csu/check_fds.c

   1 /* Copyright (C) 2000-2017 Free Software Foundation, Inc.
   2    This file is part of the GNU C Library.
   3
   4    The GNU C Library is free software; you can redistribute it and/or
   5    modify it under the terms of the GNU Lesser General Public
   6    License as published by the Free Software Foundation; either
   7    version 2.1 of the License, or (at your option) any later version.
   8
   9    The GNU C Library is distributed in the hope that it will be useful,
  10    but WITHOUT ANY WARRANTY; without even the implied warranty of
  11    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
  12    Lesser General Public License for more details.
  13
  14    You should have received a copy of the GNU Lesser General Public
  15    License along with the GNU C Library; if not, see
  16    <http://www.gnu.org/licenses/>.  */
  17
  18 #include <errno.h>
  19 #include <fcntl.h>
  20 #include <paths.h>
  21 #include <unistd.h>
  22 #include <sys/stat.h>
  23 #include <sys/sysmacros.h>
  24
  25 /* Try to get a machine dependent instruction which will make the
  26    program crash.  This is used in case everything else fails.  */
  27 #include <abort-instr.h>
  28 #ifndef ABORT_INSTRUCTION
  29 /* No such instruction is available.  */
  30 # define ABORT_INSTRUCTION
  31 #endif
  32
  33 #include <device-nrs.h>
  34 #include <not-cancel.h>
  35
  36
  37 /* Should other OSes (e.g., Hurd) have different versions which can
  38    be written in a better way?  */
  39 static void
  40 check_one_fd (int fd, int mode)
  41 {
  42   /* Note that fcntl() with this parameter is not a cancellation point.  */
  43   if (__builtin_expect (__libc_fcntl (fd, F_GETFD), 0) == -1
  44       && errno == EBADF)
  45     {
  46       const char *name;
  47       dev_t dev;
  48
  49       /* For writable descriptors we use /dev/full.  */
  50       if ((mode & O_ACCMODE) == O_WRONLY)
  51         {
  52           name = _PATH_DEV "full";
  53           dev = makedev (DEV_FULL_MAJOR, DEV_FULL_MINOR);
  54         }
  55       else
  56         {
  57           name = _PATH_DEVNULL;
  58           dev = makedev (DEV_NULL_MAJOR, DEV_NULL_MINOR);
  59         }
  60
  61       /* Something is wrong with this descriptor, it's probably not
  62          opened.  Open /dev/null so that the SUID program we are
  63          about to start does not accidentally use this descriptor.  */
  64       int nullfd = __open_nocancel (name, mode, 0);
  65
  66       /* We are very paranoid here.  With all means we try to ensure
  67          that we are actually opening the /dev/null device and nothing
  68          else.
  69
  70          Note that the following code assumes that STDIN_FILENO,
  71          STDOUT_FILENO, STDERR_FILENO are the three lowest file
  72          decsriptor numbers, in this order.  */
  73       struct stat64 st;
  74       if (__builtin_expect (nullfd != fd, 0)
  75           || __builtin_expect (__fxstat64 (_STAT_VER, fd, &st), 0) != 0
  76           || __builtin_expect (S_ISCHR (st.st_mode), 1) == 0
  77           || st.st_rdev != dev)
  78         /* We cannot even give an error message here since it would
  79            run into the same problems.  */
  80         while (1)
  81           /* Try for ever and ever.  */
  82           ABORT_INSTRUCTION;
  83     }
  84 }
  85
  86
  87 void
  88 __libc_check_standard_fds (void)
  89 {
  90   /* Check all three standard file descriptors.  The O_NOFOLLOW flag
  91      is really paranoid but some people actually are.  If /dev/null
  92      should happen to be a symlink to somewhere else and not the
  93      device commonly known as "/dev/null" we bail out.  */
  94   check_one_fd (STDIN_FILENO, O_WRONLY | O_NOFOLLOW);
  95   check_one_fd (STDOUT_FILENO, O_RDONLY | O_NOFOLLOW);
  96   check_one_fd (STDERR_FILENO, O_RDONLY | O_NOFOLLOW);
  97 }