EXP34-C. Do not dereference null pointers

Attempting to dereference a null pointer results in undefined behavior, typically abnormal program termination.

Noncompliant Code Example

In this noncompliant code example, input_str is copied into dynamically allocated memory referenced by str. If malloc() fails, it returns a null pointer that is assigned to str. When str is dereferenced in memcpy(), the program behaves in an unpredictable manner.

size_t size = strlen(input_str)+1;
str = (char *)malloc(size);
memcpy(str, input_str, size);
/* ... */
free(str);
str = NULL;

Compliant Solution

To correct this error, ensure the pointer returned by malloc() is not null. This also ensures compliance with MEM32-C. Detect and handle memory allocation errors.

size_t size = strlen(input_str)+1;
str = (char *)malloc(size);
if (str == NULL) {
  /* Handle Allocation Error */
}
memcpy(str, input_str, size);
/* ... */
free(str);
str = NULL;

Noncompliant Code Example

This noncompliant code example can be found in drivers/net/tun.c and affects Linux kernel 2.6.30 [Goodin 2009].

static unsigned int tun_chr_poll(struct file *file, poll_table * wait)  {
  struct tun_file *tfile = file->private_data;
  struct tun_struct *tun = __tun_get(tfile);
  struct sock *sk = tun->sk;
  unsigned int mask = 0;

  if (!tun)
    return POLLERR;

  DBG(KERN_INFO "%s: tun_chr_poll\n", tun->dev->name);

  poll_wait(file, &tun->socket.wait, wait);

  if (!skb_queue_empty(&tun->readq))
    mask |= POLLIN | POLLRDNORM;

  if (sock_writeable(sk) ||
     (!test_and_set_bit(SOCK_ASYNC_NOSPACE, &sk->sk_socket->flags) &&
     sock_writeable(sk)))
    mask |= POLLOUT | POLLWRNORM;

  if (tun->dev->reg_state != NETREG_REGISTERED)
    mask = POLLERR;

  tun_put(tun);
  return mask;
}

The vulnerability occurs because sk is initialized to tun->sk before checking if tun is equal to NULL. Of course, this should be done first because the GCC compiler (in this case) optimize it and completely remove the if (!tun) check because it is performed after the assignment. As a result, the above vulnerability can result in a null pointer dereference exploit.

Normally, null pointer dereference results in access violation and abnormal program termination. However, it is possible to permit null pointer dereferencing on several operating systems, for example, using mmap(2) with the MAP_FIXED flag on Linux and Mac OS X or using shmat(2) with the SHM_RND flag on Linux [Liu 2009].

Compliant Solution

This compliant solution eliminates the null pointer deference by initializing sk to tun->sk following the null pointer check.

static unsigned int tun_chr_poll(struct file *file, poll_table * wait)  {
  struct tun_file *tfile = file->private_data;
  struct tun_struct *tun = __tun_get(tfile);
  struct sock *sk;
  unsigned int mask = 0;

  if (!tun)
    return POLLERR;

  sk = tun->sk;

  DBG(KERN_INFO "%s: tun_chr_poll\n", tun->dev->name);

  poll_wait(file, &tun->socket.wait, wait);

  if (!skb_queue_empty(&tun->readq))
    mask |= POLLIN | POLLRDNORM;

  if (sock_writeable(sk) ||
     (!test_and_set_bit(SOCK_ASYNC_NOSPACE, &sk->sk_socket->flags) &&
     sock_writeable(sk)))
    mask |= POLLOUT | POLLWRNORM;

  if (tun->dev->reg_state != NETREG_REGISTERED)
    mask = POLLERR;

  tun_put(tun);
  return mask;
}

Risk Assessment

Dereferencing a null pointer results in undefined behavior, typically abnormal program termination. In some situations, however, dereferencing a null pointer can lead to the execution of arbitrary code [Jack 07, van Sprundel 06]. The indicated severity is for this more severe case; on platforms where it is not possible to exploit a null pointer dereference to execute arbitrary code, the actual severity is low.

Automated Detection

The LDRA tool suite Version 7.6.0 can detect violations of this rule.

Fortify SCA Version 5.0 can detect violations of this rule.

Splint Version 3.1.1 can detect violations of this rule.

Compass/ROSE can detect violations of this rule. In particular, Rose ensures that any pointer returned by malloc(), calloc(), or realloc() is first checked for NULL before being used (otherwise it is free()-d). Rose does not handle cases where an allocation is assigned to an lvalue that is not a variable (such as a struct member or C++ function call returning a reference.)

The Coverity Prevent Version 5.0 CHECKED_RETURN, NULL_RETURNS, REVERSE_INULL and FORWARD_NULL checkers can all find violations of this rule. The CHECKED_RETURN finds instances where a pointer is checked against NULL and then later dereferenced. The NULL_RETURNS checker identifies functions that can return a null pointer but are not checked. The REVERSE_INULL identifies code that dereferences a pointer and then checks the pointer against NULL. The FORWARD_NULL checker can find the instances where NULL is explicitly dereferenced or a pointer is checked against null but then dereferenced anyway. Coverity Prevent cannot discover all violations of this rule, so further verification is necessary.

Klocwork can detect violations of this rule with the NPD.* and RNPD.* checkers. See Klocwork Cross Reference

Related Vulnerabilities

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

Other Languages

This rule appears in the C++ Secure Coding Standard as EXP34-CPP. Ensure a null pointer is not dereferenced.

Bibliography

[Goodin 2009]
[ISO/IEC 9899:1999] Section 6.3.2.3, "Pointers"
[ISO/IEC PDTR 24772] "HFC Pointer casting and pointer type changes" and "XYH Null Pointer Dereference"
[Jack 07]
[Liu 2009]
[MITRE 07] CWE ID 476, "NULL Pointer Dereference"
[van Sprundel 06]
[Viega 05] Section 5.2.18, "Null-pointer dereference"

EXP33-C. Do not reference uninitialized memory      03. Expressions (EXP)      EXP35-C. Do not access or modify an array in the result of a function call after a subsequent sequence point