DCL30-C. Do not refer to an object outside of its lifetime

An object has a storage duration that determines its lifetime. There are three storage durations: static, automatic, and allocated.

According to [[ISO/IEC 9899-1999]]:

The lifetime of an object is the portion of program execution during which storage is guaranteed to be reserved for it. An object exists, has a constant address, and retains its last-stored value throughout its lifetime. If an object is referred to outside of its lifetime, the behavior is undefined. The value of a pointer becomes indeterminate when the object it points to reaches the end of its lifetime.

Attempting to access an object outside of its lifetime could result in exploitable vulnerabilities.

Non-Compliant Code Example 1

This non-compliant code example declares the variable p as a pointer to a constant char with file scope. The value of str is assigned to p within the dont_do_this() function. However, str has automatic storage duration so the lifetime of str ends when the dont_do_this() function exits.

char const *p;
void dont_do_this() {
    char const str[] = "This will change";
    p = str; /* dangerous */
    /* ... */
}

void innocuous() {
    char const str[] = "Surprise, surprise";
}
/* ... */
dont_do_this();
innocuous();
/* now, it is likely that p is pointing to "Surprise, surprise" */

As a result of this undefined behavior, it is likely that p will refer to the string literal "Surprise, surprise" after the call to the innocuous() function.

Compliant Solution 1

In this compliant solution, the pointer to the constant char p is moved within the this_is_OK() to prevent this variable from being accessed outside of the function.

void this_is_OK() {
    char const str[] = "Everything OK";
    char const *p = str;
    /* ... */
}
/* pointer p is now inaccessible outside the scope of string str */

Non-Compliant Code Example 2

In this example, the function func() incorrectly returns a pointer to a local stack variable.

char *func() {
   char a[10] ;
   /* Operate on a */
   return &a[0];
}

Compiling with appropriate warning levels should generate a warning when a local stack variable is returned from a function.

Compliant Solution 2

Correcting this example depends on the intent of the programmer. If the intent is to modify the value of a and have that modification persist outside of the scope of func(), then the desired behavior can be achieved by declaring a elsewhere and passing it as a parameter to func().

void func(char a[]) {
   /* Operate on a */
   return;
}

Risk Assessment

Referencing an object outside of its lifetime could result in an attacker being able to run arbitrary code.

Related Vulnerabilities

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

Automated Detection

The Coverity Prevent RETURN_LOCAL checker finds many instances where a function will return a pointer to a local stack variable.

References

[[ISO/IEC 9899-1999]] Section 6.2.4, "Storage durations of objects," and Section 7.20.3, "Memory management functions"