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In this noncompliant code example, integer values returned by parseint(getdata()) are stored into an array of INTBUFSIZE elements of type int called buf [Dowd 2006]. If data is available for insertion into buf (which is indicated by havedata()) and buf_ptr has not been incremented past buf + sizeof(buf), an integer value is stored at the address referenced by buf_ptr. However, the sizeof operator returns the total number of bytes in buf, which is typically a multiple of the number of elements in buf. This value is scaled to the size of an integer and added to buf. As a result, the check to make sure integers are not written past the end of buf is incorrect, and a buffer overflow is possible.

int buf[INTBUFSIZE];
int *buf_ptr = buf;

while (havedata() && buf_ptr < (buf + sizeof(buf))) {
    *buf_ptr++ = parseint(getdata());
}

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int buf[INTBUFSIZE];
int *buf_ptr = buf;

while (havedata() && buf_ptr < &buf[INTBUFSIZE]) {
  *buf_ptr++ = parseint(getdata());
}

This solution works because the C standard guarantees Standard guarantees the address of buf[INTBUFSIZE] even though no such element exists.

Noncompliant Code Example

The following This noncompliant code example is based on a flaw in the OpenBSD operating system. An integer, skip, is added as an offset to a pointer of type struct big. The adjusted pointer is then used as a destination address in a call to memset(). However, when skip is added to the struct big pointer, it is automatically scaled by the size of struct big, which is 32 bytes (assuming 4-byte integers, 8-byte long long integers, and no structure padding). This scaling results in the call to memset() writing to unintended memory.

struct big {
  unsigned long long ull_1; /* typicallyTypically 8 bytes */
  unsigned long long ull_2; /* typicallyTypically 8 bytes */
  unsigned long long ull_3; /* typicallyTypically 8 bytes */
  int si_4; /* typicallyTypically 4 bytes */
  int si_5; /* typicallyTypically 4 bytes */
};
/* ... */
 
int f(void) {
  size_t skip = offsetof(struct big, ull_2);
  struct big *s = (struct big *)malloc(sizeof(struct big));
  if (!s) {
   return -1; /* HandleIndicate malloc() errorfailure */
  }

  memset(s + skip, 0, sizeof(struct big) - skip);
  /* ... */
  free(s);
  s = NULL;
  
  return 0;
}

A similar situation occurred in OpenBSD's make command [Murenin 2007].

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To correct this example, the struct big pointer is cast as a char *. This , which causes skip to be scaled by a factor of 1.:

struct big {
  unsigned long long ull_1; /* typicallyTypically 8 bytes */
  unsigned long long ull_2; /* typicallyTypically 8 bytes */
  unsigned long long ull_3; /* typicallyTypically 8 bytes */
  int si_4; /* typicallyTypically 4 bytes */
  int si_5; /* typicallyTypically 4 bytes */
};
/* ... */
 
int f(void) {
  size_t skip = offsetof(struct big, ull_2);
  struct big *s = (struct big *)malloc(sizeof(struct big));
  if (!s) {
    return -1; /* HandleIndicate malloc() errorfailure */
  }

  memset((char *)s + skip, 0, sizeof(struct big) - skip);
/* ... */
  free(s);
  s = NULL;

  return 0;
}

Risk Assessment

Failure to understand and properly use pointer arithmetic can allow an attacker to execute arbitrary code.

Automated Detection

How long is 4 yards plus 3 feet? It is obvious from elementary arithmetic that any answer involving 7 is wrong, as the student did not take the units into account. The right method is to convert both numbers to reflect the same units.

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Related Vulnerabilities

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

Related Guidelines

Bibliography

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