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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.

According to the C Standard, section 6.2.4, paragraph 2 [ISO/IEC 9899:20112024],

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 If a pointer becomes indeterminate when value is used in an evaluation after the object it the pointer points to (or just past) reaches the end of its lifetime, the behavior is undefined.

Attempting Do not attempt to access an object outside of its lifetime can result in . Attempting to do so is undefined behavior and can lead to an exploitable vulnerability. (See also undefined behavior 9  of Appendix in the C Standard, Annex J.)

Noncompliant Code Example (

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Differing Storage Durations)

In this noncompliant code sampleexample, the address of local the variable c_str with automatic storage duration is assigned to the variable p, which has file scopestatic storage duration. The assignment itself is legalvalid, but it is illegal invalid for c_str to go out of scope while p holds its address, as happens at the end of dont_do_this().

#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" */
return 0;
}

Compliant Solution (

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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 */

AlternatelyAlternatively, both p and c_str could be declared with static scopestorage duration.

Compliant Solution (Differing

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Storage Durations)

If it is necessary for p to be defined with file scope, but with static storage duration but c_str with a more limited scopeduration, 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 noncompliant code sample, the function init_array() returns a pointer to a local stack variable, which could be accessed by the caller.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 warningsdiagnostic messages. (See MSC00-C. Compile cleanly at high warning levels.)

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

Noncompliant Code Example (Output Parameter)

In this noncompliant code sampleexample, 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 Upon the return of squirrel_away()'s return, it is illegal for local to go out of scope, 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 execute arbitrary code.

Automated Detection

Related Vulnerabilities

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

Related Guidelines

Key here (explains table format and definitions)

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ISO/IEC 9899:2011 Section 6.2.4, "Storage durations of objects," and section 7.22.3, "Memory management functions"

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ISO/IEC PDTR 24772 "DCM Dangling references to stack frames"

MISRA Rule 8.6

CERT-CWE Mapping Notes

Key here for mapping notes

CWE-562 and DCL30-C

DCL30-C = Union( CWE-562, list) where list =

• Assigning a stack pointer to an argument (thereby letting it outlive the current function

Bibliography

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