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 #include <config.h>
  47
  48 #ifndef _GNU_SOURCE
  49 #define _GNU_SOURCE
  50 #endif
  51
  52 #include <stdlib.h>
  53 #include <stdio.h>
  54 #include <string.h>
  55
  56 #include <sys/types.h>
  57 #include <sys/stat.h>
  58 #include <sys/ioctl.h>
  59 #include <fcntl.h>
  60 #ifdef HAVE_TERMIOS_H
  61 #include <termios.h>
  62 #endif
  63 #include <unistd.h>
  64
  65 #define LINUX_CONS_SAVER_C
  66 #include "cons.saver.h"
  67 #include "../src/tty/win.h"
  68
  69 static void
  70 send_contents (char *buffer, unsigned int columns, unsigned int rows)
  71 {
  72   unsigned char begin_line = 0, end_line = 0;
  73   unsigned int lastline, lc_index, x;
  74   unsigned char message, outbuf[1024], *p;
  75   unsigned short bytes;
  76
  77   lc_index = 2 * rows * columns;
  78   for (lastline = rows; lastline > 0; lastline--)
  79     for (x = 0; x < columns; x++)
  80       {
  81         lc_index -= 2;
  82         if (buffer [lc_index] != ' ')
  83           goto out;
  84       }
  85 out:
  86
  87   message = CONSOLE_CONTENTS;
  88   write (1, &message, 1);
  89
  90   read (0, &begin_line, 1);
  91   read (0, &end_line, 1);
  92   if (begin_line > lastline)
  93     begin_line = lastline;
  94   if (end_line > lastline)
  95     end_line = lastline;
  96
  97   lc_index = (end_line - begin_line) * columns;
  98   bytes = lc_index;
  99   if (lc_index != bytes)
 100     bytes = 0;
 101   write (1, &bytes, 2);
 102   if (! bytes)
 103     return;
 104
 105   p = outbuf;
 106   for (lc_index = 2 * begin_line * columns;
 107        lc_index < 2 * end_line * columns;
 108        lc_index += 2)
 109     {
 110       *p++ = buffer [lc_index];
 111       if (p == outbuf + sizeof (outbuf))
 112         {
 113           write (1, outbuf, sizeof (outbuf));
 114           p = outbuf;
 115         }
 116     }
 117
 118   if (p != outbuf)
 119     write (1, outbuf, p - outbuf);
 120 }
 121
 122 static void __attribute__ ((noreturn))
 123 die (void)
 124 {
 125   unsigned char zero = 0;
 126   write (1, &zero, 1);
 127   exit (3);
 128 }
 129
 130 int
 131 main (int argc, char **argv)
 132 {
 133   unsigned char action = 0, console_flag = 3;
 134   int console_fd, vcsa_fd, console_minor, buffer_size;
 135   struct stat st;
 136   uid_t uid, euid;
 137   char *buffer, *tty_name, console_name [16], vcsa_name [16];
 138   const char *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 }