EXP34-C. Do not dereference null pointers

Dereferencing a null pointer is undefined behavior.

On many platforms, dereferencing a null pointer results in abnormal program termination, but this is not required by the standard. See "Clever Attack Exploits Fully-Patched Linux Kernel" [Goodin 2009] for an example of a code execution exploit that resulted from a null pointer dereference.

Noncompliant Code Example

This noncompliant code example is derived from a real-world example taken from a vulnerable version of the libpng library as deployed on a popular ARM-based cell phone [Jack 2007]. The  libpng library allows applications to read, create, and manipulate PNG (Portable Network Graphics) raster image files. The libpng library implements its own wrapper to malloc() that returns a null pointer on error or on being passed a 0-byte-length argument.

This code also violates ERR33-C. Detect and handle standard library errors.

#include <png.h> /* From libpng */
#include <string.h>
 
void func(png_structp png_ptr, int length, const void *user_data) { 
  png_charp chunkdata;
  chunkdata = (png_charp)png_malloc(png_ptr, length + 1);
  /* ... */
  memcpy(chunkdata, user_data, length);
  /* ... */
 }

If length has the value −1, the addition yields 0, and png_malloc() subsequently returns a null pointer, which is assigned to chunkdata. The chunkdata pointer is later used as a destination argument in a call to memcpy(), resulting in user-defined data overwriting memory starting at address 0. In the case of the ARM and XScale architectures, the 0x0 address is mapped in memory and serves as the exception vector table; consequently, dereferencing 0x0 did not cause an abnormal program termination.

Compliant Solution

This compliant solution ensures that the pointer returned by png_malloc() is not null. It also uses the unsigned type size_t to pass the length parameter, ensuring that negative values are not passed to func().

#include <png.h> /* From libpng */
#include <string.h>

 void func(png_structp png_ptr, size_t length, const void *user_data) { 
  png_charp chunkdata;
  if (length == SIZE_MAX) {
    /* Handle error */
  }
  chunkdata = (png_charp)png_malloc(png_ptr, length + 1);
  if (NULL == chunkdata) {
    /* Handle error */
  }
  /* ... */
  memcpy(chunkdata, user_data, length);
  /* ... */

 }

Noncompliant Code Example

In this noncompliant code example, input_str is copied into dynamically allocated memory referenced by c_str. If malloc() fails, it returns a null pointer that is assigned to c_str. When c_str is dereferenced in memcpy(), the program exhibits undefined behavior.  Additionally, if input_str is a null pointer, the call to strlen() dereferences a null pointer, also resulting in undefined behavior. This code also violates ERR33-C. Detect and handle standard library errors.

#include <string.h>
#include <stdlib.h>
 
void f(const char *input_str) {
  size_t size = strlen(input_str) + 1;
  char *c_str = (char *)malloc(size);
  memcpy(c_str, input_str, size);
  /* ... */
  free(c_str);
  c_str = NULL;
  /* ... */
}

Compliant Solution

This compliant solution ensures that both input_str and the pointer returned by malloc() are not null. 

#include <string.h>
#include <stdlib.h>
 
void f(const char *input_str) {
  size_t size;
  char *c_str;
 
  if (NULL == input_str) {
    /* Handle error */
  }
  
  size = strlen(input_str) + 1;
  c_str = (char *)malloc(size);
  if (NULL == c_str) {
    /* Handle error */
  }
  memcpy(c_str, input_str, size);
  /* ... */
  free(c_str);
  c_str = NULL;
  /* ... */
}

Noncompliant Code Example

This noncompliant code example is from a version of 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 sk pointer is initialized to tun->sk before checking if tun is a null pointer. Because null pointer dereferencing is undefined behavior, the compiler (GCC in this case) can optimize away the if (!tun) check because it is performed after tun->sk is accessed, implying that tun is non-null. As a result, this noncompliant code example is vulnerable to a null pointer dereference exploit, because null pointer dereferencing can be permitted on several platforms, for example, by using mmap(2) with the MAP_FIXED flag on Linux and Mac OS X or by using the shmat() POSIX function with the SHM_RND flag [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;

  /* The remaining code is omitted because it is unchanged... */

}

Risk Assessment

Dereferencing a null pointer is undefined behavior, typically abnormal program termination. In some situations, however, dereferencing a null pointer can lead to the execution of arbitrary code [Jack 2007], [van Sprundel 2006]. 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

Related Vulnerabilities

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

Related Guidelines

Bibliography