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SBAT: Current proposal

.sbat section

the .sbat section has the following fields:

field meaning
component_name the name we're comparing
component_generation the generation number for the comparison
vendor_name human readable vendor name
vendor_package_name human readable package name
vendor_version human readable package version (maybe machine parseable too, not specified here)
vendor_url url to look stuff up, contact, whatever.

SBAT EFI variable

The SBAT EFI variable (SbatLevel-605dab50-e046-4300-abb6-3dd810dd8b23) is structured as a series ASCII CSV records:

sbat,1
component_name,component_generation
component_name,component_generation

with the first record being our structure version number expressed as: {'s', 'b', 'a', 't', ',', '1', '\n'}

Validation Rules

  • If an SBAT variable is set, binaries validated directly by shim (i.e. not with the API) get validated using SBAT
  • If a binary validated by the API does have a .sbat section, it also gets validating using SBAT
  • If a binary is enrolled in db by its hash rather than by certificate, the validation result is logged only, not enforced
  • When validating SBAT, any component_name that's in both SBAT and .sbat gets compared
  • component_generation in .sbat must be >= component_generation of the identical component_name in SBAT
  • That version comparison includes the "sbat" entry
  • Component versions must be positive integers.

Example universe starting point

For grub, a build from a fresh checkout of upstream might have the following in .sbat:

sbat,1,SBAT Version,sbat,1,https://github.com/rhboot/shim/blob/main/SBAT.md
grub,1,Free Software Foundation,grub,2.04,https://www.gnu.org/software/grub/

A Fedora build believed to have exactly the same set of vulnerabilities plus one that was never upstream might have:

sbat,1,SBAT Version,sbat,1,https://github.com/rhboot/shim/blob/main/SBAT.md
grub,1,Free Software Foundation,grub,2.04,https://www.gnu.org/software/grub/
grub.fedora,1,The Fedora Project,grub2,2.04-31.fc33,https://src.fedoraproject.org/rpms/grub2

Likewise, Red Hat has various builds for RHEL 7 and RHEL 8, all of which have something akin to the following in .sbat:

sbat,1,SBAT Version,sbat,1,https://github.com/rhboot/shim/blob/main/SBAT.md
grub,1,Free Software Foundation,grub,2.02,https://www.gnu.org/software/grub/
grub.fedora,1,Red Hat Enterprise Linux,grub2,2.02-0.34.fc24,mail:[email protected]
grub.rhel,1,Red Hat Enterprise Linux,grub2,2.02-0.34.el7_2,mail:[email protected]

The Debian package believed to have the same set of vulnerabilities as upstream might have:

sbat,1,SBAT Version,sbat,1,https://github.com/rhboot/shim/blob/main/SBAT.md
grub,1,Free Software Foundation,grub,2.04,https://www.gnu.org/software/grub/
grub.debian,1,Debian,grub2,2.04-12,https://packages.debian.org/source/sid/grub2

Another party known for less than high quality software who carry a bunch of out of tree grub patches on top of a very old grub version from before any of the upstream vulns were committed to the tree. They haven't ever had the upstream vulns, and in fact have never shipped any vulnerabilities. Their grub .sbat might have the following (which we'd be very suspect of signing, but hey, suppose it turns out to be correct):

sbat,1,SBAT Version,sbat,1,https://github.com/rhboot/shim/blob/main/SBAT.md
grub.acme,1,Acme Corporation,grub,1.96-8191,https://acme.arpa/packages/grub

At the same time, we're all shipping the same shim-16 codebase, and in our shim builds, we all have the following in .sbat:

sbat,1,SBAT Version,sbat,1,https://github.com/rhboot/shim/blob/main/SBAT.md

The SBAT data we've all agreed on and the UEFI CA has distributed is:

sbat,1
shim,1
grub,1
grub.fedora,1

Which is literally the byte array:

{
  's', 'b', 'a', 't', ',', '1', '\n',
  's', 'h', 'i', 'm', ',', '1', '\n',
  'g', 'r', 'u', 'b', ',', '1', '\n',
  'g', 'r', 'u', 'b', '.', 'f', 'e', 'd', 'o', 'r', 'a', ',', '1', '\n',
}

Along comes bug 0

Let's say we find a bug in Fedora that's serious, but it's only in Fedora's patches, because we're the dumb ones. Fedora issues a new shim build with the following .sbat info:

sbat,1,SBAT Version,sbat,1,https://github.com/rhboot/shim/blob/main/SBAT.md
grub,1,Free Software Foundation,grub,2.04,https://www.gnu.org/software/grub/
grub.fedora,2,The Fedora Project,grub2,2.04-32.fc33,https://src.fedoraproject.org/rpms/grub2

For some (clearly insane) reason, 9 RHEL builds are also produced with the same fixes and the same data in .sbat that Fedora has. The RHEL 7.2 update (just one example, same one as the RHEL example above) has the following in its .sbat data:

sbat,1,SBAT Version,sbat,1,https://github.com/rhboot/shim/blob/main/SBAT.md
grub,1,Free Software Foundation,grub,2.02,https://www.gnu.org/software/grub/
grub.fedora,2,Red Hat Enterprise Linux,grub2,2.02-0.34.fc24,mail:[email protected]
grub.rhel,2,Red Hat Enterprise Linux,grub2,2.02-0.34.el7_2.1,mail:[email protected]

The UEFI CA issues a new SBAT update which looks like:

sbat,1
shim,1
grub,1
grub.fedora,2

Along comes bug 1

Another kind security researcher shows up with a serious bug, and this one was in upstream grub-0.94 and every version after that, and is shipped by all vendors.

At this point, each vendor updates their grub builds, and updates the component_generation in .sbat to 2. The upstream build now looks like:

sbat,1,SBAT Version,sbat,1,https://github.com/rhboot/shim/blob/main/SBAT.md
grub,2,Free Software Foundation,grub,2.05,https://www.gnu.org/software/grub/

But Fedora's now looks like:

sbat,1,SBAT Version,sbat,1,https://github.com/rhboot/shim/blob/main/SBAT.md
grub,2,Free Software Foundation,grub,2.04,https://www.gnu.org/software/grub/
grub.fedora,2,The Fedora Project,grub2,2.04-33.fc33,https://src.fedoraproject.org/rpms/grub2

Other distros either rebase on 2.05 or theirs change similarly to Fedora's. We now have two options for Acme Corp:

  • add a grub.acme,1 entry to SBAT
  • have Acme Corp add grub,2,Free Software Foundation,grub,1.96,https://www.gnu.org/software/grub/ to their new build's .sbat

We talk to Acme and they agree to do the latter, thus saving flash real estate to be developed on another day. Their binary now looks like:

sbat,1,SBAT Version,sbat,1,https://github.com/rhboot/shim/blob/main/SBAT.md
grub,2,Free Software Foundation,grub,1.96,https://www.gnu.org/software/grub/
grub.acme,2,Acme Corporation,grub,1.96-8192,https://acme.arpa/packages/grub

The UEFI CA issues an update which looks like:

sbat,1
shim,1
grub,2
grub.fedora,2

Acme Corp gets with the program

Acme at this point discovers some features have been added to grub and they want them. They ship a new grub build that's completely rebased on top of upstream and has no known vulnerabilities. Its .sbat data looks like:

sbat,1,SBAT Version,sbat,1,https://github.com/rhboot/shim/blob/main/SBAT.md
grub,2,Free Software Foundation,grub,2.05,https://www.gnu.org/software/grub/
grub.acme,2,Acme Corporation,grub,2.05-1,https://acme.arpa/packages/grub

Someone was wrong on the internet and bug 2

Debian discovers that they actually shipped bug 0 as well (woops). They produce a new build which fixes it and has the following in .sbat:

sbat,1,SBAT Version,sbat,1,https://github.com/rhboot/shim/blob/main/SBAT.md
grub,2,Free Software Foundation,grub,2.04,https://www.gnu.org/software/grub/
grub.debian,2,Debian,grub2,2.04-13,https://packages.debian.org/source/sid/grub2

Before the UEFI CA has released an update, though, another upstream issue is found. Everybody updates their builds as they did for bug 1. Debian also updates theirs, as they would, and their new build has:

sbat,1,SBAT Version,sbat,1,https://github.com/rhboot/shim/blob/main/SBAT.md
grub,3,Free Software Foundation,grub,2.04,https://www.gnu.org/software/grub/
grub.debian,2,Debian,grub2,2.04-13,https://packages.debian.org/source/sid/grub2

And the UEFI CA issues an update to SBAT which has:

sbat,1
shim,1
grub,3
grub.fedora,2

Two key things here:

  • grub.debian still got updated to 2 in their .sbat data, because a vulnerability was fixed that is only covered by that updated number.
  • There is still no SBAT update for grub.debian, because there's no binary that needs it which is not covered by updating grub to 3.