src/cons.saver.c

   1 /* This program should be setuid vcsa and /dev/vcsa* should be
   2    owned by the same user too.
   3    Partly rewritten by Jakub Jelinek <jakub@redhat.com>.  */
   4
   5 /* General purpose Linux console screen save/restore server
   6    Copyright (C) 1994, 1998, 1999, 2000, 2001, 2002, 2003, 2004, 2005,
   7    2006, 2007 Free Software Foundation, Inc.
   8    Original idea from Unix Interactive Tools version 3.2b (tty.c)
   9    This code requires root privileges.
  10    You may want to make the cons.saver setuid root.
  11    The code should be safe even if it is setuid but who knows?
  12
  13    This program is free software; you can redistribute it and/or modify
  14    it under the terms of the GNU General Public License as published by
  15    the Free Software Foundation; either version 2 of the License, or
  16    (at your option) any later version.
  17
  18    This program is distributed in the hope that it will be useful,
  19    but WITHOUT ANY WARRANTY; without even the implied warranty of
  20    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  21    GNU General Public License for more details.
  22
  23    You should have received a copy of the GNU General Public License
  24    along with this program; if not, write to the Free Software
  25    Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA.  */
  26
  27 /* This code does _not_ need to be setuid root. However, it needs
  28    read/write access to /dev/vcsa* (which is priviledged
  29    operation). You should create user vcsa, make cons.saver setuid
  30    user vcsa, and make all vcsa's owned by user vcsa.
  31
  32    Seeing other peoples consoles is bad thing, but believe me, full
  33    root is even worse. */
  34
  35 /** \file cons.saver.c
  36  *  \brief Source: general purpose Linux console screen save/restore server
  37  *
  38  *  This code does _not_ need to be setuid root. However, it needs
  39  *  read/write access to /dev/vcsa* (which is priviledged
  40  *  operation). You should create user vcsa, make cons.saver setuid
  41  *  user vcsa, and make all vcsa's owned by user vcsa.
  42  *  Seeing other peoples consoles is bad thing, but believe me, full
  43  *  root is even worse.
  44  */
  45
  46 #ifdef HAVE_CONFIG_H
  47 #include <config.h>
  48 #endif
  49
  50 #ifndef _GNU_SOURCE
  51 #define _GNU_SOURCE
  52 #endif
  53
  54 #include <stdlib.h>
  55 #include <stdio.h>
  56 #include <string.h>
  57
  58 #include <sys/types.h>
  59 #include <sys/stat.h>
  60 #include <sys/ioctl.h>
  61 #include <fcntl.h>
  62 #ifdef HAVE_TERMIOS_H
  63 #include <termios.h>
  64 #endif
  65 #include <unistd.h>
  66
  67 #define LINUX_CONS_SAVER_C
  68 #include "cons.saver.h"
  69
  70 static void
  71 send_contents (char *buffer, unsigned int columns, unsigned int rows)
  72 {
  73   unsigned char begin_line = 0, end_line = 0;
  74   unsigned int lastline, index, x;
  75   unsigned char message, outbuf[1024], *p;
  76   unsigned short bytes;
  77
  78   index = 2 * rows * columns;
  79   for (lastline = rows; lastline > 0; lastline--)
  80     for (x = 0; x < columns; x++)
  81       {
  82         index -= 2;
  83         if (buffer [index] != ' ')
  84           goto out;
  85       }
  86 out:
  87
  88   message = CONSOLE_CONTENTS;
  89   write (1, &message, 1);
  90
  91   read (0, &begin_line, 1);
  92   read (0, &end_line, 1);
  93   if (begin_line > lastline)
  94     begin_line = lastline;
  95   if (end_line > lastline)
  96     end_line = lastline;
  97
  98   index = (end_line - begin_line) * columns;
  99   bytes = index;
 100   if (index != bytes)
 101     bytes = 0;
 102   write (1, &bytes, 2);
 103   if (! bytes)
 104     return;
 105
 106   p = outbuf;
 107   for (index = 2 * begin_line * columns;
 108        index < 2 * end_line * columns;
 109        index += 2)
 110     {
 111       *p++ = buffer [index];
 112       if (p == outbuf + sizeof (outbuf))
 113         {
 114           write (1, outbuf, sizeof (outbuf));
 115           p = outbuf;
 116         }
 117     }
 118
 119   if (p != outbuf)
 120     write (1, outbuf, p - outbuf);
 121 }
 122
 123 static void __attribute__ ((noreturn))
 124 die (void)
 125 {
 126   unsigned char zero = 0;
 127   write (1, &zero, 1);
 128   exit (3);
 129 }
 130
 131 int
 132 main (int argc, char **argv)
 133 {
 134   unsigned char action = 0, console_flag = 3;
 135   int console_fd, vcsa_fd, console_minor, buffer_size;
 136   struct stat st;
 137   uid_t uid, euid;
 138   char *buffer, *tty_name, console_name [16], vcsa_name [16], *p, *q;
 139   struct winsize winsz;
 140
 141   close (2);
 142
 143   if (argc != 2)
 144     die ();
 145
 146   tty_name = argv [1];
 147   if (strnlen (tty_name, 15) == 15
 148       || strncmp (tty_name, "/dev/", 5))
 149     die ();
 150
 151   setsid ();
 152   uid = getuid ();
 153   euid = geteuid ();
 154
 155   if (seteuid (uid) < 0)
 156     die ();
 157   console_fd = open (tty_name, O_RDONLY);
 158   if (console_fd < 0)
 159     die ();
 160   if (fstat (console_fd, &st) < 0 || ! S_ISCHR (st.st_mode))
 161     die ();
 162   if ((st.st_rdev & 0xff00) != 0x0400)
 163     die ();
 164   console_minor = (int) (st.st_rdev & 0x00ff);
 165   if (console_minor < 1 || console_minor > 63)
 166     die ();
 167   if (st.st_uid != uid)
 168     die ();
 169
 170   switch (tty_name [5])
 171     {
 172     /* devfs */
 173     case 'v': p = "/dev/vc/%d"; q = "/dev/vcc/a%d"; break;
 174     /* /dev/ttyN */
 175     case 't': p = "/dev/tty%d"; q = "/dev/vcsa%d"; break;
 176     default: die (); break;
 177     }
 178
 179   snprintf (console_name, sizeof (console_name), p, console_minor);
 180   if (strncmp (console_name, tty_name, sizeof (console_name)) != 0)
 181     die ();
 182
 183   if (seteuid (euid) < 0)
 184     die ();
 185
 186   snprintf (vcsa_name, sizeof (vcsa_name), q, console_minor);
 187   vcsa_fd = open (vcsa_name, O_RDWR);
 188   if (vcsa_fd < 0)
 189     die ();
 190   if (fstat (vcsa_fd, &st) < 0 || ! S_ISCHR (st.st_mode))
 191     die ();
 192
 193   if (seteuid (uid) < 0)
 194     die ();
 195
 196   winsz.ws_col = winsz.ws_row = 0;
 197   if (ioctl (console_fd, TIOCGWINSZ, &winsz) < 0
 198       || winsz.ws_col <= 0 || winsz.ws_row <= 0
 199       || winsz.ws_col >= 256 || winsz.ws_row >= 256)
 200     die ();
 201
 202   buffer_size = 4 + 2 * winsz.ws_col * winsz.ws_row;
 203   buffer = calloc (buffer_size, 1);
 204   if (buffer == NULL)
 205     die ();
 206
 207   write (1, &console_flag, 1);
 208
 209   while (console_flag && read (0, &action, 1) == 1)
 210     {
 211       switch (action)
 212         {
 213         case CONSOLE_DONE:
 214           console_flag = 0;
 215           continue;
 216         case CONSOLE_SAVE:
 217           if (seteuid (euid) < 0
 218               || lseek (vcsa_fd, 0, 0) != 0
 219               || fstat (console_fd, &st) < 0 || st.st_uid != uid
 220               || read (vcsa_fd, buffer, buffer_size) != buffer_size
 221               || fstat (console_fd, &st) < 0 || st.st_uid != uid)
 222             memset (buffer, 0, buffer_size);
 223           if (seteuid (uid) < 0)
 224             die ();
 225           break;
 226         case CONSOLE_RESTORE:
 227           if (seteuid (euid) >= 0
 228               && lseek (vcsa_fd, 0, 0) == 0
 229               && fstat (console_fd, &st) >= 0 && st.st_uid == uid)
 230             write (vcsa_fd, buffer, buffer_size);
 231           if (seteuid (uid) < 0)
 232             die ();
 233           break;
 234         case CONSOLE_CONTENTS:
 235           send_contents (buffer + 4, winsz.ws_col, winsz.ws_row);
 236           break;
 237         }
 238
 239       write (1, &console_flag, 1);
 240     }
 241
 242   exit (0);
 243 }