src/consaver/cons.saver.c

   1 /*
   2    General purpose Linux console screen save/restore server
   3
   4    This program should be setuid vcsa and /dev/vcsa* should be
   5    owned by the same user too.
   6
   7    Original idea from Unix Interactive Tools version 3.2b (tty.c)
   8    This code requires root privileges.
   9    You may want to make the cons.saver setuid root.
  10    The code should be safe even if it is setuid but who knows?
  11
  12    Partly rewritten by Jakub Jelinek <jakub@redhat.com>.
  13
  14    Copyright (C) 1994, 1998, 1999, 2000, 2001, 2002, 2003, 2004, 2005,
  15    2006, 2007, 2011
  16    The Free Software Foundation, Inc.
  17
  18    This file is part of the Midnight Commander.
  19
  20    The Midnight Commander is free software: you can redistribute it
  21    and/or modify it under the terms of the GNU General Public License as
  22    published by the Free Software Foundation, either version 3 of the License,
  23    or (at your option) any later version.
  24
  25    The Midnight Commander is distributed in the hope that it will be useful,
  26    but WITHOUT ANY WARRANTY; without even the implied warranty of
  27    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  28    GNU General Public License for more details.
  29
  30    You should have received a copy of the GNU General Public License
  31    along with this program.  If not, see <http://www.gnu.org/licenses/>.
  32  */
  33
  34 /* This code does _not_ need to be setuid root. However, it needs
  35    read/write access to /dev/vcsa* (which is priviledged
  36    operation). You should create user vcsa, make cons.saver setuid
  37    user vcsa, and make all vcsa's owned by user vcsa.
  38
  39    Seeing other peoples consoles is bad thing, but believe me, full
  40    root is even worse. */
  41
  42 /** \file cons.saver.c
  43  *  \brief Source: general purpose Linux console screen save/restore server
  44  *
  45  *  This code does _not_ need to be setuid root. However, it needs
  46  *  read/write access to /dev/vcsa* (which is priviledged
  47  *  operation). You should create user vcsa, make cons.saver setuid
  48  *  user vcsa, and make all vcsa's owned by user vcsa.
  49  *  Seeing other peoples consoles is bad thing, but believe me, full
  50  *  root is even worse.
  51  */
  52
  53 #include <config.h>
  54
  55 #ifndef _GNU_SOURCE
  56 #define _GNU_SOURCE
  57 #endif
  58
  59 #include <stdlib.h>
  60 #include <stdio.h>
  61 #include <string.h>
  62
  63 #include <sys/types.h>
  64 #include <sys/stat.h>
  65 #ifdef HAVE_SYS_IOCTL_H
  66 #include <sys/ioctl.h>
  67 #endif
  68 #include <fcntl.h>
  69 #include <termios.h>
  70 #include <unistd.h>
  71
  72 #define LINUX_CONS_SAVER_C
  73 #include "cons.saver.h"
  74
  75 /*** global variables ****************************************************************************/
  76
  77 /*** file scope macro definitions ****************************************************************/
  78
  79 /*** file scope type declarations ****************************************************************/
  80
  81 /*** file scope variables ************************************************************************/
  82
  83 /*** file scope functions ************************************************************************/
  84 /* --------------------------------------------------------------------------------------------- */
  85
  86 static void
  87 send_contents (char *buffer, unsigned int columns, unsigned int rows)
  88 {
  89     unsigned char begin_line = 0, end_line = 0;
  90     unsigned int lastline, lc_index, x;
  91     unsigned char message, outbuf[1024], *p;
  92     unsigned short bytes;
  93
  94     lc_index = 2 * rows * columns;
  95     for (lastline = rows; lastline > 0; lastline--)
  96         for (x = 0; x < columns; x++)
  97         {
  98             lc_index -= 2;
  99             if (buffer[lc_index] != ' ')
 100                 goto out;
 101         }
 102   out:
 103
 104     message = CONSOLE_CONTENTS;
 105     if (write (1, &message, 1) != 1)
 106         return;
 107     if (read (0, &begin_line, 1) != 1)
 108         return;
 109     if (read (0, &end_line, 1) != 1)
 110         return;
 111     if (begin_line > lastline)
 112         begin_line = lastline;
 113     if (end_line > lastline)
 114         end_line = lastline;
 115
 116     lc_index = (end_line - begin_line) * columns;
 117     bytes = lc_index;
 118
 119     if (write (1, &bytes, 2) != 2)
 120         return;
 121     if (bytes == 0)
 122         return;
 123
 124     p = outbuf;
 125     for (lc_index = 2 * begin_line * columns; lc_index < 2 * end_line * columns; lc_index += 2)
 126     {
 127         *p++ = buffer[lc_index];
 128         if (p == outbuf + sizeof (outbuf))
 129         {
 130             if (write (1, outbuf, sizeof (outbuf)) != sizeof (outbuf))
 131                 return;
 132             p = outbuf;
 133         }
 134     }
 135
 136     if (p != outbuf)
 137         if (write (1, outbuf, p - outbuf) < (p - outbuf))
 138             return;
 139 }
 140
 141 /* --------------------------------------------------------------------------------------------- */
 142
 143 static void __attribute__ ((noreturn)) die (void)
 144 {
 145     unsigned char zero = 0;
 146     ssize_t ret;
 147     ret = write (1, &zero, 1);
 148     (void) ret;
 149     exit (3);
 150 }
 151
 152 /* --------------------------------------------------------------------------------------------- */
 153 /*** public functions ****************************************************************************/
 154 /* --------------------------------------------------------------------------------------------- */
 155
 156 int
 157 main (int argc, char **argv)
 158 {
 159     unsigned char action = 0, console_flag = 3;
 160     int console_fd, vcsa_fd, console_minor, buffer_size;
 161     struct stat st;
 162     uid_t uid, euid;
 163     char *buffer, *tty_name, console_name[16], vcsa_name[16];
 164     const char *p, *q;
 165     struct winsize winsz;
 166
 167     close (2);
 168
 169     if (argc != 2)
 170         die ();
 171
 172     tty_name = argv[1];
 173     if (strnlen (tty_name, 15) == 15 || strncmp (tty_name, "/dev/", 5))
 174         die ();
 175
 176     setsid ();
 177     uid = getuid ();
 178     euid = geteuid ();
 179
 180     if (seteuid (uid) < 0)
 181         die ();
 182     console_fd = open (tty_name, O_RDONLY);
 183     if (console_fd < 0)
 184         die ();
 185     if (fstat (console_fd, &st) < 0 || !S_ISCHR (st.st_mode))
 186         die ();
 187     if ((st.st_rdev & 0xff00) != 0x0400)
 188         die ();
 189     console_minor = (int) (st.st_rdev & 0x00ff);
 190     if (console_minor < 1 || console_minor > 63)
 191         die ();
 192     if (st.st_uid != uid)
 193         die ();
 194
 195     switch (tty_name[5])
 196     {
 197         /* devfs */
 198     case 'v':
 199         p = "/dev/vc/%d";
 200         q = "/dev/vcc/a%d";
 201         break;
 202         /* /dev/ttyN */
 203     case 't':
 204         p = "/dev/tty%d";
 205         q = "/dev/vcsa%d";
 206         break;
 207     default:
 208         die ();
 209         break;
 210     }
 211
 212     snprintf (console_name, sizeof (console_name), p, console_minor);
 213     if (strncmp (console_name, tty_name, sizeof (console_name)) != 0)
 214         die ();
 215
 216     if (seteuid (euid) < 0)
 217         die ();
 218
 219     snprintf (vcsa_name, sizeof (vcsa_name), q, console_minor);
 220     vcsa_fd = open (vcsa_name, O_RDWR);
 221     if (vcsa_fd < 0)
 222         die ();
 223     if (fstat (vcsa_fd, &st) < 0 || !S_ISCHR (st.st_mode))
 224         die ();
 225
 226     if (seteuid (uid) < 0)
 227         die ();
 228
 229     winsz.ws_col = winsz.ws_row = 0;
 230     if (ioctl (console_fd, TIOCGWINSZ, &winsz) < 0
 231         || winsz.ws_col <= 0 || winsz.ws_row <= 0 || winsz.ws_col >= 256 || winsz.ws_row >= 256)
 232         die ();
 233
 234     buffer_size = 4 + 2 * winsz.ws_col * winsz.ws_row;
 235     buffer = calloc (buffer_size, 1);
 236     if (buffer == NULL)
 237         die ();
 238
 239     if (write (1, &console_flag, 1) != 1)
 240         die ();
 241
 242     while (console_flag && read (0, &action, 1) == 1)
 243     {
 244         switch (action)
 245         {
 246         case CONSOLE_DONE:
 247             console_flag = 0;
 248             continue;
 249         case CONSOLE_SAVE:
 250             if (seteuid (euid) < 0
 251                 || lseek (vcsa_fd, 0, 0) != 0
 252                 || fstat (console_fd, &st) < 0 || st.st_uid != uid
 253                 || read (vcsa_fd, buffer, buffer_size) != buffer_size
 254                 || fstat (console_fd, &st) < 0 || st.st_uid != uid)
 255                 memset (buffer, 0, buffer_size);
 256             if (seteuid (uid) < 0)
 257                 die ();
 258             break;
 259         case CONSOLE_RESTORE:
 260             if (seteuid (euid) >= 0
 261                 && lseek (vcsa_fd, 0, 0) == 0 && fstat (console_fd, &st) >= 0 && st.st_uid == uid)
 262                 if (write (vcsa_fd, buffer, buffer_size) != buffer_size)
 263                     die ();
 264             if (seteuid (uid) < 0)
 265                 die ();
 266             break;
 267         case CONSOLE_CONTENTS:
 268             send_contents (buffer + 4, winsz.ws_col, winsz.ws_row);
 269             break;
 270         }
 271
 272         if (write (1, &console_flag, 1) != 1)
 273             die ();
 274     }
 275
 276     exit (0);
 277 }
 278
 279 /* --------------------------------------------------------------------------------------------- */