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diff --git a/Documentation/security/vboot/index.md b/Documentation/security/vboot/index.md new file mode 100644 index 0000000000..97420893e5 --- /dev/null +++ b/Documentation/security/vboot/index.md @@ -0,0 +1,324 @@ +# vboot - Verified Boot Support + +Google's verified boot support consists of: + +* A root of trust +* Special firmware layout +* Firmware verification +* Firmware measurements +* A firmware update mechanism +* Specific build flags +* Signing the coreboot image + +Google's vboot verifies the firmware and places measurements within the TPM. + +*** + +## Root of Trust + +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 GCC compiler must reliably translate the code into machine code + without inserting any additional code (virus, backdoor, etc.) +* The CPU must reliably execute the reset sequence and instructions as + documented by the CPU manufacturer. +* The SPI flash must provide only the code programmed into it to the CPU + without providing any alternative reset vector or code sequence. +* The SPI flash must honor the write-protect input and protect the specified + portion of the SPI flash from all erase and write accesses. + +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. + +*** + +## Firmware Layout + +Several sections are added to the firmware layout to support vboot: + +* Read-only section +* Google Binary Blob (GBB) area +* Read/write section A +* Read/write section B + +The following sections describe the various portions of the flash layout. + +### Read-Only Section + +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: + +* Vital Product Data (VPD) area +* Firmware ID area +* Google Binary Blob (GBB) area +* coreboot file system containing read-only recovery firmware + +### Google Binary Blob (GBB) Area + +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. + +### Recovery Firmware + +The recovery firmware is contained within a coreboot file system and consists of: + +* reset vector +* bootblock +* verstage +* romstage +* postcar +* ramstage +* payload +* flash map file +* config file +* processor specific files: + * Microcode + * fspm.bin + * fsps.bin + +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. + +### Read/Write Section + +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: + +* romstage +* postcar +* ramstage +* payload +* config file +* processor specific files: + * Microcode + * fspm.bin + * fsps.bin + +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. + +*** + +## Firmware Updates + +The read/write sections exist in one of three states: + +* Invalid +* Ready to boot +* Successfully booted + + +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 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. + +*** + +## Build Flags + +The following *Kconfig* values need to be selected to enable vboot: + +* COLLECT_TIMESTAMPS +* 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: + +* `VBOOT_VBNV_CMOS` +* `VBOOT_VBNV_EC` +* `VBOOT_VBNV_FLASH` + +More non-volatile storage features may be found in `security/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*. + +*** + +## Signing the coreboot Image + +The following 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`. + +```bash +#!/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 +``` + +*** + +## Boot Flow + +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. + +*** + +## Chromebook Special Features + +Google's Chromebooks have some special features: + +* Developer mode +* Write-protect screw + +### Developer Mode + +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. + +### Write Protect Screw + +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! |