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
  68 static void
  69 send_contents (char *buffer, unsigned int columns, unsigned int rows)
  70 {
  71   unsigned char begin_line = 0, end_line = 0;
  72   unsigned int lastline, index, x;
  73   unsigned char message, outbuf[1024], *p;
  74   unsigned short bytes;
  75
  76   index = 2 * rows * columns;
  77   for (lastline = rows; lastline > 0; lastline--)
  78     for (x = 0; x < columns; x++)
  79       {
  80         index -= 2;
  81         if (buffer [index] != ' ')
  82           goto out;
  83       }
  84 out:
  85
  86   message = CONSOLE_CONTENTS;
  87   write (1, &message, 1);
  88
  89   read (0, &begin_line, 1);
  90   read (0, &end_line, 1);
  91   if (begin_line > lastline)
  92     begin_line = lastline;
  93   if (end_line > lastline)
  94     end_line = lastline;
  95
  96   index = (end_line - begin_line) * columns;
  97   bytes = index;
  98   if (index != bytes)
  99     bytes = 0;
 100   write (1, &bytes, 2);
 101   if (! bytes)
 102     return;
 103
 104   p = outbuf;
 105   for (index = 2 * begin_line * columns;
 106        index < 2 * end_line * columns;
 107        index += 2)
 108     {
 109       *p++ = buffer [index];
 110       if (p == outbuf + sizeof (outbuf))
 111         {
 112           write (1, outbuf, sizeof (outbuf));
 113           p = outbuf;
 114         }
 115     }
 116
 117   if (p != outbuf)
 118     write (1, outbuf, p - outbuf);
 119 }
 120
 121 static void __attribute__ ((noreturn))
 122 die (void)
 123 {
 124   unsigned char zero = 0;
 125   write (1, &zero, 1);
 126   exit (3);
 127 }
 128
 129 int
 130 main (int argc, char **argv)
 131 {
 132   unsigned char action = 0, console_flag = 3;
 133   int console_fd, vcsa_fd, console_minor, buffer_size;
 134   struct stat st;
 135   uid_t uid, euid;
 136   char *buffer, *tty_name, console_name [16], vcsa_name [16];
 137   const char *p, *q;
 138   struct winsize winsz;
 139
 140   close (2);
 141
 142   if (argc != 2)
 143     die ();
 144
 145   tty_name = argv [1];
 146   if (strnlen (tty_name, 15) == 15
 147       || strncmp (tty_name, "/dev/", 5))
 148     die ();
 149
 150   setsid ();
 151   uid = getuid ();
 152   euid = geteuid ();
 153
 154   if (seteuid (uid) < 0)
 155     die ();
 156   console_fd = open (tty_name, O_RDONLY);
 157   if (console_fd < 0)
 158     die ();
 159   if (fstat (console_fd, &st) < 0 || ! S_ISCHR (st.st_mode))
 160     die ();
 161   if ((st.st_rdev & 0xff00) != 0x0400)
 162     die ();
 163   console_minor = (int) (st.st_rdev & 0x00ff);
 164   if (console_minor < 1 || console_minor > 63)
 165     die ();
 166   if (st.st_uid != uid)
 167     die ();
 168
 169   switch (tty_name [5])
 170     {
 171     /* devfs */
 172     case 'v': p = "/dev/vc/%d"; q = "/dev/vcc/a%d"; break;
 173     /* /dev/ttyN */
 174     case 't': p = "/dev/tty%d"; q = "/dev/vcsa%d"; break;
 175     default: die (); break;
 176     }
 177
 178   snprintf (console_name, sizeof (console_name), p, console_minor);
 179   if (strncmp (console_name, tty_name, sizeof (console_name)) != 0)
 180     die ();
 181
 182   if (seteuid (euid) < 0)
 183     die ();
 184
 185   snprintf (vcsa_name, sizeof (vcsa_name), q, console_minor);
 186   vcsa_fd = open (vcsa_name, O_RDWR);
 187   if (vcsa_fd < 0)
 188     die ();
 189   if (fstat (vcsa_fd, &st) < 0 || ! S_ISCHR (st.st_mode))
 190     die ();
 191
 192   if (seteuid (uid) < 0)
 193     die ();
 194
 195   winsz.ws_col = winsz.ws_row = 0;
 196   if (ioctl (console_fd, TIOCGWINSZ, &winsz) < 0
 197       || winsz.ws_col <= 0 || winsz.ws_row <= 0
 198       || winsz.ws_col >= 256 || winsz.ws_row >= 256)
 199     die ();
 200
 201   buffer_size = 4 + 2 * winsz.ws_col * winsz.ws_row;
 202   buffer = calloc (buffer_size, 1);
 203   if (buffer == NULL)
 204     die ();
 205
 206   write (1, &console_flag, 1);
 207
 208   while (console_flag && read (0, &action, 1) == 1)
 209     {
 210       switch (action)
 211         {
 212         case CONSOLE_DONE:
 213           console_flag = 0;
 214           continue;
 215         case CONSOLE_SAVE:
 216           if (seteuid (euid) < 0
 217               || lseek (vcsa_fd, 0, 0) != 0
 218               || fstat (console_fd, &st) < 0 || st.st_uid != uid
 219               || read (vcsa_fd, buffer, buffer_size) != buffer_size
 220               || fstat (console_fd, &st) < 0 || st.st_uid != uid)
 221             memset (buffer, 0, buffer_size);
 222           if (seteuid (uid) < 0)
 223             die ();
 224           break;
 225         case CONSOLE_RESTORE:
 226           if (seteuid (euid) >= 0
 227               && lseek (vcsa_fd, 0, 0) == 0
 228               && fstat (console_fd, &st) >= 0 && st.st_uid == uid)
 229             write (vcsa_fd, buffer, buffer_size);
 230           if (seteuid (uid) < 0)
 231             die ();
 232           break;
 233         case CONSOLE_CONTENTS:
 234           send_contents (buffer + 4, winsz.ws_col, winsz.ws_row);
 235           break;
 236         }
 237
 238       write (1, &console_flag, 1);
 239     }
 240
 241   exit (0);
 242 }