FIO32-C. Do not perform operations on devices that are only appropriate for files

File names on many operating systems, including Windows and UNIX, may be used to access special files, which are actually devices. Reserved MS-DOS device names include AUX, CON, PRN, COM1, and LPT1. Device files on UNIX systems are used to apply access rights and to direct operations on the files to the appropriate device drivers.

Performing operations on device files that are intended for ordinary character or binary files can result in crashes and denial-of-service attacks. For example, when Windows attempts to interpret the device name as a file resource, it performs an invalid resource access that usually results in a crash [Howard 2002].

Device files in UNIX can be a security risk when an attacker can access them in an unauthorized way. For instance, if attackers can read or write to the /dev/kmem device, they may be able to alter their priority, UID, or other attributes of their process or simply crash the system. Similarly, access to disk devices, tape devices, network devices, and terminals being used by other processes can lead to problems [Garfinkel 1996].

On Linux, it is possible to lock certain applications by attempting to open devices rather than files. Consider the following example:

/dev/mouse
/dev/console
/dev/tty0
/dev/zero

A Web browser that failed to check for these devices would allow an attacker to create a website with image tags such as <IMG src="file:///dev/mouse"> that would lock the user's mouse.

Noncompliant Code Example

In this noncompliant code example, the user can specify a locked device or a FIFO (first-in, first-out) file name, causing the program to hang on the call to fopen():

#include <stdio.h>
 
void func(const char *file_name) {
  FILE *file;
  if ((file = fopen(file_name, "wb")) == NULL) {
    /* Handle error */
  }

  /* Operate on the file */

  fclose(file);
}

Compliant Solution (POSIX)

POSIX defines the O_NONBLOCK flag to open(), which ensures that delayed operations on a file do not hang the program [IEEE Std 1003.1:2013].

When opening a FIFO with O_RDONLY or O_WRONLY set:

• If O_NONBLOCK is set, an open() for reading-only shall return without delay. An open() for writing-only shall return an error if no process currently has the file open for reading.
• If O_NONBLOCK is clear, an open() for reading-only shall block the calling thread until a thread opens the file for writing. An open() for writing-only shall block the calling thread until a thread opens the file for reading.

When opening a block special or character special file that supports non-blocking opens:

• If O_NONBLOCK is set, the open() function shall return without blocking for the device to be ready or available. Subsequent behavior of the device is device-specific.
• If O_NONBLOCK is clear, the open() function shall block the calling thread until the device is ready or available before returning.

Otherwise, the behavior of O_NONBLOCK is unspecified.

Once the file is open, programmers can use the POSIX lstat() and fstat() functions to obtain information about a named file and the S_ISREG() macro to determine if the file is a regular file. (See FIO05-C. Identify files using multiple file attributes.)

Because the behavior of O_NONBLOCK on subsequent calls to read() or write() is unspecified, it is advisable to disable the flag after it has been determined that the file in question is not a special device.

When available (Linux 2.1.126+, FreeBSD, Solaris 10, POSIX.1-2008), the O_NOFOLLOW should also be used. (See POS01-C. Check for the existence of links when dealing with files.) When O_NOFOLLOW is not available, symbolic link checks should use the method from POS35-C. Avoid race conditions while checking for the existence of a symbolic link.

#include <stdio.h>
#include <sys/types.h>
#include <sys/stat.h>
#ifdef O_NOFOLLOW
  #define OPEN_FLAGS O_NOFOLLOW | O_NONBLOCK
#else
  #define OPEN_FLAGS O_NONBLOCK
#endif

void func(const char *file_name) {
  struct stat orig_st;
  struct stat open_st;
  int fd;
  int flags;

  if ((lstat(file_name, &orig_st) != 0) ||
      (!S_ISREG(orig_st.st_mode))) {
    /* Handle error */
  }

  /* A TOCTOU race condition exists here; see below */

  fd = open(file_name, OPEN_FLAGS | O_WRONLY);
  if (fd == -1) {
    /* Handle error */
  }

  if (fstat(fd, &open_st) != 0) {
    /* Handle error */
  }

  if ((orig_st.st_mode != open_st.st_mode) ||
      (orig_st.st_ino  != open_st.st_ino) ||
      (orig_st.st_dev  != open_st.st_dev)) {
    /* The file was tampered with */
  }

  /* 
   * Optional: drop the O_NONBLOCK now that we are sure
   * this is a good file.
   */
  if ((flags = fcntl(fd, F_GETFL)) == -1) {
    /* Handle error */
  }

  if (fcntl(fd, F_SETFL, flags & ~O_NONBLOCK) != 0) {
    /* Handle error */
  }

  /* Operate on the file */

  close(fd);
}

This code contains an intractable TOCTOU (time-of-check, time-of-use) race condition under which an attacker can alter the file referenced by file_name following the call to lstat() but before the call to open(). The switch will be discovered after the file is opened, but opening the file cannot be prevented in the case where this action itself causes undesired behavior.

Essentially, an attacker can switch out a file for one of the file types shown in the following table with the specified effect.

File Types and Effects

This TOCTOU race condition can be prevented if the effected files are maintained in a secure directory. (See FIO15-C. Ensure that file operations are performed in a secure directory.)

Noncompliant Code Example (Windows)

This noncompliant code example uses the GetFileType() API: 

#include <Windows.h>
 
void func(const TCHAR *file_name) {
  HANDLE hFile = CreateFile(file_name,
                            GENERIC_READ | GENERIC_WRITE, 0, 
                            NULL, OPEN_EXISTING,
                            FILE_ATTRIBUTE_NORMAL, NULL);
  if (hFile == INVALID_HANDLE_VALUE) {
    /* Handle error */
  } else if (GetFileType(hFile) != FILE_TYPE_DISK) {
    /* Handle error */
    CloseHandle(hFile);
  } else {
    /* Operate on the file */
    CloseHandle(hFile);
  }
}

Although it may be tempting to use the Win32 GetFileType() function, it is a dangerous API to use in this case. If the file name given identifies a named pipe, and that pipe is currently blocking on a read request, the call to GetFileType() will block until the read request completes. An attacker can then effectively launch a denial-of-service attack on the application. Furthermore, the act of opening a file handle may cause further action to be taken, such as line states being set to their default voltage when opening a serial device.

Compliant Solution (Windows)

Microsoft documents a list of reserved identifiers that represent devices and have a device namespace to be used specifically by devices [MSDN]. This compliant solution tests the given file name against these constructs:

#include <ctype.h>
#include <stdbool.h>
#include <stdlib.h>
#include <string.h>
#include <stdio.h>
 
static bool isReservedName(const char *path) {
  /* This list of reserved names comes from MSDN */
  static const char *reserved[] = { "nul", "con", "prn", "aux",
                                    "com1", "com2", "com3",
                                    "com4", "com5", "com6",
                                    "com7", "com8", "com9",
                                    "lpt1", "lpt2", "lpt3",
                                    "lpt4", "lpt5", "lpt6",
                                    "lpt7", "lpt8", "lpt9" };
  char *lower;
  char *temp;
  bool ret = false;
  
  /*
   * First, check to see if this is a device namespace, which
   * always starts with \\.\, since device namespaces are not
   * legal file paths.
   */
  temp = strstr(path, "\\\\.\\");
  if (temp == path) {
    return true;
  }
 
  /*
   * Since Windows uses a case-insensitive file system, operate
   * on a lowercase version of the given filename. Note: this
   * ignores globalization issues and assumes ASCII
   * characters.
   */
  lower = (char *)malloc(strlen(path) + 1);
  if (!lower) {
    return false;
  }
  
  temp = lower;
  while (*path) {
    *lower++ = tolower(*path++);
  }
  lower = temp;
 
  /* Compare against the list of ancient reserved names */
  for (size_t i = 0; !ret &&
       i < sizeof(reserved) / sizeof(*reserved); ++i) {    
    if (0 == strcmp(lower, reserved[i])) {
      ret = true;
    }
  }
 
  free(lower);
  return ret;
} 

Risk Assessment

Allowing operations that are appropriate only for files to be performed on devices can result in denial-of-service attacks or more serious exploits depending on the platform.

Automated Detection

Related Vulnerabilities

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

Related Guidelines

Bibliography