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CIS 228 Grub Basics and Boot Security

CIS 228 Grub Basics and Boot Security. How we get there. Physical security . Lock it up! BIOS Password Disable BIOS alternate boot devices Disable interactive boot: vi /etc/sysconfig/init PROMPT=no Password protected GRUB boot Console locking: yum install vlock, xlock

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CIS 228 Grub Basics and Boot Security

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  1. CIS 228 Grub Basics andBoot Security How we get there.

  2. Physical security • Lock it up! • BIOS Password • Disable BIOS alternate boot devices • Disable interactive boot: vi /etc/sysconfig/init PROMPT=no • Password protected GRUB boot • Console locking: yum install vlock, xlock • Lock desktop GUI, or no Xwindows at all. • Disable Ctrl/Alt/Del (procedure varies)

  3. Grand Unified Bootloader • Replaces LILO, which replaced SYSLINUX, which replaced LOADLIN. You still see these latter on “live” and install CD’s. • Works different from LILO in that changes are automatic after edit (LILO requires issuing of a command to regenerate MBR bootstrap) GRUB works in stages. Stage architecture allows GRUB to be large (~20-30K) and therefore fairly complex and highly configurable, compared to most bootloaders, which are sparse and simple to fit within the limitations of the Partition Table. • Stage 1 is located in the MBR and points (chainloads) to Stage 2, since the MBR is too small to contain all of the needed data. • Stage 2 points to the configuration file (/boot/grub/grub.conf –or- menu.lst), which contains user interface and and OS options . Stage 2 can be located anywhere on the disk. If Stage 2 cannot find its configuration table, GRUB will cease the boot sequence and present the user with a GRUB command line for manual configuration. • Stage 1.5 also exists and might be used if the boot information is small enough to fit in the area immediately after MBR.

  4. Boot Floppy/USB • cd /boot/grubcat stage1 stage2 > /dev/fd0 • cd /boot/grub/dd if=stage1 of=/dev/fd0 bs=512 count=1dd if=stage2 of=/dev/fd0 bs=512 seek=1

  5. Grub Versions • 0.97 • 1.97 • 1.98 • 1.99 • 2.0?

  6. Grub Configuration • /boot/grub/grub.conf -or- menu.lst. The following can also be placed on a single line • Defaults section default=0 …Default OS entry starting with 0 timeout=0 …Time to hit spacebar splashimage=(hd0,0)/boot/grub/splash.xpm.gz … boot display image, ESC to bypass hiddenmenu password=<encrypted password> • OS section title <any string> root (hd0,0) … optional, where OS image (kernel) is stored (hd0=sda, 0=partition 1) kernel /boot/vmlinux-<version> <options> root=/dev/sda2 … where / is -or- kernel (hd0,0)/boot/vmlinux-<version> <options> root=/dev/sda2 initrd /boot/initramfs-<version> (/boot/initrd in some distros) -or- initrd (hd0,0)/boot/initramfs-<version> lock .. Password protected entry For non-LINUX OS (i.e. Windows), or non-compliant OSes rootnoverify (hd0,0) chainloader (hd0,0)+1 … chain to OS specific boot loader (ntldr for Windows)

  7. GRUB Command Line • You need to know the following: - The partition containing the kernel (i.e. /boot) – root, find commands - The partition, path and filename of the initrd file (i.e. /boot) - Within that partition, the directory path and filename of the kernel - The partition containing /sbin/init (i.e. root=/dev/sdax on kernel statement) - ESC Key gets you to command line view past splashscreen • Example: The partition containing the kernel = /dev/hda1, or (hd0,0) in grub-speak Directory path and filename of the kernel = /vmlinuz-i686-up-4GBThe partition containing /sbin/init is /dev/sda2 grub> root (hd0,0)grub> kernel /vmlinuz-i686-up-4GB root=/dev/sda9 grub> boot -OR- grub> kernel (hd0,0)/vmlinuz-i686-up-4GB root=/dev/sda2grub> boot · any initrd image must match linux kernel image

  8. Boot Options Boot a foreign language distro • grub> root (hd0,0)grub> kernel /vmlinuz-i686-up-4GB root=/dev/sda2 lang=usgrub> boot Single partition • grub> root (hd0,0)grub> kernel /boot/vmlinuz-i686-up-4GB root=/dev/sda1grub> boot Maintenance mode • grub> root (hd0,0)grub> kernel /boot/vmlinuz-i686-up-4GB root=/dev/sda1 singlegrub> boot Unknown OS • grub> root (hd0,0) grub> find /sbin/init grub> find /vmlinuz* grub> find /boot/vmlinuz* grub> null (hd0,0)/vmlinuz

  9. (re)generating the MBR LINUX • grub-install /dev/sda • grub-install --root-directory=/boot /dev/sda Find unknown OS • grub> find /boot/grub/stage1 grub> root (hdX,Y)grub> setup (hd0) … writes the MBRquit • ms-sys …Windows MBR from LINUX Windows • fdisk /mbr (DOS) • bootsect (Win 7) • fixmbr (XP) • bootrec /mbr (Vista/Win 7)

  10. grub password • /etc/grub.conf - default=0 - timeout=15 - password GrbPwd4SysAd$ • Use the up-arrow and down-arrow keys to select which entry is highlighted. Press enter to boot the selected OS or 'p' to enter a password to unlock the next set of features. • “lock” directive (under title) prevents insecure booting of a system, like mtce mode

  11. Generating the password • grub-crypt - Password: GrbPwd4SysAd$ - Retype password: GrbPwd4SysAd$ - ^9^32kwzzX./3WISQ0C • /etc/grub.conf - default=0 - timeout=15 - password --encrypted ^9^32kwzzX./3WISQ0C Also: • grub-crypt --sha-256 • grub-crypt --md5 –OR- grub-md5-crypt • grub> md5crypt

  12. Grub 2 • Now with Debian/Ubuntu 9.10 (1.97), 11.10 (1.98) 12.04 (1.99), Fedora Core 16. Script driven Requires issue of command to update-grub togenerate MBR like LILO Password protection procedures more complex. In beta on 1.97, 1.98. Limited in Ubuntu 12.04 LTS Shift key gets you to command line display past splash-screen (replaces ESC in Grub 1) Partitioning numbering begins with 1, not 0 as in Grub 1 Hard drive numbering remains the same • GRUB 2 places files in three locations: /boot/grub2/grub.cfg - main configuration file that replaces menu.lst/grub.conf. DO NOT EDIT! /etc/grub.d/ - directory contains (bash) GRUB scripts used as building blocks for grub.cfg file built with update-grub command. /etc/default/grub - contains GRUB2 menu settings read by the GRUB scripts and written into grub.cfg. Customizeable part of GRUB, similar to the old menu.lst/grub.conf minus the actual boot entries.

  13. Grub 2 scripts Typical Ubuntu Scripts (note numbering sequence): • 00_header script that loads GRUB settings from /etc/default/grub. • 05_debian_theme defines the background, colors and themes. Script name changes on other distros. • 10_linux loads the menu entries for the installed distribution. • 20_memtest86+ loads the memtest utility. • 30_os-prober script that scans the hard disks for other OSes and adds them to the boot menu. • 40_custom is a template that you can use to create additional entries to the boot menu. This entry is never modified by automated OS detection.

  14. Script Examples: • 11_otherOS #!/bin/sh -eecho "Adding my custom Linux to GRUB 2"cat << EOFmenuentry "My custom Linux" {set root=(hd0,5)linux /boot/vmlinuzinitrd /boot/initrd.img}EOF • 12_windows #!/bin/sh -eecho "Adding Windows 8 to GRUB 2 menu"cat << EOFmenuentry "Windows 8" {set root=(hd0,1)chainloader (hd0,1)+1}EOF

  15. Grub2 commands • update-grub (/dev/sda)– updates OS list , generates grub.cfg. Front end to grub(2)-mkconfig 40_custom not modified by this command. • grub(2)-mkconfig /boot/grub2/grub.cfg - generates grub.cfg • grub(2)-install (/dev/sda) – generates mbr

  16. See also • UEFI – see EFI shim. 32 vs 64 bit. Secure Boot Procedures vary. • Windows 7 dual boot using bcdedit visual bcdedit tool, or EasyBCD • Windows (XP and before) dual boot: boot.ini: c:\linux.mbr=“Linux” … where linux.mbr is the hd0 mbr

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