Variable length arrays (VLAs), a conditionally supported language feature, are essentially the same as traditional C arrays except that they are declared with a size that is not a constant integer expression and can be declared only at block scope or function prototype scope and no linkage. When supported, a variable length array can be declared

{ /* Block scope */
  char vla[size];
}

where the integer expression size and the declaration of vla are both evaluated at runtime. If the size argument supplied to a variable length array is not a positive integer value, the behavior is undefined. (See undefined behavior 75.)  Additionally, if the magnitude of the argument is excessive, the program may behave in an unexpected way. An attacker may be able to leverage this behavior to overwrite critical program data [Griffiths 2006]. The programmer must ensure that size arguments to variable length arrays, especially those derived from untrusted data, are in a valid range.

Because variable length arrays are a conditionally supported feature of C11, their use in portable code should be guarded by testing the value of the macro __STDC_NO_VLA__. Implementations that do not support variable length arrays indicate it by setting __STDC_NO_VLA__ to the integer constant 1.

Noncompliant Code Example

In this noncompliant code example, a variable length array of size size is declared. The size is declared as size_t in compliance with INT01-C. Use rsize_t or size_t for all integer values representing the size of an object.

#include <stddef.h>

extern void do_work(int *array, size_t size);
 
void func(size_t size) {
  int vla[size];
  do_work(vla, size);
}

However, the value of size may be zero or excessive, potentially giving rise to a security vulnerability.

Compliant Solution

This compliant solution ensures the size argument used to allocate vla is in a valid range (between 1 and a programmer-defined maximum); otherwise, it uses an algorithm that relies on dynamic memory allocation. The solution also avoids unsigned integer wrapping that, given a sufficiently large value of size, would cause malloc to allocate insufficient storage for the array.

#include <stdint.h>
#include <stdlib.h>
 
enum { MAX_ARRAY = 1024 };
extern void do_work(int *array, size_t size);
 
void func(size_t size) {
  if (0 == size || SIZE_MAX / sizeof(int) < size) {
    /* Handle error */
    return;
  }
  if (size < MAX_ARRAY) {
    int vla[size];
    do_work(vla, size);
  } else {
    int *array = (int *)malloc(size * sizeof(int));
    if (array == NULL) {
      /* Handle error */
    }
    do_work(array, size);
    free(array);
  }
}

Noncompliant Code Example (sizeof)

The following noncompliant code example defines A to be a variable length array and then uses the sizeof operator to compute its size at runtime. When the function is called with an argument greater than SIZE_MAX / (N1 * sizeof (int)), the runtime sizeof expression may wrap around, yielding a result that is smaller than the mathematical product N1 * n2 * sizeof (int). The call to malloc(), when successful, will then allocate storage for fewer than n2 elements of the array, causing one or more of the final memset() calls in the for loop to write past the end of that storage.

#include <stdlib.h>
#include <string.h>
 
enum { N1 = 4096 };

void *func(size_t n2) {
  typedef int A[n2][N1];

  A *array = malloc(sizeof(A));
  if (!array) {
    /* Handle error */
    return NULL;
  }

  for (size_t i = 0; i != n2; ++i) {
    memset(array[i], 0, N1 * sizeof(int));
  }

  return array;
}

Compliant Solution (sizeof)

This compliant solution prevents sizeof wrapping by detecting the condition before it occurs and avoiding the subsequent computation when the condition is detected.

#include <stdint.h>
#include <stdlib.h>
#include <string.h>
 
enum { N1 = 4096 };

void *func(size_t n2) {
  if (n2 > SIZE_MAX / (N1 * sizeof(int))) {
    /* Prevent sizeof wrapping */
    return NULL;
  }

  typedef int A[n2][N1];

  A *array = malloc(sizeof(A));
  if (!array) {
    /* Handle error */
    return NULL;
  } 

  for (size_t i = 0; i != n2; ++i) {
    memset(array[i], 0, N1 * sizeof(int));
  }
  return array;
}

Implementation Details

Microsoft

Variable length arrays are not supported by Microsoft compilers.

Risk Assessment

Failure to properly specify the size of a variable length array may allow arbitrary code execution or result in stack exhaustion.

Automated Detection

Tool	Version	Checker	Description
Coverity		REVERSE_NEGATIVE	Fully implemented
LDRA tool suite		621 S	Enhanced enforcement
Parasoft C/C++test		CERT_C-ARR32-a	Ensure the size of the variable length array is in valid range
Polyspace Bug Finder		Memory allocated with tainted size Tainted size of variable length array	Size argument to memory function is from an unsecure source Size of the variable-length array (VLA) is from an unsecure source and may be zero, negative, or too large
PRQA QA-C		1051, 2052	Partially implemented
Cppcheck		negativeArraySize	Context sensitive analysis Will warn only if given size is negative
TrustInSoft Analyzer		alloca_bounds	Exhaustively verified.

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)

CERT-CWE Mapping Notes

Key here for mapping notes

CWE-129 and ARR32-C

Intersection( CWE-188, EXP39-C) = Ø

ARR32-C addresses specifying the size of a variable-length array (VLA). CWE-129 addresses invalid array indices, not array sizes.

CWE-758 and ARR32-C

Independent( INT34-C, INT36-C, MSC37-C, FLP32-C, EXP33-C, EXP30-C, ERR34-C, ARR32-C)

CWE-758 = Union( ARR32-C, list) where list =

• Undefined behavior that results from anything other than too large a VLA dimension.

CWE-119 and ARR32-C

• Intersection( CWE-119, ARR32-C) = Ø

• ARR32-C is not about providing a valid buffer but reading/writing outside it. It is about providing an invalid buffer, or one that exhausts the stack.

Bibliography