9. 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 (see section 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 (see section 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 (see section md5crypt). For example, run the grub shell (see section 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 (see section 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.

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