Difference between revisions of "GRUB2"

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'''[https://www.gnu.org/software/grub/grub.html GRUB2]''' is a modular, multiboot-capable bootloader for many operating systems that can be used as a payload for coreboot.  
+
'''[https://www.gnu.org/software/grub/grub.html GRUB 2]''' is a modular, multiboot-capable bootloader for many operating systems that can be used as a payload for coreboot.  
 
+
  
 
== Status ==
 
== Status ==
 +
GRUB 2 can be launched:
 +
* Directly by coreboot as a payload
 +
* Directly by SeaBIOS as a payload
 +
* By SeaBIOS, on disk, as it would with a normal BIOS.
  
* The mainline version of GRUB2 has a [http://grub.enbug.org/CoreBoot wiki page on the coreboot port] (Update: no longer available)
+
Recent git versions have improved memory management that removes the memory limitations when ran as a payload.
* Additional information about our former GRUB2 effort (which was part of Google Summer of Code 2007) can be found in the history of this page. Don't expect any link there to work.
+
* As an alternative, you could consider using [[FILO]]. Both FILO and GRUB2 have various advantages and disadvantages. Which of the two is better suited depends on your requirements.
+
* Yet another alternative is to not put GRUB into the BIOS ROM, but have it run from your disk as you would with a vendor BIOS. For that, you can use [[SeaBIOS]] as payload, which will then be able to run either GRUB1 or GRUB2 from your disk.
+
  
 +
== features ==
 +
=== Interesting features sumarry ===
 +
* It can open and boot from encrypted LUKS partitions.
 +
* It can verify the signatures of files ( interesting for initramfs and kernels )
 +
* has a "cbmemc" command that can see cbmem
 +
* it's compatible with the coreboot framebuffer.
 +
* it has some cmos commands that permits to interact with the nvram, like:
 +
** cmostest
 +
** cmosclear
 +
** cmosset
 +
** cmosdump
 +
* it can loads and run coreboot payloads from cbfs(both compressed and uncompressed), its memdisk, or any other filesystem it can read...
  
== Compiling GRUB2 for being use as a payload ==
+
=== Security ===
See [[Talk:GRUB2]] and [https://lists.gnu.org/archive/html/grub-devel/2011-06/msg00003.html here] for more details.
+
==== signed kernels ====
 +
GRUB is capable of running only trusted(signed) kernels.
 +
* it supports both RSA and DSA gpg keys
  
=== Compiling ===
+
Here's a little howto.
  bzr branch http://bzr.savannah.gnu.org/r/grub/trunk/grub
+
 
 +
First generate a key:
 +
$ gpg --gen-key
 +
gpg (GnuPG) 2.0.19; Copyright (C) 2012 Free Software Foundation, Inc.
 +
This is free software: you are free to change and redistribute it.
 +
There is NO WARRANTY, to the extent permitted by law.
 +
 +
Please select what kind of key you want:
 +
    (1) RSA and RSA (default)
 +
    (2) DSA and Elgamal
 +
    (3) DSA (sign only)
 +
    (4) RSA (sign only)
 +
Your selection? 3
 +
DSA keys may be between 1024 and 3072 bits long.
 +
What keysize do you want? (2048) 3072
 +
Requested keysize is 3072 bits
 +
Please specify how long the key should be valid.
 +
          0 = key does not expire
 +
      <n>  = key expires in n days
 +
      <n>w = key expires in n weeks
 +
      <n>m = key expires in n months
 +
      <n>y = key expires in n years
 +
Key is valid for? (0)
 +
Key does not expire at all
 +
Is this correct? (y/N) y
 +
 +
GnuPG needs to construct a user ID to identify your key.
 +
 +
Real name: Denis 'GNUtoo' Carikli
 +
Email address: GNUtoo@no-log.org
 +
Comment: Kernel signing key
 +
You selected this USER-ID:
 +
    "Denis 'GNUtoo' Carikli (Kernel signing key) <GNUtoo@no-log.org>"
 +
 +
Change (N)ame, (C)omment, (E)mail or (O)kay/(Q)uit? o
 +
You need a Passphrase to protect your secret key.
 +
 +
We need to generate a lot of random bytes. It is a good idea to perform
 +
some other action (type on the keyboard, move the mouse, utilize the
 +
disks) during the prime generation; this gives the random number
 +
generator a better chance to gain enough entropy.
 +
gpg: WARNING: some OpenPGP programs can't handle a DSA key with this digest
 +
size
 +
gpg: key C86D4C64 marked as ultimately trusted
 +
public and secret key created and signed.
 +
 +
gpg: checking the trustdb
 +
gpg: 3 marginal(s) needed, 1 complete(s) needed, PGP trust model
 +
gpg: depth: 0  valid:  2  signed:  0  trust: 0-, 0q, 0n, 0m, 0f, 2u
 +
pub  3072D/C86D4C64 2013-03-13
 +
      Key fingerprint = 7244 AC33 F9A7 9AE8 30DE  8996 9097 B48D C86D 4C64
 +
  uid                  Denis 'GNUtoo' Carikli (Kernel signing key)
 +
<GNUtoo@no-log.org>
 +
 +
Note that this key cannot be used for encryption.  You may want to use
 +
the command "--edit-key" to generate a subkey for this purpose.
 +
Then sign the kernels and initramfs:
 +
cd /boot
 +
sudo -E gpg --detach-sign vmlinuz-linux-libre-pae
 +
sudo -E gpg --detach-sign initramfs-linux-libre-pae.img
 +
 
 +
gpg --export  > boot.key
 +
Then you can put the key on the memdisk (advised) or the boot partition for test purposes only.
 +
Then in GRUB do (for testing purposes):
 +
trust boot.key
 +
set check_signatures=enforce
 +
to only boot correctly signed kernels and initramfs...
 +
 
 +
Then load kernel and initramfs as usual...
 +
 
 +
==== Trisquel, Ubuntu, Debian ====
 +
We want automatics hooks to sign our kernel so we don't have to do it manually each time...
 +
The following howto was tested on trisquel 6
 +
Generate the key as root(sudo su) like we just explained, but without a password
 +
In debian based distributions you can hook the kernel build to sign the result:
 +
Add the following to /etc/kernel/postinst.d/yy-update-signatures
 +
#! /bin/sh
 +
set -e
 +
 +
version="$1"
 +
 +
rm -f /boot/vmlinuz-${version}.sig
 +
gpg --detach-sign /boot/vmlinuz-${version}
 +
rm -f /boot/initrd.img-${version}.sig
 +
gpg --detach-sign /boot/initrd.img-${version}
 +
Then do:
 +
chmod +x /etc/kernel/postinst.d/yy-update-signatures
 +
Then do:
 +
gpg --export  > /boot/boot.key
 +
 
 +
Then modify /etc/grub.d/10_linux to use bash instead of sh like that:
 +
#! /bin/bash
 +
And also modify to that:
 +
<pre>
 +
case x`uname -m` in
 +
    xi?86 | xx86_64)
 +
list=`for i in /boot/vmlinuz-* /vmlinuz-* /boot/kernel-* ; do
 +
                  if grub_file_is_not_garbage "$i" ; then echo -n "$i " ; fi
 +
              done` ;;
 +
    *)
 +
list=`for i in /boot/vmlinuz-* /boot/vmlinux-* /vmlinuz-* /vmlinux-* /boot/kernel-* ; do
 +
                  if grub_file_is_not_garbage "$i" ; then echo -n "$i " ; fi
 +
    done` ;;
 +
esac
 +
</pre>
 +
To look like that:
 +
<pre>
 +
case x`uname -m` in
 +
    xi?86 | xx86_64)
 +
list=`for i in /boot/vmlinuz-* /vmlinuz-* /boot/kernel-* ; do
 +
                  if [[ "$i" != /boot/*.sig ]] ; then
 +
                      if grub_file_is_not_garbage "$i" ; then echo -n "$i " ; fi
 +
                  fi
 +
              done` ;;
 +
    *)
 +
list=`for i in /boot/vmlinuz-* /boot/vmlinux-* /vmlinuz-* /vmlinux-* /boot/kernel-* ; do
 +
                  if grub_file_is_not_garbage "$i" ; then echo -n "$i " ; fi
 +
    done` ;;
 +
esac
 +
</pre>
 +
 
 +
==== LUKS disks openning ====
 +
GRUB is capable of opening LUKS disks like that:
 +
grub> ls
 +
(ata2) (ata2,msdos3) (ata2,msdos2) (ata2,msdos1) (usb0) (usb0,msdos1) (ata6) (memdisk)
 +
grub> cryptomount (ata2,msdos3)
 +
Attempting to decrypt master key...
 +
Enter passphrase for ata2,msdos3 (431439b0870f40a3bfe8f3ca3aa7072a):
 +
Slot 0 opened
 +
grub> ls
 +
(crypto0) (ata2) (ata2,msdos3) (ata2,msdos2) (ata2,msdos1) (usb0) (usb0,msdos1) (ata6) (memdisk)
 +
grub> set root=crypto0
 +
grub> ls /
 +
lost+found/ boot/ var/ dev/ run/ etc/ tmp/ sys/ proc/ usr/ lib/ sbin/ bin/ home/ mnt/ opt/ root/ srv/ media/
 +
 
 +
Note that you have to type the password and so it's better to have some kind of output (VGA, Serial etc...)
 +
 
 +
=== Other features ===
 +
==== Payloads launching ====
 +
GRUB is capable of launching coreboot payloads. See the "Payloads" section of this page
 +
 
 +
== grub.cfg ==
 +
=== Serial ===
 +
==== On a real serial port ====
 +
To enable serial, add the following on top of your grub.cfg:
 +
serial --speed=115200 --word=8 --parity=no --stop=1
 +
terminal_input --append  serial
 +
terminal_output --append serial
 +
 
 +
==== On an usb serial or usb debug adapter ====
 +
To enable serial, first find out the name of your usb serial port trough:
 +
insmod nativedisk # needed not to get the disk disapearing when insmoding the *hci
 +
insmod ehci
 +
insmod ohci
 +
insmod uhci
 +
insmod usb
 +
insmod usbserial_pl2303
 +
insmod usbserial_ftdi
 +
insmod usbserial_usbdebug
 +
terminal_output
 +
The terminal_output command should print it:
 +
grub> terminal_output
 +
Active output terminals:
 +
serial_usb1 gfxterm
 +
Available output terminals:
 +
console vga_text serial
 +
Here we can see "serial_usb1" so we now know that its name is usb1
 +
 
 +
Then add the following on top of your grub.cfg:
 +
insmod nativedisk
 +
insmod ehci
 +
insmod ohci
 +
insmod uhci
 +
insmod usb
 +
insmod usbserial_pl2303
 +
insmod usbserial_ftdi
 +
insmod usbserial_usbdebug
 +
serial --speed=115200 --word=8 --parity=no --stop=1 usb1
 +
terminal_output --append serial_usb1
 +
terminal_input --append serial_usb1
 +
 
 +
The following chips/protocols are supported:
 +
* usbdebug
 +
* ftdi
 +
* pl2303
 +
 
 +
=== Other things ===
 +
Append that in your configuration:
 +
terminal_input --append at_keyboard #add keyboard support.
 +
#set timeout=1 #you may want to set a timeout
 +
#set pager=1 # you may want to use the pager or not
 +
play 480 440 1 #play a beep at startup
 +
set prefix=(memdisk)/boot/grub
 +
In case of native graphics you may want the following:
 +
gfxpayload=keep
 +
terminal_output --append gfxterm
 +
 
 +
=== Payloads ===
 +
Here is how to load the SeaBIOS payload from the memdisk.
 +
menuentry 'SeaBios' {
 +
set root='memdisk'
 +
echo    'Loading SeaBios ...'
 +
chainloader /bios.bin.elf
 +
}
 +
see in "creating the GRUB payload" how to include the file in the memdisk...
 +
 
 +
=== Distributions ===
 +
Here's an example on how to load the parabola distribution on the Lenovo X60 from the fifth partition.
 +
menuentry 'Parabola GNU/Linux-libre GNU/Linux, with Linux librepae kernel [Serial]' {
 +
        insmod ahci
 +
        insmod part_msdos
 +
set root='ahci0,msdos5'
 +
echo 'Loading Linux librepae kernel ...'
 +
linux /vmlinuz-linux-libre-pae root=/dev/mapper/root ro cryptdevice=/dev/sda6:root idle=halt pcie_aspm=force console=ttyS0,115200
 +
echo 'Loading initial ramdisk ...'
 +
initrd /initramfs-linux-libre-pae.img
 +
}
 +
 
 +
=== Scanning for grub.cfg on local Hard Drives. ===
 +
 
 +
menuentry 'Scan for OS on internal HDD' {
 +
insmod regexp
 +
insmod ahci
 +
insmod part_msdos
 +
for x in (ahci0,*) ; do
 +
if [ -f "$x/grub/grub.cfg" ] ; then
 +
menuentry "Load Config from $x" $x {
 +
root=$2
 +
configfile /grub/grub.cfg
 +
}
 +
fi
 +
if [ -f "$x/boot/grub/grub.cfg" ] ; then
 +
menuentry "Load Config from $x" $x {
 +
root=$2
 +
configfile /boot/grub/grub.cfg
 +
}
 +
fi
 +
done
 +
}
 +
 
 +
== Compiling ==
 +
git clone git://git.savannah.gnu.org/grub.git grub
 
  cd grub
 
  cd grub
 
  ./autogen.sh
 
  ./autogen.sh
 
  ./configure --with-platform=coreboot
 
  ./configure --with-platform=coreboot
 
  make
 
  make
 +
sudo make install #install the utilities
  
=== creating the grub payload (with a memdisk) ===
+
== creating the GRUB payload==
cd grub-core
+
Create a target directory:
  tar cvf ../memdisk.tar serial.mod terminal.mod normal.mod ahci.mod all_video.mod ata.mod boot.mod cat.mod chain.mod configfile.mod crypto.mod elf.mod ext2.mod extcmd.mod fshelp.mod help.mod linux.mod memdisk.mod minicmd.mod multiboot2.mod pata.mod part_msdos.mod gettext.mod --transform 's#^#/boot/grub/i386-coreboot/#'
+
  mkdir memdisk
tar uvf ../memdisk.tar grub.cfg  --transform 's#^#/boot/grub/#'
+
Then copy your grub.cfg in:
../grub-mkimage -d . -O i386-coreboot -o ../../grub2.elf memdisk tar ehci ohci uhci at_keyboard usb_keyboard -m ../memdisk.tar
+
  memdisk/boot/grub/grub.cfg
ls -l -h ../../grub2.elf
+
  
=== creating the grub payload (without a memdisk) ===
+
Then adapt and run that script:
  cd grub-core
+
  #!/bin/sh
  modules="serial terminal normal ahci all_video ata boot cat chain configfile crypto elf  ext2 extcmd fshelp help linux memdisk minicmd multiboot2 pata part_msdos gettext"
+
rm -f grub2-x60.elf
  ../grub-mkimage -d . -O i386-coreboot -o ../../grub2.elf memdisk tar ehci ohci uhci at_keyboard usb_keyboard ${modules}
+
  #copy the payloads you want
  ls -l -h ../../grub2.elf
+
  cp ../../seabios-x60/out/bios.bin.elf ./memdisk/
 +
  cp ../../coreboot-qemu/payloads/nvramcui/nvramcui.elf ./memdisk/
 +
  cp ../../coreboot-x60/payloads/coreinfo/build/coreinfo.elf ./memdisk/
 +
cp ../../memtest86+-4.20/memtest ./memdisk/memtest.elf
 +
#and some files
 +
cp ../../coreboot-x60/bootsplash.jpg  ./memdisk/
 +
cd memdisk
 +
grub-mkstandalone -O i386-coreboot -o ../grub2-x60.elf $(find -type f)
 +
echo "--RESULT--"
 +
  ls -l -h ../grub2-x60.elf
  
=== creating the grub payload (with a memdisk for the config file) ===
+
== combining with coreboot ==
cd grub-core
+
=== As a SeaBIOS payload ===
tar cvf ../memdisk.tar grub.cfg  --transform 's#^#/boot/grub/#'
+
modules="serial terminal  normal ahci all_video ata boot cat chain configfile crypto elf  ext2 extcmd fshelp help linux memdisk minicmd multiboot2 pata part_msdos gettext"
+
../grub-mkimage -d . -O i386-coreboot -o ../../grub2.elf memdisk tar ehci ohci uhci at_keyboard usb_keyboard ${modules} -m ../memdisk.tar
+
ls -l -h ../../grub2.elf
+
 
+
=== combining with coreboot ===
+
 
  build/cbfstool build/coreboot.rom add-payload -n img/grub2 -f grub2.elf -t raw
 
  build/cbfstool build/coreboot.rom add-payload -n img/grub2 -f grub2.elf -t raw
 
  build/cbfstool build/coreboot.rom print
 
  build/cbfstool build/coreboot.rom print
That way it will be possible to run grub2 as a payload after SeaBIOS: The advantage is that it's less risky. At runtime press F12 and you'll have the grub2 option.
+
That way it will be possible to run GRUB as a payload after SeaBIOS:  
 +
The advantage is that it's less risky. At runtime press F12 and you'll  
 +
have the GRUB option.
 +
 
 +
=== As a Coreboot payload ===
 +
Advantages: faster, can be used for security
 +
 
 +
Disadvantages: more risky if you have no way to recover
 +
 
 +
==== Howto ====
 +
 
 +
In make menuconfig of coreboot, select the path of grub2.elf.
 +
 
 +
Also make sure you have some kinds of output such as VGA or serial (it
 +
needs to be activated in both coreboot and GRUB)
 +
 
 +
== Before flashing ==
 +
You should try the grub2.elf on qemu before flashing it to a real mainboard/laptop.

Latest revision as of 03:03, 21 February 2014

GRUB 2 is a modular, multiboot-capable bootloader for many operating systems that can be used as a payload for coreboot.

Status

GRUB 2 can be launched:

  • Directly by coreboot as a payload
  • Directly by SeaBIOS as a payload
  • By SeaBIOS, on disk, as it would with a normal BIOS.

Recent git versions have improved memory management that removes the memory limitations when ran as a payload.

features

Interesting features sumarry

  • It can open and boot from encrypted LUKS partitions.
  • It can verify the signatures of files ( interesting for initramfs and kernels )
  • has a "cbmemc" command that can see cbmem
  • it's compatible with the coreboot framebuffer.
  • it has some cmos commands that permits to interact with the nvram, like:
    • cmostest
    • cmosclear
    • cmosset
    • cmosdump
  • it can loads and run coreboot payloads from cbfs(both compressed and uncompressed), its memdisk, or any other filesystem it can read...

Security

signed kernels

GRUB is capable of running only trusted(signed) kernels.

  • it supports both RSA and DSA gpg keys

Here's a little howto.

First generate a key:

$ gpg --gen-key
gpg (GnuPG) 2.0.19; Copyright (C) 2012 Free Software Foundation, Inc.
This is free software: you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law.

Please select what kind of key you want:
   (1) RSA and RSA (default)
   (2) DSA and Elgamal
   (3) DSA (sign only)
   (4) RSA (sign only)
Your selection? 3
DSA keys may be between 1024 and 3072 bits long.
What keysize do you want? (2048) 3072
Requested keysize is 3072 bits
Please specify how long the key should be valid.
         0 = key does not expire
      <n>  = key expires in n days
      <n>w = key expires in n weeks
      <n>m = key expires in n months
      <n>y = key expires in n years
Key is valid for? (0) 
Key does not expire at all
Is this correct? (y/N) y

GnuPG needs to construct a user ID to identify your key.

Real name: Denis 'GNUtoo' Carikli
Email address: GNUtoo@no-log.org
Comment: Kernel signing key
You selected this USER-ID:
    "Denis 'GNUtoo' Carikli (Kernel signing key) <GNUtoo@no-log.org>"

Change (N)ame, (C)omment, (E)mail or (O)kay/(Q)uit? o
You need a Passphrase to protect your secret key.

We need to generate a lot of random bytes. It is a good idea to perform
some other action (type on the keyboard, move the mouse, utilize the
disks) during the prime generation; this gives the random number
generator a better chance to gain enough entropy.
gpg: WARNING: some OpenPGP programs can't handle a DSA key with this digest 
size
gpg: key C86D4C64 marked as ultimately trusted
public and secret key created and signed.

gpg: checking the trustdb
gpg: 3 marginal(s) needed, 1 complete(s) needed, PGP trust model
gpg: depth: 0  valid:   2  signed:   0  trust: 0-, 0q, 0n, 0m, 0f, 2u
pub   3072D/C86D4C64 2013-03-13
      Key fingerprint = 7244 AC33 F9A7 9AE8 30DE  8996 9097 B48D C86D 4C64
uid                  Denis 'GNUtoo' Carikli (Kernel signing key) 
<GNUtoo@no-log.org>

Note that this key cannot be used for encryption.  You may want to use
the command "--edit-key" to generate a subkey for this purpose.

Then sign the kernels and initramfs:

cd /boot
sudo -E gpg --detach-sign vmlinuz-linux-libre-pae
sudo -E gpg --detach-sign initramfs-linux-libre-pae.img
gpg --export  > boot.key

Then you can put the key on the memdisk (advised) or the boot partition for test purposes only. Then in GRUB do (for testing purposes):

trust boot.key
set check_signatures=enforce

to only boot correctly signed kernels and initramfs...

Then load kernel and initramfs as usual...

Trisquel, Ubuntu, Debian

We want automatics hooks to sign our kernel so we don't have to do it manually each time... The following howto was tested on trisquel 6 Generate the key as root(sudo su) like we just explained, but without a password In debian based distributions you can hook the kernel build to sign the result: Add the following to /etc/kernel/postinst.d/yy-update-signatures

#! /bin/sh
set -e

version="$1"

rm -f /boot/vmlinuz-${version}.sig
gpg --detach-sign /boot/vmlinuz-${version}
rm -f /boot/initrd.img-${version}.sig
gpg --detach-sign /boot/initrd.img-${version}

Then do:

chmod +x /etc/kernel/postinst.d/yy-update-signatures

Then do:

gpg --export  > /boot/boot.key

Then modify /etc/grub.d/10_linux to use bash instead of sh like that:

#! /bin/bash

And also modify to that:

 case x`uname -m` in
     xi?86 | xx86_64)
 	list=`for i in /boot/vmlinuz-* /vmlinuz-* /boot/kernel-* ; do
                   if grub_file_is_not_garbage "$i" ; then echo -n "$i " ; fi
               done` ;;
     *) 
 	list=`for i in /boot/vmlinuz-* /boot/vmlinux-* /vmlinuz-* /vmlinux-* /boot/kernel-* ; do
                   if grub_file_is_not_garbage "$i" ; then echo -n "$i " ; fi
 	     done` ;;
 esac

To look like that:

 case x`uname -m` in
     xi?86 | xx86_64)
 	list=`for i in /boot/vmlinuz-* /vmlinuz-* /boot/kernel-* ; do
                   if [[ "$i" != /boot/*.sig ]] ; then 
                       if grub_file_is_not_garbage "$i" ; then echo -n "$i " ; fi
                   fi
               done` ;;
     *) 
 	list=`for i in /boot/vmlinuz-* /boot/vmlinux-* /vmlinuz-* /vmlinux-* /boot/kernel-* ; do
                   if grub_file_is_not_garbage "$i" ; then echo -n "$i " ; fi
 	     done` ;;
 esac

LUKS disks openning

GRUB is capable of opening LUKS disks like that:

grub> ls 
(ata2) (ata2,msdos3) (ata2,msdos2) (ata2,msdos1) (usb0) (usb0,msdos1) (ata6) (memdisk)
grub> cryptomount (ata2,msdos3)
Attempting to decrypt master key...
Enter passphrase for ata2,msdos3 (431439b0870f40a3bfe8f3ca3aa7072a):
Slot 0 opened
grub> ls
(crypto0) (ata2) (ata2,msdos3) (ata2,msdos2) (ata2,msdos1) (usb0) (usb0,msdos1) (ata6) (memdisk) 
grub> set root=crypto0
grub> ls /
lost+found/ boot/ var/ dev/ run/ etc/ tmp/ sys/ proc/ usr/ lib/ sbin/ bin/ home/ mnt/ opt/ root/ srv/ media/

Note that you have to type the password and so it's better to have some kind of output (VGA, Serial etc...)

Other features

Payloads launching

GRUB is capable of launching coreboot payloads. See the "Payloads" section of this page

grub.cfg

Serial

On a real serial port

To enable serial, add the following on top of your grub.cfg:

serial --speed=115200 --word=8 --parity=no --stop=1
terminal_input --append  serial
terminal_output --append serial

On an usb serial or usb debug adapter

To enable serial, first find out the name of your usb serial port trough:

insmod nativedisk # needed not to get the disk disapearing when insmoding the *hci
insmod ehci
insmod ohci
insmod uhci
insmod usb
insmod usbserial_pl2303
insmod usbserial_ftdi
insmod usbserial_usbdebug
terminal_output

The terminal_output command should print it:

grub> terminal_output 
Active output terminals:
serial_usb1 gfxterm 
Available output terminals:
console vga_text serial 

Here we can see "serial_usb1" so we now know that its name is usb1

Then add the following on top of your grub.cfg:

insmod nativedisk
insmod ehci
insmod ohci
insmod uhci
insmod usb
insmod usbserial_pl2303
insmod usbserial_ftdi
insmod usbserial_usbdebug
serial --speed=115200 --word=8 --parity=no --stop=1 usb1
terminal_output --append serial_usb1
terminal_input --append serial_usb1

The following chips/protocols are supported:

  • usbdebug
  • ftdi
  • pl2303

Other things

Append that in your configuration:

terminal_input --append at_keyboard #add keyboard support.
#set timeout=1 #you may want to set a timeout
#set pager=1 # you may want to use the pager or not
play 480 440 1 #play a beep at startup
set prefix=(memdisk)/boot/grub

In case of native graphics you may want the following:

gfxpayload=keep
terminal_output --append gfxterm

Payloads

Here is how to load the SeaBIOS payload from the memdisk.

menuentry 'SeaBios' {
	set root='memdisk'
	echo    'Loading SeaBios ...'
	chainloader /bios.bin.elf
}

see in "creating the GRUB payload" how to include the file in the memdisk...

Distributions

Here's an example on how to load the parabola distribution on the Lenovo X60 from the fifth partition.

menuentry 'Parabola GNU/Linux-libre GNU/Linux, with Linux librepae kernel [Serial]' {
        insmod ahci
        insmod part_msdos
	set root='ahci0,msdos5'
	echo	'Loading Linux librepae kernel ...'
	linux	/vmlinuz-linux-libre-pae root=/dev/mapper/root ro cryptdevice=/dev/sda6:root idle=halt pcie_aspm=force console=ttyS0,115200
	echo	'Loading initial ramdisk ...'
	initrd	/initramfs-linux-libre-pae.img
}

Scanning for grub.cfg on local Hard Drives.

menuentry 'Scan for OS on internal HDD' {
	insmod regexp
	insmod ahci
	insmod part_msdos
	for x in (ahci0,*) ; do
		if [ -f "$x/grub/grub.cfg" ] ; then
			menuentry "Load Config from $x" $x { 
				root=$2
				configfile /grub/grub.cfg
			}
		fi
		if [ -f "$x/boot/grub/grub.cfg" ] ; then
			menuentry "Load Config from $x" $x {
				root=$2
				configfile /boot/grub/grub.cfg
			}
		fi
	done
}

Compiling

git clone git://git.savannah.gnu.org/grub.git grub
cd grub
./autogen.sh
./configure --with-platform=coreboot
make
sudo make install #install the utilities

creating the GRUB payload

Create a target directory:

mkdir memdisk

Then copy your grub.cfg in:

memdisk/boot/grub/grub.cfg

Then adapt and run that script:

#!/bin/sh
rm -f grub2-x60.elf
#copy the payloads you want
cp ../../seabios-x60/out/bios.bin.elf ./memdisk/
cp ../../coreboot-qemu/payloads/nvramcui/nvramcui.elf ./memdisk/
cp ../../coreboot-x60/payloads/coreinfo/build/coreinfo.elf ./memdisk/
cp ../../memtest86+-4.20/memtest ./memdisk/memtest.elf
#and some files
cp ../../coreboot-x60/bootsplash.jpg  ./memdisk/
cd memdisk
grub-mkstandalone -O i386-coreboot -o ../grub2-x60.elf $(find -type f)
echo "--RESULT--"
ls -l -h ../grub2-x60.elf

combining with coreboot

As a SeaBIOS payload

build/cbfstool build/coreboot.rom add-payload -n img/grub2 -f grub2.elf -t raw
build/cbfstool build/coreboot.rom print

That way it will be possible to run GRUB as a payload after SeaBIOS: The advantage is that it's less risky. At runtime press F12 and you'll have the GRUB option.

As a Coreboot payload

Advantages: faster, can be used for security

Disadvantages: more risky if you have no way to recover

Howto

In make menuconfig of coreboot, select the path of grub2.elf.

Also make sure you have some kinds of output such as VGA or serial (it needs to be activated in both coreboot and GRUB)

Before flashing

You should try the grub2.elf on qemu before flashing it to a real mainboard/laptop.