Firmware security

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This page is a draft in active flux ...
Please leave suggestions on the talk page.

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Scope

This page describes the role of Open Firmware in BitFrost security on XO.

Goals

  1. Run recovery firmware if primary firmware is bad
  2. No access to ok prompt without developer key
  3. Firmware update images must be signed
  4. Boot images must be signed
  5. Unactivated laptops will only boot the activation image
  6. Boot alternate OS image if primary OS image is bad

Files

The files listed below are on NAND FLASH in JFFS2. The zip archives listed below must be created without compression (-n option) and without paths (-j option). Implementation notes and rationale are in italics.

  • Primary images are in /boot, secondary images are in /boot-alt
    At the start of an upgrade process, boot is copied to boot-alt; this doesn't need to be atomic. When upgrade is complete and validated, the upgraded files will be moved atomically to /boot. The atomic substitution requires /boot to be a symlink to the 'real' boot directory. See Early Boot for more details on the upgrade process.
  • The activation lease is named "/security/lease.sig".
    The activation lease is in the firmware activation lease format with hashname "sha"
  • The developer key is named "/security/develop.sig".
    The developer key is in the firmware developer key format with hashname "sha"
    Once the developer key has been read for the first time, it is copied by firmware into reserved space in the flash.
  • The normal OS image is in "/boot/runos.zip", containing "data.img" and "data.sig", and "/boot/runrd.zip", containing "data.img" and "data.sig". An activation OS image is in "/boot/actos.zip", containing "data.img" and "data.sig", and "/boot/actrd.zip", containing "data.img" and "data.sig".
    • data.img in runos.zip and actos.zip is one of the load formats that OFW supports, e.g. Linux bzImage
    • data.img in runrd.zip and actrd.zip is a ramdisk image in a format supported by the kernel in runos.zip/actos.zip (typically initramfs)
    • data.sig is in the firmware key format with hashname "sha256"
    • runrd.zip/actrd.zip may be omitted, in which case runos.zip/actos.zip is booted without a ramdisk.
      Rationale: The ramdisk is only used during activation and certain upgrades. As an optimization, we can skip authenticating and loading the ramdisk when it is not needed in order to simplify and speed up boot. In practice runrd.zip is likely to be a symlink which can be easily deleted and recreated when needed.
  • The new firmware image is "/boot/bootfw.zip", containing "data.img" and "data.sig"
    • data.img is the usual OFW image format
    • data.sig is in the firmware key format with two lines, providing both "sha256" and "rmd160" signatures.
  • The backup files in /boot-alt have the same names and are in the same formats as /boot.

On USB disk or SD card, the files are as follows:

  • Only one directory, /boot, containing runos.zip, bootfw.zip, and an optional runrd.zip as before
    Security dictates that we boot from an authenticated filesystem. Rather than have OFW authenticate the full USB/SD contents, we'll boot from an authenticated ramdisk, which can then authenticate whatever other files it needs (if any) from the USB/SD disk. For this reason, there should always be a ramdisk, but it may be monolithic with the kernel in runos.zip rather than in a separate runrd.zip file.
    We should be careful to ensure that the files are authenticated after they have been loaded into memory, to prevent attacks involving switching files between authentication and later use.

Process

  1. If OFW fails to come up correctly, a firmware recovery procedure is attempted - details TBD.
  2. In the following, the "primary" images are the files in /boot, and the "secondary" images are the files in /boot-alt, unless the "O" gamepad key is held down during boot, in which case the roles reverse: the primary files come from /boot-alt, and the secondary files come from /boot.
  3. If the SPI FLASH lockout latch is already set (as with a warm boot), OFW skips to step 7.
  4. OFW checks for a new firmware image in the /boot directory on an attached USB, then SD, device. If one exists and verifies, OFW reflashes itself and reboots.
    Upgrade USB keys may contain firmware-only upgrades.
  5. OFW checks for a new firmware image in the primary directory in the NAND flash. If one exists and verifies, OFW reflashes itself and reboots.
  6. OFW locks out further SPI FLASH writing with the hardware lock.
  7. If a valid developer key is present, OFW enters non-secure mode, where it behaves as it currently does. Otherwise ...
  8. If the activation key is present and valid, the boot filenames will be runos.zip and (if present) runrd.zip. Otherwise, the boot filenames will be actos.zip and (if present) actrd.zip.
  9. If the activation key is present and valid, we will attempt to verify and boot from the boot files in /boot on an attached USB, then SD, device.
  10. OFW verifies and boots from the boot files in the primary directory.
  11. OFW verifies and boots from the boot files in the secondary directory.
  12. If none of the above booting steps succeed, OFW displays an error screen and halts.
  13. During OS startup, the OS should check the firmware version and if it matches or is greater than the version in the new firmware image file on disk, the OS should delete or rename that file so the firmware won't see it the next time.

Usage notes

  • After boot, userland can determine the source from the OFW device tree - /ofw/chosen/bootpath is the OFW device specifier of the kernel file, /ofw/chosen/ramdisk is the device specifier of the initrd file (if any), and /ofw/chosen/bootargs is the cmdline. This can be used to determine whether activation is needed (actos.zip) or whether booting is being performed from the secondary source (boot-alt/*).
  • Although outside the scope of this spec, there are primary and secondary filesystem roots in /run/a and /run/b corresponding to the kernels in /boot and /boot-alt. /boot/runrd.img will typically be absent to speed boot. However, /boot-alt/runrd.img will typically be required in order to switch to /run/b so that kernel and userland match. When we clone /boot into /boot-alt at the beginning of an upgrade, we link in an appropriate /boot-alt/runrd.img (See Early Boot for more information.)
  • When the alternate kernel is booted and we've switched into /run/b, we can either:
    1. add a /boot/runrd.zip link so that if we reboot into the primary we can switch the filesystem back /run/a, or
    2. Swap /boot and /boot-alt, making future boots start this kernel. This option is preferred. We should ensure that we've done another upgrade before we try to boot into a different kernel again.
  • We will typically use hard or soft links to avoid storing multiple os and ramdisk images. The current plan is to actually have only one kernel and one ramdisk image; the ramdisk will look at how it was invoked to determine whether this is an upgrade, activation, or alternate boot.

Summary

Plan of Record
Case File Expiry Signed Object Signature (ASCII) Computation Key Effect on Success
OS/Ramdisk Images /boot{,-alt}/ {run,act}{os,rd}.zip - data.img in .zip data.sig in .zip sha256->rsassa-pss-2048 OLPC_OS_Key Load image
Firmware Images /boot/bootfw.zip - data.img in .zip data.sig in .zip (2 lines) sha256->rsassa-pss-2048, rmd160->rsassa-pkcs-v1_5-2048 OLPC_FW_Key Update firmware
Developer /security/develop.sig - <SN>:<UU>:Time0 keydata on sig line sha256->rsassa-pss-2048 OLPC_DEVELOP_Key Unlock firmware
Lease /security/lease.sig time on sig line <SN>:<UU>:<Expiry> keydata on sig line sha256->rsassa-pss-2048, check time OLPC_LEASE_Key Use runos not actos

Rationale:

  • .sig, not .key, because it contains signatures, not keys
  • no need to reiterate the name for the .img file; it just complicates the code
  • prefer RSASSA-PSS signatures (see PKCS #1) due to known attacks on RSASSA-PKCS [1], [2]
  • ripemd160 protects against false-positive flaws in signature verification algorithm; algorithm choice is constrained by those supported by PKCS #11.
  • "Time0" is in same 16-character format as a legitimate time, but it is ignored.
  • I'm still thinking through this, so don't be surprised if I change my mind (wmb)

Notes

  • I am assuming that the primary use of USB/SD boot is to do OS, firmware, or activity upgrades where bandwidth is a limitation. These can easily be done with the mechanism provided by just sticking a (properly signed) "magic upgrade key" into the USB port and power-cycling.
  • USB probing is time-consuming and perilous (although SD is not). At some point (after manufacturing processes are fixed) we'll only try to boot from USB if the 'X' game key is pressed during boot.
  • We could be more user-friendly by detecting "failed boot after linux kernel invocation" in some way, and automatically booting from the backup in this case. This seems like post-FRS work.
  • Firmware RTC *must be UTC*. Quanta must set the RTC to UTC at the factory; antitheft server must sync to UTC during antitheft interaction.
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