FwupdPlugin – 1.0: Plugin: UEFI SBAT
Description: Reference for FwupdPlugin-1.0: Plugin: UEFI SBAT

#### Plugin: UEFI SBAT

## Introduction

SBAT is a generation number based revocation mechanism that is used as a successsor to dbx checksum blocklisting.

See https://github.com/rhboot/shim/blob/main/SBAT.md for more details.

## Firmware Format

The daemon will decompress the cabinet archive and extract a signed PE/COFF firmware blob with a `.sbata` section containing the revocation CSV data.

This plugin supports the following protocol ID:

- `org.uefi.sbat`

## GUID Generation

These devices use the distribution-specific DeviceInstanceId values, e.g.

- `UEFI\OS_fedora` (only quirk)
- `UEFI\OS_fedora&VAR_SbatLevelRT`

## Update Behavior

The device version is calculated using the same heuristics as for the revocation PE file, which is complicated slightly by there being potentially multiple components (each with a version) in the payload, for instance, `shim`, `grub` and `sd-boot`.

These revocation components may or may not exist in the ESP, and may not exist in the existing platform UEFI `SbatLevelRT` variable, and so it is impossible to create a set of child-devices for each item like we might do for a USB device for example.

Additionally, assigning a single version is complicated further because per-vendor overrides may be present in the SBAT entries in either the `SbatLevelRT` key or the signed revocation file.

To overcome these limitations (and to force a round peg in a square hole) we sum up each generation number contained in the SBAT sections, including the per-vendor value where provided.

The semver major value is the SBAT revision, the minor value is the sum of all the upstream component generation numbers, and the micro value is the sum of all the downstream component generation numbers.

For example:

A single component:

    sbat,1
    -> "version 1.0.0"

Multiple components:

    sbat,1
    grub,4
    -> "version 1.4.0"

Multiple components, with a distro specific override:

    sbat,1
    grub,4
    sd-boot,2
    grub.fedora,2
    grub.ubuntu,2
    -> "version 1.7.4"

This ensures that we can have a single version number representing the device current state, and also a single version number that represents the most up to date per-distro revocation data.

For this reason different distributions should have a different AppStream namespace for the revocation data, for instance `org.fedoraproject.sbat.firmware` — which makes sense as each vendor will be signing the revocation binary with its own key anyway.

## Deployment Concerns

Before deploying the contents of the `.sbata` COFF section to the `SbatLevelRT` UEFI variable we should check that we wouldn’t be soft bricking the users PC by deploying a SBAT policy that cannot allow the current shim and second stage bootloader to run on next reboot.

Although the `SbatLevelRT` UEFI variable is deleted when SecureBoot is turned off, this is not something we should be asking end users to do.

By locating each `BootXXXX` file path on the ESP we can load the `.sbat` section and only deploy the revocation if the generation number of the EFI binary is greater than or equal to the specific generation number specified in the revocation.

For instance, if there is a boot entry `Boot0001 -> shim.efi` we would:

- Read `ESP/shim.efi` COFF section `.sbat`:

  sbat,1 shim,4 shim.rh,3 shim.fedora,3

- Read `ESP/grubx64.efi` (this filename is hardcoded into `ESP/shim.efi`) COFF section `.sbat`

  sbat,1 grub,3 grub.rh,2

- Verify that the `.sbata` section in the `revocations.efi` payload is not newer that each of the file-defined SBAT values:

  sbat,1 shim,2 grub,3 grub.debian,4

## Vendor ID Security

The vendor ID is set from the operating system vendor as discovered from `os-release`, in this instance set to `OS:fedora`

## External Interface Access

This plugin requires read access to `/sys/firmware/efi/efivars` and read/write access to the ESP.

## Version Considerations

This plugin has been available since fwupd version `2.0.0`.

## Testing

    $ sudo fwupdtool --plugins uefi-sbat  -vv install-blob revocations.efi.signed
    $ sudo fwupdtool --plugins uefi-sbat  -vv reboot-cleanup
    $ sudo fwupdtool firmware-build ../plugins/uefi-sbat/revocation.builder.xml revocation.efi
    Decompressing…           [************                           ]
    SBAT level is too old on /boot/efi/EFI/fedora/grubx64.efi: ESP file /boot/efi/EFI/fedora/shimx64.efi has SBAT entry sbat v1, but revocation has v2

## Owners

Anyone can submit a pull request to modify this plugin, but the following people should be consulted before making major or functional changes:

- Peter Jones: `vathpela`