>(shape);
if(square) {
int length = square->getLength();
} else {
// handle error
}
}
References
Microsoft Learn: Type conversions and type safety.
Common Weakness Enumeration: CWE-843.