403 lines
13 KiB
HTML
403 lines
13 KiB
HTML
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<!DOCTYPE html>
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<html>
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<head>
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<title>vboot - Verified Boot Support</title>
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</head>
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<body>
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<h1>vboot - Verified Boot Support</h1>
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<p>
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Google's verified boot support consists of:
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</p>
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<ul>
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<li>A root of trust</li>
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<li>Special firmware layout</li>
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<li>Firmware verification</li>
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<li>Firmware measurements</li>
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<li>A firmware update mechanism</li>
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<li>Specific build flags</li>
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<li>Signing the coreboot image</li>
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</ul>
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Google's vboot verifies the firmware and places measurements
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within the TPM.
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<hr>
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<h1>Root of Trust</h1>
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<p>
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When using vboot, the root-of-trust is basically the read-only portion of the
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SPI flash. The following items factor into the trust equation:
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</p>
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<ul>
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<li>The GCC compiler must reliably translate the code into machine code
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without inserting any additional code (virus, backdoor, etc.)
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</li>
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<li>The CPU must reliably execute the reset sequence and instructions as
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documented by the CPU manufacturer.
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</li>
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<li>The SPI flash must provide only the code programmed into it to the CPU
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without providing any alternative reset vector or code sequence.
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</li>
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<li>The SPI flash must honor the write-protect input and protect the
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specified portion of the SPI flash from all erase and write accesses.
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</li>
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</ul>
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<p>
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The firmware is typically protected using the write-protect pin on the SPI
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flash part and setting some of the write-protect bits in the status register
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during manufacturing. The protected area is platform specific and for x86
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platforms is typically 1/4th of the SPI flash
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part size. Because this portion of the SPI flash is hardware write protected,
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it is not possible to update this portion of the SPI flash in the field,
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without altering the system to eliminate the ground connection to the SPI flash
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write-protect pin. Without hardware modifications, this portion of the SPI
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flash maintains the manufactured state during the system's lifetime.
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</p>
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<hr>
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<h1>Firmware Layout</h1>
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<p>
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Several sections are added to the firmware layout to support vboot:
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</p>
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<ul>
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<li>Read-only section</li>
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<li>Google Binary Blob (GBB) area</li>
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<li>Read/write section A</li>
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<li>Read/write section B</li>
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</ul>
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<p>
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The following sections describe the various portions of the flash layout.
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</p>
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<h2>Read-Only Section</h2>
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<p>
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The read-only section contains a coreboot file system (CBFS) that contains all
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of the boot firmware necessary to perform recovery for the system. This
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firmware is typically protected using the write-protect pin on the SPI flash
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part and setting some of the write-protect bits in the status register during
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manufacturing. The protected area is typically 1/4th of the SPI flash part
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size and must cover the entire read-only section which consists of:
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</p>
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<ul>
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<li>Vital Product Data (VPD) area</li>
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<li>Firmware ID area</li>
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<li>Google Binary Blob (GBB) area</li>
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<li>coreboot file system containing read-only recovery firmware</li>
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</ul>
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<h2>Google Binary Blob (GBB) Area</h2>
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<p>
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The GBB area is part of the read-only section. This area contains a 4096 or
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8192 bit public root RSA key that is used to verify the VBLOCK area to obtain
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the firmware signing key.
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</p>
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<h2>Recovery Firmware</h2>
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<p>
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The recovery firmware is contained within a coreboot file system and consists
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of:
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</p>
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<ul>
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<li>reset vector</li>
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<li>bootblock</li>
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<li>verstage</li>
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<li>romstage</li>
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<li>postcar</li>
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<li>ramstage</li>
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<li>payload</li>
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<li>flash map file</li>
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<li>config file</li>
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<li>processor specific files:
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<ul>
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<li>Microcode</li>
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<li>fspm.bin</li>
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<li>fsps.bin</li>
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</ul>
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</li>
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</ul>
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<p>
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The recovery firmware is written during manufacturing and typically contains
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code to write the storage device (eMMC device or hard disk). The recovery
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image is usually contained on a socketed device such as a USB flash drive or
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an SD card. Depending upon the payload firmware doing the recovery, it may
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be possible for the user to interact with the system to specify the recovery
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image path. Part of the recovery is also to write the A and B areas of the
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SPI flash device to boot the system.
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</p>
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<h2>Read/Write Section</h2>
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<p>
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The read/write sections contain an area which contains the firmware signing
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key and signature and an area containing a coreboot file system with a subset
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of the firmware. The firmware files in FW_MAIN_A and FW_MAIN_B are:
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</p>
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<ul>
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<li>romstage</li>
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<li>postcar</li>
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<li>ramstage</li>
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<li>payload</li>
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<li>config file</li>
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<li>processor specific files:
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<ul>
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<li>Microcode</li>
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<li>fspm.bin</li>
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<li>fsps.bin</li>
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</ul>
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</li>
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</ul>
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<p>
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The firmware subset enables most issues to be fixed in the field with firmware
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updates. The firmware files handle memory and most of silicon initialization.
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These files also produce the tables which get passed to the operating system.
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</p>
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<hr>
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<h1>Firmware Updates</h1>
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<p>
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The read/write sections exist in one of three states:
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</p>
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<ul>
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<li>Invalid</li>
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<li>Ready to boot</li>
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<li>Successfully booted</li>
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</ul>
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<table border="1">
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<tr bgcolor="#ffc0c0">
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<td>
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Where is this state information written?
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<br/>CMOS?
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<br/>RW_NVRAM?
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<br/>RW_FWID_*
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</td>
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</tr>
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</table>
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<p>
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Firmware updates are handled by the operating system by writing any read/write
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section that is not in the "successfully booted" state. Upon the next reboot,
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vboot determines the section to boot. If it finds one in the "ready to boot"
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state then it attempts to boot using that section. If the boot fails then
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vboot marks the section as invalid and attempts to fall back to a read/write
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section in the "successfully booted" state. If vboot is not able to find a
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section in the "successfully booted" state then vboot enters recovery mode.
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</p>
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<p>
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Only the operating system is able to transition a section from the "ready to
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boot" state to the "successfully booted" state. The transition is typically
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done after after the operating system has been running for a while indicating
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that successful boot was possible and the operating system is stable.
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</p>
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<p>
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Note that as long as the SPI write protection is in place then the system is
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always recoverable. If the flash update fails then the system will continue
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to boot using the previous read/write area. The same is true if coreboot
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passes control to the payload or the operating system and then the boot fails.
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In the worst case, the SPI flash gets totally corrupted in which case vboot
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fails the signature checks and enters recovery mode. There are no times where
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the SPI flash is exposed and the reset vector or part of the recovery firmware
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gets corrupted.
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</p>
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<hr>
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<h1>Build Flags</h1>
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<p>
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The following Kconfig values need to be selected to enable vboot:
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</p>
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<ul>
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<li>COLLECT_TIMESTAMPS</li>
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<li>VBOOT</li>
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</ul>
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<p>
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The starting stage needs to be specified by selecting either
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VBOOT_STARTS_IN_BOOTBLOCK or VBOOT_STARTS_IN_ROMSTAGE.
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</p>
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<p>
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If vboot starts in bootblock then vboot may be built as a separate stage by
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selecting VBOOT_SEPARATE_VERSTAGE. Additionally, if static RAM is too small
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to fit both verstage and romstage then selecting VBOOT_RETURN_FROM_VERSTAGE
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enables bootblock to reuse the RAM occupied by verstage for romstage.
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</p>
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<p>
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Non-volatile flash is needed for vboot operation. This flash area may be in
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CMOS, the EC, or in a read/write area of the SPI flash device. Select one of
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the following:
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</p>
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<ul>
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<li>VBOOT_VBNV_CMOS</li>
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<li>VBOOT_VBNV_EC</li>
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<li>VBOOT_VBNV_FLASH</li>
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</ul>
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<p>
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More non-volatile storage features may be found in src/vboot/Kconfig.
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</p>
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<p>
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A TPM is also required for vboot operation. TPMs are available in
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drivers/i2c/tpm and drivers/pc80/tpm.
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</p>
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<p>
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In addition to adding the coreboot files into the read-only region, enabling
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vboot causes the build script to add the read/write files into coreboot file
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systems in FW_MAIN_A and FW_MAIN_B.
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</p>
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<hr>
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<h1>Signing the coreboot Image</h1>
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<p>
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The follow command script is an example of how to sign the coreboot image file.
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This script is used on the Intel Galileo board and creates the GBB area and
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inserts it into the coreboot image. It also updates the VBLOCK areas with the
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firmware signing key and the signature for the FW_MAIN firmware. More details
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are available in 3rdparty/vboot/README.
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</p>
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<pre><code>#!/bin/sh
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#
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# The necessary tools were built and installed using the following commands:
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#
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# pushd 3rdparty/vboot
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# make
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# sudo make install
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# popd
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#
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# The keys were made using the following command
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#
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# 3rdparty/vboot/scripts/keygeneration/create_new_keys.sh \
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# --4k --4k-root --output $PWD/keys
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#
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#
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# The "magic" numbers below are derived from the GBB section in
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# src/mainboard/intel/galileo/vboot.fmd.
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#
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# GBB Header Size: 0x80
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# GBB Offset: 0x611000, 4KiB block number: 1553 (0x611)
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# GBB Length: 0x7f000, 4KiB blocks: 127 (0x7f)
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# COREBOOT Offset: 0x690000, 4KiB block number: 1680 (0x690)
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# COREBOOT Length: 0x170000, 4KiB blocks: 368 (0x170)
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#
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# 0x7f000 (GBB Length) = 0x80 + 0x100 + 0x1000 + 0x7ce80 + 0x1000
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#
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# Create the GBB area blob
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# Parameters: hwid_size,rootkey_size,bmpfv_size,recoverykey_size
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#
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gbb_utility -c 0x100,0x1000,0x7ce80,0x1000 gbb.blob
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#
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# Copy from the start of the flash to the GBB region into the signed flash
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# image.
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#
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# 1553 * 4096 = 0x611 * 0x1000 = 0x611000, size of area before GBB
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#
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dd conv=fdatasync ibs=4096 obs=4096 count=1553 \
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if=build/coreboot.rom of=build/coreboot.signed.rom
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#
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# Append the empty GBB area to the coreboot.rom image.
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#
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# 1553 * 4096 = 0x611 * 0x1000 = 0x611000, offset to GBB
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#
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dd conv=fdatasync obs=4096 obs=4096 seek=1553 if=gbb.blob \
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of=build/coreboot.signed.rom
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#
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# Append the rest of the read-only region into the signed flash image.
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#
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# 1680 * 4096 = 0x690 * 0x1000 = 0x690000, offset to COREBOOT area
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# 368 * 4096 = 0x170 * 0x1000 = 0x170000, length of COREBOOT area
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#
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dd conv=fdatasync ibs=4096 obs=4096 skip=1680 seek=1680 count=368 \
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if=build/coreboot.rom of=build/coreboot.signed.rom
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#
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# Insert the HWID and public root and recovery RSA keys into the GBB area.
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#
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gbb_utility \
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--set --hwid='Galileo' \
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-r $PWD/keys/recovery_key.vbpubk \
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-k $PWD/keys/root_key.vbpubk \
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build/coreboot.signed.rom
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#
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# Sign the read/write firmware areas with the private signing key and update
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# the VBLOCK_A and VBLOCK_B regions.
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#
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3rdparty/vboot/scripts/image_signing/sign_firmware.sh \
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build/coreboot.signed.rom \
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$PWD/keys \
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build/coreboot.signed.rom
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</code></pre>
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<hr>
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<h1>Boot Flow</h1>
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<p>
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The reset vector exist in the read-only area and points to the bootblock entry
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point. The only copy of the bootblock exists in the read-only area of the SPI
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flash. Verstage may be part of the bootblock or a separate stage. If separate
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then the bootblock loads verstage from the read-only area and transfers control
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to it.
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</p>
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<p>
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Upon first boot, verstage attempts to verify the read/write section A. It gets
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the public root key from the GBB area and uses that to verify the VBLOCK area
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in read-write section A. If the VBLOCK area is valid then it extracts the
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firmware signing key (1024-8192 bits) and uses that to verify the FW_MAIN_A
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area of read/write section A. If the verification is successful then verstage
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instructs coreboot to use the coreboot file system in read/write section A for
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the contents of the remaining boot firmware (romstage, postcar, ramstage and
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|
|
the payload).
|
||
|
|
</p>
|
||
|
|
|
||
|
|
<p>
|
||
|
|
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.
|
||
|
|
</p>
|
||
|
|
|
||
|
|
<hr>
|
||
|
|
<h1>Chromebook Special Features</h1>
|
||
|
|
<p>
|
||
|
|
Google's Chromebooks have some special features:
|
||
|
|
</p>
|
||
|
|
<ul>
|
||
|
|
<li>Developer mode</li>
|
||
|
|
<li>Write-protect screw</li>
|
||
|
|
</ul>
|
||
|
|
|
||
|
|
<h2>Developer Mode</h2>
|
||
|
|
<p>
|
||
|
|
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.
|
||
|
|
</p>
|
||
|
|
|
||
|
|
<h2>Write Protect Screw</h2>
|
||
|
|
<p>
|
||
|
|
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!
|
||
|
|
</p>
|
||
|
|
|
||
|
|
<hr>
|
||
|
|
<p>Modified: 2 May 2017</p>
|
||
|
|
</body>
|
||
|
|
</html>
|