grub.info: Security

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Protecting your computer from cracking

   You may be interested in how to prevent ordinary users from doing
whatever they like, if you share your computer with other people. So
this chapter describes how to improve the security of GRUB.
   One thing which could be a security hole is that the user can do too
many things with GRUB, because GRUB allows one to modify its
configuration and run arbitrary commands at run-time. For example, the
user can even read `/etc/passwd' in the command-line interface by the
command `cat' (*note cat::). So it is necessary to disable all the
interactive operations.
   Thus, GRUB provides a "password" feature, so that only administrators
can start the interactive operations (i.e. editing menu entries and
entering the command-line interface). To use this feature, you need to
run the command `password' in your configuration file (*note
password::), like this:
     password --md5 PASSWORD
   If this is specified, GRUB disallows any interactive control, until
you press the key <p> and enter a correct password.  The option `--md5'
tells GRUB that `PASSWORD' is in MD5 format.  If it is omitted, GRUB
assumes the `PASSWORD' is in clear text.
   You can encrypt your password with the command `md5crypt' (*note
md5crypt::). For example, run the grub shell (*note Invoking the grub
shell::), and enter your password:
     grub> md5crypt
     Password: **********
     Encrypted: $1$U$JK7xFegdxWH6VuppCUSIb.
   Then, cut and paste the encrypted password to your configuration
file.
   Also, you can specify an optional argument to `password'. See this
example:
     password PASSWORD /boot/grub/menu-admin.lst
   In this case, GRUB will load `/boot/grub/menu-admin.lst' as a
configuration file when you enter the valid password.
   Another thing which may be dangerous is that any user can choose any
menu entry. Usually, this wouldn't be problematic, but you might want to
permit only administrators to run some of your menu entries, such as an
entry for booting an insecure OS like DOS.
   GRUB provides the command `lock' (*note lock::). This command always
fails until you enter the valid password, so you can use it, like this:
     title Boot DOS
     lock
     rootnoverify (hd0,1)
     makeactive
     chainload +1
   You should insert `lock' right after `title', because any user can
execute commands in an entry until GRUB encounters `lock'.
   You can also use the command `password' instead of `lock'. In this
case the boot process will ask for the password and stop if it was
entered incorrectly.  Since the `password' takes its own PASSWORD
argument this is useful if you want different passwords for different
entries.