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#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;
}

Furthermore, this code also violates ARR39-C. Do not add or subtract a scaled integer to a pointer, where array is a pointer to the two-dimensional array, where it should really be a pointer to the latter dimension instead. This means that the memset() call does out-of-bounds writes on all of its invocations except the first.

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. The code also uses an additional typedef to fix the type of array so that memset() never writes past the two-dimensional array.

#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 A1[N1];
  typedef A1 A[n2][N1];

  AA1 *array = (A1*) 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;
}

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

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)

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

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