+Google's verified boot support consists of: +
++When using vboot, the root-of-trust is basically the read-only portion of the +SPI flash. The following items factor into the trust equation: +
++The firmware is typically protected using the write-protect pin on the SPI +flash part and setting some of the write-protect bits in the status register +during manufacturing. The protected area is platform specific and for x86 +platforms is typically 1/4th of the SPI flash +part size. Because this portion of the SPI flash is hardware write protected, +it is not possible to update this portion of the SPI flash in the field, +without altering the system to eliminate the ground connection to the SPI flash +write-protect pin. Without hardware modifications, this portion of the SPI +flash maintains the manufactured state during the system's lifetime. +
+ ++Several sections are added to the firmware layout to support vboot: +
++The following sections describe the various portions of the flash layout. +
+ ++The read-only section contains a coreboot file system (CBFS) that contains all +of the boot firmware necessary to perform recovery for the system. This +firmware is typically protected using the write-protect pin on the SPI flash +part and setting some of the write-protect bits in the status register during +manufacturing. The protected area is typically 1/4th of the SPI flash part +size and must cover the entire read-only section which consists of: +
++The GBB area is part of the read-only section. This area contains a 4096 or +8192 bit public root RSA key that is used to verify the VBLOCK area to obtain +the firmware signing key. +
+ ++The recovery firmware is contained within a coreboot file system and consists +of: +
++The recovery firmware is written during manufacturing and typically contains +code to write the storage device (eMMC device or hard disk). The recovery +image is usually contained on a socketed device such as a USB flash drive or +an SD card. Depending upon the payload firmware doing the recovery, it may +be possible for the user to interact with the system to specify the recovery +image path. Part of the recovery is also to write the A and B areas of the +SPI flash device to boot the system. +
+ + ++The read/write sections contain an area which contains the firmware signing +key and signature and an area containing a coreboot file system with a subset +of the firmware. The firmware files in FW_MAIN_A and FW_MAIN_B are: +
++The firmware subset enables most issues to be fixed in the field with firmware +updates. The firmware files handle memory and most of silicon initialization. +These files also produce the tables which get passed to the operating system. +
+ ++The read/write sections exist in one of three states: +
+|
+Where is this state information written?
+ CMOS? + RW_NVRAM? + RW_FWID_* + |
+
+Firmware updates are handled by the operating system by writing any read/write +section that is not in the "successfully booted" state. Upon the next reboot, +vboot determines the section to boot. If it finds one in the "ready to boot" +state then it attempts to boot using that section. If the boot fails then +vboot marks the section as invalid and attempts to fall back to a read/write +section in the "successfully booted" state. If vboot is not able to find a +section in the "successfully booted" state then vboot enters recovery mode. +
+ ++Only the operating system is able to transition a section from the "ready to +boot" state to the "successfully booted" state. The transition is typically +done after after the operating system has been running for a while indicating +that successful boot was possible and the operating system is stable. +
+ ++Note that as long as the SPI write protection is in place then the system is +always recoverable. If the flash update fails then the system will continue +to boot using the previous read/write area. The same is true if coreboot +passes control to the payload or the operating system and then the boot fails. +In the worst case, the SPI flash gets totally corrupted in which case vboot +fails the signature checks and enters recovery mode. There are no times where +the SPI flash is exposed and the reset vector or part of the recovery firmware +gets corrupted. +
+ ++The following Kconfig values need to be selected to enable vboot: +
++The starting stage needs to be specified by selecting either +VBOOT_STARTS_IN_BOOTBLOCK or VBOOT_STARTS_IN_ROMSTAGE. +
+ ++If vboot starts in bootblock then vboot may be built as a separate stage by +selecting VBOOT_SEPARATE_VERSTAGE. Additionally, if static RAM is too small +to fit both verstage and romstage then selecting VBOOT_RETURN_FROM_VERSTAGE +enables bootblock to reuse the RAM occupied by verstage for romstage. +
+ ++Non-volatile flash is needed for vboot operation. This flash area may be in +CMOS, the EC, or in a read/write area of the SPI flash device. Select one of +the following: +
++More non-volatile storage features may be found in src/vboot/Kconfig. +
+ ++A TPM is also required for vboot operation. TPMs are available in +drivers/i2c/tpm and drivers/pc80/tpm. +
+ ++In addition to adding the coreboot files into the read-only region, enabling +vboot causes the build script to add the read/write files into coreboot file +systems in FW_MAIN_A and FW_MAIN_B. +
+ ++The follow command script is an example of how to sign the coreboot image file. +This script is used on the Intel Galileo board and creates the GBB area and +inserts it into the coreboot image. It also updates the VBLOCK areas with the +firmware signing key and the signature for the FW_MAIN firmware. More details +are available in 3rdparty/vboot/README. +
+ +#!/bin/sh
+#
+# The necessary tools were built and installed using the following commands:
+#
+# pushd 3rdparty/vboot
+# make
+# sudo make install
+# popd
+#
+# The keys were made using the following command
+#
+# 3rdparty/vboot/scripts/keygeneration/create_new_keys.sh \
+# --4k --4k-root --output $PWD/keys
+#
+#
+# The "magic" numbers below are derived from the GBB section in
+# src/mainboard/intel/galileo/vboot.fmd.
+#
+# GBB Header Size: 0x80
+# GBB Offset: 0x611000, 4KiB block number: 1553 (0x611)
+# GBB Length: 0x7f000, 4KiB blocks: 127 (0x7f)
+# COREBOOT Offset: 0x690000, 4KiB block number: 1680 (0x690)
+# COREBOOT Length: 0x170000, 4KiB blocks: 368 (0x170)
+#
+# 0x7f000 (GBB Length) = 0x80 + 0x100 + 0x1000 + 0x7ce80 + 0x1000
+#
+# Create the GBB area blob
+# Parameters: hwid_size,rootkey_size,bmpfv_size,recoverykey_size
+#
+gbb_utility -c 0x100,0x1000,0x7ce80,0x1000 gbb.blob
+
+#
+# Copy from the start of the flash to the GBB region into the signed flash
+# image.
+#
+# 1553 * 4096 = 0x611 * 0x1000 = 0x611000, size of area before GBB
+#
+dd conv=fdatasync ibs=4096 obs=4096 count=1553 \
+ if=build/coreboot.rom of=build/coreboot.signed.rom
+
+#
+# Append the empty GBB area to the coreboot.rom image.
+#
+# 1553 * 4096 = 0x611 * 0x1000 = 0x611000, offset to GBB
+#
+dd conv=fdatasync obs=4096 obs=4096 seek=1553 if=gbb.blob \
+ of=build/coreboot.signed.rom
+
+#
+# Append the rest of the read-only region into the signed flash image.
+#
+# 1680 * 4096 = 0x690 * 0x1000 = 0x690000, offset to COREBOOT area
+# 368 * 4096 = 0x170 * 0x1000 = 0x170000, length of COREBOOT area
+#
+dd conv=fdatasync ibs=4096 obs=4096 skip=1680 seek=1680 count=368 \
+ if=build/coreboot.rom of=build/coreboot.signed.rom
+
+#
+# Insert the HWID and public root and recovery RSA keys into the GBB area.
+#
+gbb_utility \
+ --set --hwid='Galileo' \
+ -r $PWD/keys/recovery_key.vbpubk \
+ -k $PWD/keys/root_key.vbpubk \
+ build/coreboot.signed.rom
+
+#
+# Sign the read/write firmware areas with the private signing key and update
+# the VBLOCK_A and VBLOCK_B regions.
+#
+3rdparty/vboot/scripts/image_signing/sign_firmware.sh \
+ build/coreboot.signed.rom \
+ $PWD/keys \
+ build/coreboot.signed.rom
++The reset vector exist in the read-only area and points to the bootblock entry +point. The only copy of the bootblock exists in the read-only area of the SPI +flash. Verstage may be part of the bootblock or a separate stage. If separate +then the bootblock loads verstage from the read-only area and transfers control +to it. +
+ ++Upon first boot, verstage attempts to verify the read/write section A. It gets +the public root key from the GBB area and uses that to verify the VBLOCK area +in read-write section A. If the VBLOCK area is valid then it extracts the +firmware signing key (1024-8192 bits) and uses that to verify the FW_MAIN_A +area of read/write section A. If the verification is successful then verstage +instructs coreboot to use the coreboot file system in read/write section A for +the contents of the remaining boot firmware (romstage, postcar, ramstage and +the payload). +
+ ++If verification fails for the read/write area and the other read/write area is +not valid vboot falls back to the read-only area to boot into system recovery. +
+ ++Google's Chromebooks have some special features: +
++Developer mode allows the user to use coreboot to boot another operating system. +This may be a another (beta) version of Chrome OS, or another flavor of +GNU/Linux. Use of developer mode does not void the system warranty. Upon +entry into developer mode, all locally saved data on the system is lost. +This prevents someone from entering developer mode to subvert the system +security to access files on the local system or cloud. +
+ ++Chromebooks have a write-protect screw which provides the ground to the +write-protect pin of the SPI flash. Google specifically did this to allow +the manufacturing line and advanced developers to re-write the entire SPI flash +part. Once the screw is removed, any firmware may be placed on the device. +However, accessing this screw requires opening the case and voids the system +warranty! +
+ +Modified: 2 May 2017
+ +