grub

Towards a moderately paranoid Debian laptop setup [Update]

I was planning to set up my laptop from scratch for a while now... so I did.

Preparation

  • First, go home. No, really! Do all of this at home in a non-hostile, firewalled network. You don't want to be in a crowded place such as a conference where people can shoulder-surf your passwords, nor do you want your network traffic sniffed or MITM'd in a hostile network.
  • Backup all your data! You'll be wiping your whole drive soon, so make sure you have recent, tested backups.
  • Get the most recent Debian-installer ISO image (currently etch-beta3), as well as the MD5SUMS and MD5SUMS.sign files:
    wget http://cdimage.debian.org/cdimage/etch_di_beta3/i386/iso-cd/debian-testing-i386-binary-1.iso
    wget http://cdimage.debian.org/cdimage/etch_di_beta3/i386/iso-cd/MD5SUMS
    wget http://cdimage.debian.org/cdimage/etch_di_beta3/i386/iso-cd/MD5SUMS.sign
  • Run gpg --verify MD5SUMS.sign, which will fail but tell you the signing key ID (88C7C1F7 in this case). Get the key and re-run the verification: gpg --recv-key --keyserver subkeys.pgp.net 88C7C1F7 && gpg --verify MD5SUMS.sign. The output should now say "Good signature from [...]".
  • Now check the MD5 checksums via md5sum -c MD5SUMS. The output should contain debian-testing-i386-binary-1.iso: OK.
  • As you now have (somewhat) verified the integrity of the ISO image, burn it on a CD-R: wodim debian-testing-i386-binary-1.iso.
  • Put trusted versions of some files on a USB thumb drive (or CD-ROM); at least a firewall script, but maybe also your bashrc, bash_logout, inputrc, vimrc, muttrc.
  • Disconnect your laptop from any kinds of networks. Pull all ethernet cables. Disable WLAN (via hardware killswitch). Disable Bluetooth. Disable/remove Firewire, USB, serial, whatever.
  • Put on your tin-foil hat (optional).

BIOS

  • Set a good BIOS boot password (which you need to boot any OS). Set a (different) good BIOS boot setup password (which you need to enter the BIOS).
  • Disable all boot possibilities in the BIOS, except for CD-ROM. This means it should not be possible to boot via USB, hard drive, network, PXE, Firewire, floppy, whatever. The BIOS setup password helps to prevent tampering with this setting.
  • Finally, never rely on BIOS passwords alone for security! They can often be circumvented very easily.

Installation / Setting up full-disk encryption using dm-crypt

  • Insert the installer CD and boot in expert-mode (don't hit ENTER when you boot, but rather type "expert").
  • As for networking: select "Do not configure the network at this time". We'll fix and enable networking later.
  • Partitioning:

    • Select manual partitioning. Remove all partitions (if any). Create a 100 MB /boot (ext3) as primary partition, and make the rest of the hard drive one huge partition which has "Use as:" set to "physical volume for encryption".
    • The standard options for cipher, key size, IV mode etc. should be fine (AES, 256 bit, CBC-ESSIV-SHA256, dm-crypt).
    • After the erasing is done (this is important!), use the whole encrypted space as "physical volume for LVM". Then select "Configure the Logical Volume Manager". Create one big volume group and a bunch of logical volumes for the various partitions we'll use (lv-root, lv-usr, lv-var, lv-tmp, lv-swap, lv-home).
    • It is extremely important that your swap space is encrypted (in this case it is, as all partitions except for /boot reside on a dm-crypt device)! Never set up unencrypted swap!
  • Enable shadow passwords. Allow login as root (I feel confident that I won't do stupid things as root).
  • Choose a good root password, and a (different) good user password. Don't enter a full name for the user.
  • Choose the latest kernel (old kernels might have security issues). Do not participate in popcon.
  • Do not install any tasks (no "desktop", no "base system"). We want the smallest installation possible, and add only the packages we really need. Fewer packages means fewer security issues (statistically).
  • That's it. Eject the CD-ROM, reboot, change the BIOS to only allow booting from hard drive.

Post-installation tasks

  • Enter the USB thumb drive, copy all config-files to /root and /home/uwe. Log out and log in again to make ~/.bashrc and ~/.inputrc take effect.
  • Enable the firewall: mkdir /etc/rc.boot && cp fw_laptop /etc/rc.boot && chmod 700 /etc/rc.boot/fw_laptop && sh /etc/rc.boot/fw_laptop
  • Shut down all networked daemons (if any): /etc/init.d/foo stop.
  • Tighten home-directory permissions: chmod 700 /root /home/uwe.
  • Edit /etc/passwd: give all users except for root, sync, uucp and your user account /usr/sbin/nologin as login shell. None of these accounts really needs a valid login shell (nologin will log any login attempts for those accounts).
  • Edit /etc/group: remove your user account from the dialout, cdrom, and floppy group. The groups audio, video, and plugdev can stay.
  • Edit /etc/fstab: add some mount options such as ro, nosuid, noexec, or nodev as you see fit. Example:
    /dev/mapper/vg--whole-lv--root /     ext3 defaults,errors=remount-ro      0 0
    /dev/sda2                      /boot ext3 defaults,nodev,nosuid,noexec,ro 0 0
    /dev/mapper/vg--whole-lv--home /home ext3 defaults,nodev,nosuid           0 0
    /dev/mapper/vg--whole-lv--tmp  /tmp  ext3 defaults,nodev,nosuid           0 0
    /dev/mapper/vg--whole-lv--usr  /usr  ext3 defaults,nodev,ro               0 0
    /dev/mapper/vg--whole-lv--var  /var  ext3 defaults,nodev                  0 0
    /dev/mapper/vg--whole-lv--swap none  swap sw                              0 0
    /dev/scd0 /media/cdrom iso9660 noauto,nodev,nosuid,noexec,uid=uwe,gid=uwe 0 0
    
  • If you have read-only (ro) file systems, configure Apt so that it can remount them read-write when installing/removing packages. Add this to /etc/apt/apt.conf:
    DPkg
    {
      Pre-Invoke { "mount -o remount,rw /usr; mount -o remount,rw /boot"; }
      Post-Invoke { "mount -o remount,ro /usr; mount -o remount,ro /boot"; }
    }
    
  • Fix the GRUB configuration. Replace the "password foo" line (which contains the GRUB password in plain-text) from your /boot/grub/menu.lst with a "password --md5 $1$1234567890..." line, where the MD5 hash ($1$1234567890...) can be generated with grub-md5-crypt. Additionally, add such a password line after each "title" line in the GRUB config-file, so that nobody can boot any OS installed on the laptop without a password!

Networking, Upgrading and Apt-secure

  • Now that we have a small, hardened system, it should be reasonably safe to enable networking. Add this to /etc/network/interfaces:
    auto eth0
    iface eth0 inet dhcp
      pre-up /etc/rc.boot/fw_laptop
    

    Run /etc/init.d/networking restart. The firewall script will run every time the network is started.

  • Now add this (tweak as you see fit) to /etc/apt/sources.list:
    deb http://ftp.de.debian.org/debian unstable main
    deb-src http://ftp.de.debian.org/debian unstable main
  • Time for upgrading: apt-get update && apt-get dist-upgrade. All packages are GnuPG-signed and will be verified by Apt. The installer already ships the required key (for 2006), so everything should just work. Still, you should read about SecureApt.
  • Install the rest of your system now, and restore your data from backups.
  • Use sysv-rc-conf to disable all daemons you don't want to start per default: sysv-rc-conf foo off.
  • Install and set up Samhain (or any other file integrity checker): apt-get install samhain. You want to be notified if your system files are being tampered with (e.g. replaced by a rootkit).
  • Install and configure Tor for anonymous browsing. More details here.
  • Install and configure more security-related programs, e.g. logcheck, snort, rkhunter, chkrootkit, tiger, sxid, etc.

SELinux

Now install and set up SELinux. This section is based on notes from Erich Schubert (thanks!), and will soon appear in the SELinuxSetup wiki page, too.

  • Install the base packages and an SELinux policy: apt-get install selinux-basics selinux-policy-refpolicy-targeted.
  • Edit /boot/grub/menu.lst and add selinux=1 to your kernel command line to enable SELinux upon booting.
  • In /etc/pam.d/login uncomment the "session required pam_selinux.so multiple" line. Do the same in /etc/pam.d/ssh if you have ssh installed.
  • In /etc/default/rcS set FSCKFIX=yes.
  • In /etc/init.d/bootmisc.sh search for "Update motd" and comment the two lines below that line. Then rm /var/run/motd.
  • If you have exim installed, you must either install postfix or write an exim policy, as none currently exists. But even postfix needs some fixing (no pun intended ;-). Disable chroot-support (change all "chroot" fields to "n" in /etc/postfix/master.cf and execute echo 'SYNC_CHROOT="n" >> /etc/default/postfix').
  • Use check-selinux-installation to check for common SELinux problems on Debian (such as the above mentioned).
  • touch /.autorelabel. Reboot. touch /.autorelabel (again). Reboot (again).
  • Done. You should now have a working SELinux system. If no critical audit errors appear and you feel comfortable with SELinux, enable enforcing mode via setenforce 1 or by adding enforcing=1 to the kernel command line in /boot/grub/menu.lst.

Behaviour

  • Never leave your laptop unattended!
  • Always lock your terminal (using vlock) when you move more than 30 cm away from the laptop!
  • Don't run insecure and/or closed-source software (which you can never trust!). No NVIDIA/ATI drivers, no VMware, no Google Earth, no Flash Plugin (except for Gnash maybe), no Adobe Acrobat. You get the idea.
  • Keep the number of installed packages small and try to configure each of them as secure as possible.
  • Never enable networking or WLAN or Bluetooth if you don't absolutely have to.
  • Trust no one. Don't let other people use you laptop, don't give out shell accounts.

Further ideas

  • The /boot partition is still unencrypted, so an attacker can tamper with it. Boot from a CD-R, forbid booting from hard drive (BIOS). Sign/mark the CD-R physically, so you'll know when someone replaced your CD-R with his own, back-doored one.
  • Another idea is to use an additionaly USB thumb drive or CD-ROM or smartcard for two-factor authentication.
  • Install another Debian into a QEMU image. Use it as a sandbox for stuff you don't trust: qemu -snapshot -net none foo.img.
  • At all costs, disable Firewire! If possible via hardware or BIOS, or at least don't load the drivers and/or fix them (page 19).
  • Replace the proprietary, closed-source BIOS with LinuxBIOS, if possible.

That's it. You can take off that stupid tin-foil hat now.

Update 2006-09-29: Fixed typos. Mentioned sxid. Added two-factor authentication.

How to dump your BIOS/LILO/root password as plain text [Update]

This is old news by now, but still interesting IMHO. Jonathan Brossard has posted an article on BugTraq which gives a pretty good introduction to the inner workings of the BIOS (with lots of links to more detailed resources) as well as known vulnerabilities of the BIOS password mechanism.

The most interesting part is when he explains that the BIOS doesn't seem to erase its own keyboard buffer before it hands over control to the operating system. Also, current OSes (Linux, Windows, *BSD, etc.) don't seem to clear that buffer either.

This may not sound dangerous, but it actually allows anyone who can read the contents of your RAM, starting from address 0x041e, to view the keyboard buffer contents. And this buffer contains the BIOS password you type in when booting your machine (if you set/use a BIOS password, of course).

This one-liner (executed as root) should let you view your password as plain text:

dd if=/dev/mem bs=512 skip=2 count=1 | hexdump -C | head

(Only every second character belongs to the password, the rest are key scan codes, I think).

I also noticed that this same buffer also contains your LILO password, too! The same is probably true for passwords of other boot loaders such as GRUB, but I didn't test that.

Yes, reading this part of the RAM usually requires root privileges in Unix-like OSes, but as the security problem is OS-independant other OSes (e.g. DOS, or older Windows versions) might be directly affected.

But even on more secure OSes this plain-text storage of the BIOS/boot loader passwords might be a problem. Combine this with some Firewire insecurities and attackers with physical access to your machine (e.g. your unattended laptop, while you are on the toilet) might be able to read your BIOS/LILO passwords even though you locked your machine. I haven't yet tried this, but I'm pretty sure it's possible. Please post the results here if you try this.

(via Stefan 'Sec' Zehl)

Update 2006-01-09: It seems that when you use software suspend (swsuspend2) the RAM area can/will also contain your root password! Thanks nelson for reporting.

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