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Rule:  MEM50-CPPChecked automatically with code checker

Synopsis:Do not access freed memory
Language:C++
Severity Level:1
Category:Security


Description:

Evaluating a pointer including dereferencing the pointer, using it as an operand of an arithmetic operation, type casting it, and using it as the right-hand side of an assignmentinto memory that has been deallocated by a memory management function is undefined behavior. Pointers to memory that has been deallocated are called dangling pointers. Accessing a dangling pointer can result in exploitable vulnerabilities.

It is at the memory manager's discretion when to reallocate or recycle the freed memory. When memory is freed, all pointers into it become invalid, and its contents might either be returned to the operating system, making the freed space inaccessible, or remain intact and accessible. As a result, the data at the freed location can appear to be valid but change unexpectedly. Consequently, memory must not be written to or read from once it is freed.

Noncompliant Code Example ( new  and delete )

In this noncompliant code example, s is dereferenced after it has been deallocated. If this access results in a write-after-free, the vulnerability can be exploited to run arbitrary code with the permissions of the vulnerable process. Typically, dynamic memory allocations and deallocations are far removed, making it difficult to recognize and diagnose such problems.

#include <new>
   
struct S {
   void f();
};
   
void g() noexcept( false ) {
   S *s = new S;
   // ...
   delete s;
   // ...
   s->f();
}

The function g() is marked noexcept(false) to comply with MEM52-CPP. Detect and handle memory allocation errors.

Compliant Solution ( new  and delete )

In this compliant solution, the dynamically allocated memory is not deallocated until it is no longer required.

#include <new>
 
struct S {
   void f();
};
 
void g() noexcept( false ) {
   S *s = new S;
   // ...
   s->f();
   delete s;
}

Compliant Solution (Automatic Storage Duration)

When possible, use automatic storage duration instead of dynamic storage duration. Since  s is not required to live beyond the scope of  g() , this compliant solution uses automatic storage duration to limit the lifetime of  s to the scope of  g().

struct S {
   void f();
};
 
void g() {
   S s;
   // ...
   s.f();
}

Noncompliant Code Example ( std::unique_ptr )

In the following noncompliant code example, the dynamically allocated memory managed by the buff object is accessed after it has been implicitly deallocated by the object's destructor.

#include <iostream>
#include <memory>
#include <cstring>
   
int main( int argc, const char *argv[]) {
   const char *s = "" ;
   if (argc > 1) {
     enum { BufferSize = 32 };
     try {
       std::unique_ptr< char []> buff( new char [BufferSize]);
       std:: memset (buff.get(), 0, BufferSize);
       // ...
       s = std:: strncpy (buff.get(), argv[1], BufferSize - 1);
     } catch (std::bad_alloc &) {
       // Handle error
     }
   }
 
   std::cout << s << std::endl;
}

This code always creates a null-terminated byte string, despite its use of  strncpy() , because it leaves the final  char in the buffer set to 0.

Compliant Solution ( std::unique_ptr )

In this compliant solution, the lifetime of the buff object extends past the point at which the memory managed by the object is accessed.

#include <iostream>
#include <memory>
#include <cstring>
   
int main( int argc, const char *argv[]) {
   std::unique_ptr< char []> buff;
   const char *s = "" ;
 
   if (argc > 1) {
     enum { BufferSize = 32 };
     try {
       buff.reset( new char [BufferSize]);
       std:: memset (buff.get(), 0, BufferSize);
       // ...
       s = std:: strncpy (buff.get(), argv[1], BufferSize - 1);
     } catch (std::bad_alloc &) {
       // Handle error
     }
   }
 
   std::cout << s << std::endl;
}

Compliant Solution

In this compliant solution, a variable with automatic storage duration of type  std::string is used in place of the  std::unique_ptr<char[]> , which reduces the complexity and improves the security of the solution.

#include <iostream>
#include <string>
   
int main( int argc, const char *argv[]) {
   std::string str;
 
   if (argc > 1) {
     str = argv[1];
   }
 
   std::cout << str << std::endl;
}

Noncompliant Code Example ( std::string::c_str() )

In this noncompliant code example,  std::string::c_str() is being called on a temporary  std::string object. The resulting pointer will point to released memory once the  std::string object is destroyed at the end of the assignment expression, resulting in undefined behavior when accessing elements of that pointer.

#include <string>
   
std::string str_func();
void display_string( const char *);
   
void f() {
   const char *str = str_func().c_str();
   display_string(str);  /* Undefined behavior */
}

Compliant solution ( std::string::c_str() )

In this compliant solution, a local copy of the string returned by  str_func()  is made to ensure that string  str  will be valid when the call to  display_string()  is made.

#include <string>
   
std::string str_func();
void display_string( const char *s);
 
void f() {
   std::string str = str_func();
   const char *cstr = str.c_str();
   display_string(cstr);  /* ok */
}

Noncompliant Code Example

In this noncompliant code example, an attempt is made to allocate zero bytes of memory through a call to  operator new() . If this request succeeds,  operator new()  is required to return a non-null pointer value. However, according to the C++ Standard, [basic.stc.dynamic.allocation], paragraph 2 [ISO/IEC 14882-2014], attempting to dereference memory through such a pointer results in undefined behavior.

#include <new>
 
void f() noexcept( false ) {
   unsigned char *ptr = static_cast <unsigned char *>(::operator new (0));
   *ptr = 0;
   // ...
   ::operator delete (ptr);
}

Compliant Solution

The compliant solution depends on programmer intent. If the programmer intends to allocate a single  unsigned char object, the compliant solution is to use  new instead of a direct call to  operator new() , as this compliant solution demonstrates.

void f() noexcept( false ) {
   unsigned char *ptr = new unsigned char ;
   *ptr = 0;
   // ...
   delete ptr;
}

Compliant Solution

If the programmer intends to allocate zero bytes of memory (perhaps to obtain a unique pointer value that cannot be reused by any other pointer in the program until it is properly released), then instead of attempting to dereference the resulting pointer, the recommended solution is to declare ptr as a  void * , which cannot be dereferenced by a conforming implementation.

#include <new>
 
void f() noexcept( false ) {
   void *ptr = ::operator new (0);
   // ...
   ::operator delete (ptr);
}