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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 7572.)  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.

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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.

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Key here (explains table format and definitions)

CERT-CWE Mapping Notes

Key here for mapping notes

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• 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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