ARR30-C. Do not form or use out of bounds pointers or array subscripts

Potentially exploitable undefined behavior can result from any of the following:

• Using pointer arithmetic so that the result does not point into or just past the end of the same object
• Using such pointers in arithmetic expressions
• Dereferencing pointers that do not point to a valid object in memory
• Using an array subscript so that the resulting reference does not refer to an element in the array

The C standard [ISO/IEC 9899:2011] identifies the following distinct situations in which undefined behavior (UB) can arise as a result of invalid pointer operations:

Noncompliant Code Example (Forming Out-of-Bounds Pointer)

In the following noncompliant code example the function f() attempts to validate the index before using it as an offset to the statically allocated table of integers. However, the function fails to reject negative index values. When index is less than zero, the behavior of the addition expression in the return statement of the function is undefined 46. On some implementations, the addition alone can trigger a hardware trap. On other implementations, the addition may produce a result that when dereferenced can trigger a hardware trap. Other implementations still may produce a dereferenceable pointer that points to an object distinct from table. Using such a pointer to access the object may lead to information exposure or cause the wrong object to be modified.

enum { TABLESIZE = 100 };

static int table[TABLESIZE];

int* f(int index) {
  if (index < TABLESIZE)
    return table + index;

  return NULL;
}

Compliant Solution

One compliant solution is to detect and reject invalid values of index if using them in pointer arithmetic would result in an invalid pointer.

enum { TABLESIZE = 100 };

static int table[TABLESIZE];

int* f(int index) {
  if (0 <= index && index < TABLESIZE)
    return table + index;

  return NULL;
}

Compliant Solution

Another, slightly simpler and potentially more efficient compliant solution is to use an unsigned type to avoid having to check for negative values while still rejecting out-of-bounds positive values of index.

enum { TABLESIZE = 100 };

static int table[TABLESIZE];

int* f(size_t index) {
  if (index < TABLESIZE)
    return table + index;

  return NULL;
}

Noncompliant Code Example (Dereferencing Past the End Pointer)

The following noncompliant code example shows the flawed logic in the Windows Distributed Component Object Model (DCOM) Remote Procedure Call (RPC) interface that was exploited by the W32.Blaster.Worm. The error is that the while loop in the GetMachineName() function (used to extract the host name from a longer string) is not sufficiently bounded. When the character array pointed to by pwszTemp does not contain the backslash character among the first MAX_COMPUTERNAME_LENGTH_FQDN + 1 elements, the final valid iteration of the loop will dereference past the end pointer, resulting in exploitable undefined behavior 47. In this case, the actual exploit allowed the attacker to inject executable code into a running program. Economic damage from the Blaster worm has been estimated to be at least $525 million [Pethia 2003].

For a discussion of this programming error in the Common Weakness Enumeration database, see CWE-119, "Failure to constrain operations within the bounds of a memory buffer," and CWE-121, "Stack-based buffer overflow."

error_status_t _RemoteActivation(
      /* ... */, WCHAR *pwszObjectName, ... ) {
   *phr = GetServerPath(
              pwszObjectName, &pwszObjectName);
    /* ... */
}

HRESULT GetServerPath(
  WCHAR *pwszPath, WCHAR **pwszServerPath ){
  WCHAR *pwszFinalPath = pwszPath;
  WCHAR wszMachineName[MAX_COMPUTERNAME_LENGTH_FQDN+1];
  hr = GetMachineName(pwszPath, wszMachineName);
  *pwszServerPath = pwszFinalPath;
}

HRESULT GetMachineName(
  WCHAR *pwszPath,
  WCHAR wszMachineName[MAX_COMPUTERNAME_LENGTH_FQDN+1])
{
  pwszServerName = wszMachineName;
  LPWSTR pwszTemp = pwszPath + 2;
  while ( *pwszTemp != L'\\' )
    *pwszServerName++ = *pwszTemp++;
  /* ... */
}

Compliant Solution

In the following compliant solution, the while loop in the GetMachineName() function is bounded so that the loop terminates when a backslash character is found, the null termination character (L'\0') is discovered, or the end of the buffer is reached. This code does not result in a buffer overflow, even if no backslash character is found in wszMachineName.

HRESULT GetMachineName(
  wchar_t *pwszPath,
  wchar_t wszMachineName[MAX_COMPUTERNAME_LENGTH_FQDN+1])
{
  wchar_t *pwszServerName = wszMachineName;
  wchar_t *pwszTemp = pwszPath + 2;
  wchar_t *end_addr
    = pwszServerName + MAX_COMPUTERNAME_LENGTH_FQDN;
  while ( (*pwszTemp != L'\\')
     &&  ((*pwszTemp != L'\0'))
     && (pwszServerName < end_addr) )
  {
    *pwszServerName++ = *pwszTemp++;
  }

  /* ... */
}

This compliant solution is for illustrative purposes and is not necessarily the solution implemented by Microsoft. This particular "solution" may not be correct because there is no guarantee that a backslash is found.

Noncompliant Code Example (Using Past the End Index)

Similarly to the dereferencing- past-the-end-pointer error, the function insert_in_table() in the following noncompliant code example uses an otherwise valid index to attempt to store a value in an element just past the end of an array.

First, the function incorrectly validates the index pos against the size of the buffer. When the index is equal to size, the function attempts to store value in a memory location just past the end of the buffer.

Second, when the index is greater than size, the function modifies size before growing the size of the buffer. If the call to realloc() fails to increase the size of the buffer, the next call to the function with a value of pos equal to or greater than the original value of size will again attempt to store value in a memory location just past the end of the buffer or beyond.

For a discussion of this programming error in the Common Weakness Enumeration database, see CWE-122, "Heap-based buffer overflow," and CWE-129, "Improper validation of array index."

static int *table = NULL;
static size_t size = 0;

int insert_in_table(size_t pos, int value) {
  if (size < pos) {
    int *tmp;
    size = pos + 1;
    tmp = (int*)realloc(table, sizeof *table * size);
    if (NULL == tmp)
      return -1;

    table = tmp;
  }

  table[pos] = value;
  return 0;
}

Compliant Solution

The following compliant solution correctly validates the index pos by using the <= operator and avoids modifying size until it has verified that the call to realloc() was successful.

static int *table = NULL;
static size_t size = 0;

int insert_in_table(size_t pos, int value) {
  if (size <= pos) {
    int *tmp = (int*)realloc(table, sizeof *table * (pos + 1));
    if (NULL == tmp)
      return -1;   /* indicate failure */

    /* modify size only after realloc succeeds */
    size  = pos + 1;
    table = tmp;
  }

  table[pos] = value;
  return 0;
}

Noncompliant Code Example (Apparently Accessible Out-of-Range Index)

The following noncompliant code example declares matrix to consist of 7 rows and 5 columns in row-major order. The function init_matrix then iterates over all 35 elements in an attempt to initialize each to the value given by the function argument x. However, since multidimensional arrays are declared in C in row-major order, and the function iterates over the elements in column-major order, and when the value of j reaches the value COLS during the first iteration of the outer loop, the function attempts to access element matrix[0][5]. Because the type of matrix is int[7][5], the j subscript is out of range, and the access has undefined behavior 49.

static const size_t COLS = 5;
static const size_t ROWS = 7;

static int matrix[ROWS][COLS];

void init_matrix(int x) {
  for (size_t i = 0; i != COLS; ++i)
    for (size_t j = 0; j != ROWS; ++j)
      matrix[i][j] = x;
}

Compliant Solution

The following compliant solution avoids using out-of-range indices by initializing matrix elements in the same row-major order as multidimensional objects are declared in C.

static const size_t COLS = 5;
static const size_t ROWS = 7;

static int matrix[ROWS][COLS];

void init_matrix(int x) {
  for (size_t i = 0; i != ROWS; ++i)
    for (size_t j = 0; j != COLS; ++j)
      matrix[i][j] = x;
}

Noncompliant Code Example (Pointer Past Flexible Array Member)

In the following noncompliant code example the function find() attempts to iterate over the elements of the flexible array member buf, starting with the second element. However, since function g() does not allocate any storage for the member, the expression first++ in find() attempts to form a pointer just past the end of buf when there are no elements. This attempt results in undefined behavior 62.

struct S {
  size_t len;
  char   buf[];   /* flexible array member */
};

char* find(const struct S *s, int c) {
  char *first = s->buf;
  char *last  = s->buf + s->len;

  while (first++ != last)   /* undefined behavior here */
    if (*first == (unsigned char)c)
      return first;

  return NULL;
}

void g() {
  struct S *s = (struct S*)malloc(sizeof (struct S));
  s->len = 0;
  /* ... */
  char *where = find(s, '.');
  /* ... */
}

Compliant Solution

The following compliant solution avoids incrementing the pointer unless a value past the pointer's current value is known to exist.

struct S {
  size_t len;
  char   buf[];   /* flexible array member */
};

char* find(const struct S *s, int c) {
  char *first = s->buf;
  char *last  = s->buf + s->len;

  while (first != last)   /* avoid incrementing here */
    if (*++first == (unsigned char)c)
      return first;

  return NULL;
}

void g() {
  struct S *s = (struct S*)malloc(sizeof (struct S));
  s->len = 0;
  /* ... */
  char *where = find(s, '.');
  /* ... */
}

Noncompliant Code Example (Invalid Access by Library Function)

In the following noncompliant code example, the function f() calls fread() to read nitems of type wchar_t, each size bytes in size, into an array of BUFSIZ elements, wbuf. However, the expression used to compute the value of nitems fails to account for the fact that, unlike the size of char, the size of wchar_t may be greater than 1. Thus, fread() could attempt to form pointers past the end of wbuf and use them to assign values to nonexisting elements of the array. Such an attempt results in undefined behavior 109. A likely manifestation of this undefined behavior is a classic buffer overflow, which is often exploitable by code injection attacks.

For a discussion of this programming error in the Common Weakness Enumeration database, see CWE-121, "Access of memory location after end of buffer," and CWE-805, "Buffer access with incorrect length value."

void f(FILE *file) {
  wchar_t wbuf[BUFSIZ];

  const size_t size = sizeof *wbuf;
  const size_t nitems = sizeof wbuf;

  size_t nread;

  nread = fread(wbuf, size, nitems, file);
  /* ... */
}

Compliant Solution

The following compliant solution correctly computes the maximum number of items for fread() to read from the file.

void f(FILE *file) {
  wchar_t wbuf[BUFSIZ];

  const size_t size = sizeof *wbuf;
  const size_t nitems = sizeof wbuf / size;

  size_t nread;

  nread = fread(wbuf, size, nitems, file);
  /* ... */
}

Risk Assessment

Accessing out of range pointers or array subscripts for writing can result in a buffer overflow and the execution of arbitrary code with the permissions of the vulnerable process or unintended information disclosure.

Automated Detection

   for (LPWSTR pwszTemp = pwszPath + 2; *pwszTemp != L'\\';
   *pwszTemp++;)

Related Vulnerabilities

CVE-2008-1517 results from a violation of this rule. Before Mac OSX version 10.5.7, the xnu kernel accessed an array at an unverified, user-input index, allowing an attacker to execute arbitrary code by passing an index greater than the length of the array and therefore accessing outside memory [xorl 2009].

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

Related Guidelines

ISO/IEC 9899:2011 Section 6.7.5.2, "Array declarators"

ISO/IEC TR 17961 (Draft) Forming or using out-of-bounds pointers or array subscripts [invptr]

ISO/IEC TR 24772 "XYX Boundary beginning violation," "XYY Wrap-around error," and "XYZ Unchecked array indexing"

MITRE CWE: CWE-119, "Failure to constrain operations within the bounds of a memory buffer"

MITRE CWE: CWE-121, "Stack-based buffer overflow"

MITRE CWE: CWE-122, "Heap-based buffer overflow"

MITRE CWE: CWE-129, "Unchecked array indexing"

MITRE CWE: CWE-788, "Access of memory location after end of buffer"

MITRE CWE: CWE-805, "Buffer access with incorrect length value"

Bibliography

[Finlay 2003]
[Microsoft 2003]
[Pethia 2003]
[Seacord 2005a] Chapter 1, "Running with Scissors"
[Viega 2005] Section 5.2.13, "Unchecked array indexing"
[xorl 2009 ] "CVE-2008-1517: Apple Mac OS X (XNU) Missing Array Index Validation"