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

[ISO/IEC 14882-2003] Section 3.8, "Object Lifetime" describes a number of situations in which trying to access an object outside of its lifetime leads to undefined behavior.

Attempting to access an object outside of its lifetime can result in an exploitable vulnerability.

Noncompliant Code Example (

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Differing Storage Durations)

This In this noncompliant code example declares the variable p as a pointer to a constant char with file scope. The value of str , the address of the variable c_str with automatic storage duration is assigned to p within the dont_do_this() function. However, str has automatic storage duration, so the lifetime of str ends when the the variable p, which has static storage duration. The assignment itself is valid, but it is invalid for c_str to go out of scope while p holds its address, as happens at the end of dont_do_this() function exits.

#include <stdio.h>
 
const char *p;
void dont_do_this(void) {
    const char c_str[] = "This will change";
    p = c_str; /* dangerous */
    /* ... *Dangerous */
}

void innocuous(void) {
    const char str[] = "Surprise, surprise"printf("%s\n", p);
}
/* ... */

int main(void) {
  dont_do_this();
  innocuous();
/* p might be 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.

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return 0;
}

Compliant Solution (Same Storage Durations)

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

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

Compliant Solution (

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Differing Storage Durations)

If it is necessary for p to be defined with file scope, it with static storage duration but c_str with a more limited duration, then p can be set to NULL before c_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 c_str[] = "Everything OK?";
    p = c_str;
    /* ... */
    p = NULL;
}

Noncompliant Code Example (Return Values)

In this examplenoncompliant code sample, the function init_array() incorrectly returns a pointer to a local stack variable.character array with automatic storage duration, which is accessible to the caller:

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

Some compilers generate a warning diagnostic message when a pointer to an object with automatic variable storage duration is returned from a function, as in this example. Compile your Programmers should compile code at high warning levels and resolve any warnings diagnostic messages (see MSC00-CPP. Compile cleanly at high warning levels).

Compliant Solution (Return Values)

Correcting this example 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().:

#include <stddef.h>
void init_array(char array[]*array, size_t len) {
   /* Initialize array */
   return;
}

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

Noncompliant Code Example (Output Parameter)

In this noncompliant code example, the function squirrel_away() stores a pointer to local variable local into a location pointed to by function parameter ptr_param. Upon the return of squirrel_away(), the pointer ptr_param points to a variable that has an expired lifetime.

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

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

Compliant Solution (Output Parameter)

In this compliant solution, the variable local has static storage duration; consequently, ptr can be used to reference the local array within the rodent() function:

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

void rodent(void) {
  char *ptr;
  squirrel_away(&ptr);
  /* ptr is valid in this scope */
}

Risk Assessment

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

Automated Detection

The LDRA tool suite Version 7.6.0 can detect violations of this rule.

Fortify SCA Version 5.0 can detect violations when an array is declared in a function and then a pointer to that array is returned.

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Related Vulnerabilities

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

Other Languages

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Related Guidelines

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Bibliography

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[Henricson 97] Rule 5.9, "A function must never return, or in any other way give access to, references or pointers to local variables outside the scope in which they are declared."
[Lockheed Martin 05] AV Rule 111, "A function shall not return a pointer or reference to a non-static local object."
[ISO/IEC PDTR 24772] "DCM Dangling references to stack frames"
[MISRA 04] Rule 8.6 

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DCL19-CPP. Initialize automatic local variables on declaration      02. Declarations and Initialization (DCL)      DCL31-CPP. Do not define variadic functions