DCL30-C. Declare objects with appropriate storage durations

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

According to the C Standard, section 6.2.4, paragraph 2 [ISO/IEC 9899:2011],

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 can result in undefined behavior and lead to an exploitable vulnerability. (See also undefined behavior 9 in Appendix J of the C Standard.)

Noncompliant Code Example (Static Variables)

In this noncompliant code sample, the address of local variable str is assigned to the variable p, which has file scope. The assignment itself is legal, but it is illegal for str to go out of scope while p holds its address, as happens at the end of dont_do_this().

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

void innocuous(void) {
    const char str[] = "Surprise, surprise";
}
/* ... */
dont_do_this();
innocuous();
/* p might be pointing to "Surprise, surprise" */

Compliant Solution (Similar Scope)

In this compliant solution, p is declared with the same scope as str, preventing p from taking on an indeterminate value outside of this_is_OK().

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

Alternately, both p and str could be declared with static scope.

Compliant Solution (Differing Scope)

If it is necessary for p to be defined with file scope, but str with a more limited scope, then p can be set to NULL before str is destroyed. This practice prevents p from taking on an indeterminate value, although any references to p must check for NULL.

const char *p;
void is_this_OK(void) {
    const char str[] = "Everything OK?";
    p = str;
    /* ... */
    p = NULL;
}

Noncompliant Code Example (Return Values)

In this noncompliant code sample, the function init_array() returns a pointer to a local stack variable, which could be accessed by the caller:

char *init_array(void) {
   char array[10];
   /* Initialize array */
   return array;
}

Some compilers generate a warning when a pointer to an automatic variable is returned from a function, as in this example. Compile your code at high warning levels and resolve any warnings. (See MSC00-C. Compile cleanly at high warning levels.)

Compliant Solution (Return Values)

The solution, in this case, depends on the intent of the programmer. If the intent is to modify the value of array and have that modification persist outside of the scope of init_array(), the desired behavior can be achieved by declaring array elsewhere and passing it as an argument to init_array():

void init_array(char array[]) {
   /* Initialize array */
   return;
}

int main(int argc, char *argv[]) {
   char array[10];
   init_array(array);
   /* ... */
   return 0;
}

Noncompliant Code

In this noncompliant code sample, the function squirrel_away() stores a pointer to local stack variable local into a location pointed to by function parameter ptr_param. Since it can be assumed that the pointer variable to which ptr_param points remains alive upon squirrel_away()'s return, it is illegal for local to go out of scope.

void squirrel_away(char **ptr_param) {
   char local[10];
   /* Initialize array */
   *ptr_param = local;
}

void rodent() {
  char *ptr;
  squirrel_away(&ptr);
  /* ptr is live but invalid here */
}

Compliant Solution 

The variable local does not go out of scope for the entire program so, ptr is live and valid in the function rodent():

char local[10];
 
void squirrel_away(char **ptr_param) {
    /* Initialize array */
   *ptr_param = local;
}

void rodent() {
  char *ptr;
  squirrel_away(&ptr);
  /* ptr is live and valid here */
}

Risk Assessment

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

Automated Detection

Related Vulnerabilities

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

Related Guidelines

Bibliography