attestation: add initial attestation helpers, integrate into brew install

Library/Homebrew/attestation.rb

@@ -0,0 +1,138 @@
+# typed: strict
+# frozen_string_literal: true
+
+require "date"
+require "json"
+require "utils/popen"
+require "exceptions"
+
+module Homebrew
+ module Attestation
+ # @api private
+ HOMEBREW_CORE_REPO = "Homebrew/homebrew-core"
+ # @api private
+ HOMEBREW_CORE_CI_URI = "https://github.com/Homebrew/homebrew-core/.github/workflows/publish-commit-bottles.yml@refs/heads/master"
+
+ # @api private
+ BACKFILL_REPO = "trailofbits/homebrew-brew-verify"
+ # @api private
+ BACKFILL_REPO_CI_URI = "https://github.com/trailofbits/homebrew-brew-verify/.github/workflows/backfill_signatures.yml@refs/heads/main"
+
+ # No backfill attestations after this date are considered valid.
+ #
+ # This date is shortly after the backfill operation for homebrew-core
+ # completed, as can be seen here: <https://github.com/trailofbits/homebrew-brew-verify/attestations>.
+ #
+ # In effect, this means that, even if an attacker is able to compromise the backfill
+ # signing workflow, they will be unable to convince a verifier to accept their newer,
+ # malicious backfilled signatures.
+ #
+ # @api private
+ BACKFILL_CUTOFF = T.let(DateTime.new(2024, 3, 14).freeze, DateTime)
+
+ # Raised when attestation verification fails.
+ #
+ # @api private
+ class InvalidAttestationError < RuntimeError; end
+
+ # Returns a path to a suitable `gh` executable for attestation verification.
+ #
+ # @api private
+ sig { returns(Pathname) }
+ def self.gh_executable
+ # NOTE: We disable HOMEBREW_VERIFY_ATTESTATIONS when installing `gh` itself,
+ # to prevent a cycle during bootstrapping. This can eventually be resolved
+ # by vendoring a pure-Ruby Sigstore verifier client.
+ @gh_executable ||= T.let(with_env("HOMEBREW_VERIFY_ATTESTATIONS" => nil) do
+ ensure_executable!("gh")
+ end, T.nilable(Pathname))
+ end
+
+ # Verifies the given bottle against a cryptographic attestation of build provenance.
+ #
+ # The provenance is verified as originating from `signing_repo`, which is a `String`
+ # that should be formatted as a GitHub `owner/repo`.
+ #
+ # Callers may additionally pass in `signing_workflow`, which will scope the attestation
+ # down to an exact GitHub Actions workflow, in
+ # `https://github/OWNER/REPO/.github/workflows/WORKFLOW.yml@REF` format.
+ #
+ # @return [Hash] the JSON-decoded response.
+ # @raise [InvalidAttestationError] on any verification failures
+ #
+ # @api private
+ sig {
+ params(bottle: Bottle, signing_repo: String,
+ signing_workflow: T.nilable(String), subject: T.nilable(String)).returns(T::Hash[T.untyped, T.untyped])
+ }
+ def self.check_attestation(bottle, signing_repo, signing_workflow = nil, subject = nil)
+ cmd = [gh_executable, "attestation", "verify", bottle.cached_download, "--repo", signing_repo, "--format",
+ "json"]
+
+ cmd += ["--cert-identity", signing_workflow] if signing_workflow.present?
+
+ begin
+ output = Utils.safe_popen_read(*cmd)
+ rescue ErrorDuringExecution => e
+ raise InvalidAttestationError, "attestation verification failed: #{e}"
+ end
+
+ begin
+ attestations = JSON.parse(output)
+ rescue JSON::ParserError
+ raise InvalidAttestationError, "attestation verification returned malformed JSON"
+ end
+
+ # `gh attestation verify` returns a JSON array of one or more results,
+ # for all attestations that match the input's digest. We want to additionally
+ # filter these down to just the attestation whose subject matches the bottle's name.
+ subject = bottle.filename.to_s if subject.blank?
+ attestation = attestations.find do |a|
+ a.dig("verificationResult", "statement", "subject", 0, "name") == subject
+ end
+
+ raise InvalidAttestationError, "no attestation matches subject" if attestation.blank?
+
+ attestation
+ end
+
+ # Verifies the given bottle against a cryptographic attestation of build provenance
+ # from homebrew-core's CI, falling back on a "backfill" attestation for older bottles.
+ #
+ # This is a specialization of `check_attestation` for homebrew-core.
+ #
+ # @return [Hash] the JSON-decoded response
+ # @raise [InvalidAttestationError] on any verification failures
+ #
+ # @api private
+ sig { params(bottle: Bottle).returns(T::Hash[T.untyped, T.untyped]) }
+ def self.check_core_attestation(bottle)
+ begin
+ attestation = check_attestation bottle, HOMEBREW_CORE_REPO, HOMEBREW_CORE_CI_URI
+ return attestation
+ rescue InvalidAttestationError
+ odebug "falling back on backfilled attestation for #{bottle}"
+
+ # Our backfilled attestation is a little unique: the subject is not just the bottle
+ # filename, but also has the bottle's hosted URL hash prepended to it.
+ # This was originally unintentional, but has a virtuous side effect of further
+ # limiting domain separation on the backfilled signatures (by committing them to
+ # their original bottle URLs).
+ url_sha256 = Digest::SHA256.hexdigest(bottle.url)
+ subject = "#{url_sha256}--#{bottle.filename}"
+
+ backfill_attestation = check_attestation bottle, BACKFILL_REPO, BACKFILL_REPO_CI_URI, subject
+ timestamp = backfill_attestation.dig("verificationResult", "verifiedTimestamps",
+ 0, "timestamp")
+
+ raise InvalidAttestationError, "backfill attestation is missing verified timestamp" if timestamp.nil?
+
+ if DateTime.parse(timestamp) > BACKFILL_CUTOFF
+ raise InvalidAttestationError, "backfill attestation post-dates cutoff"
+ end
+ end
+
+ backfill_attestation
+ end
+ end
+end

Library/Homebrew/env_config.rb

@@ -422,6 +422,11 @@ module EnvConfig
  "useful to avoid long-running Homebrew commands being killed due to no output.",
  boolean: true,
  },
+ HOMEBREW_VERIFY_ATTESTATIONS: {
+ description: "If set, Homebrew will use the `gh` tool to verify cryptographic attestations " \
+ "of build provenance for bottles from homebrew-core.",
+ boolean: true,
+ },
  SUDO_ASKPASS: {
  description: "If set, pass the `-A` option when calling `sudo`(8).",
  },

Library/Homebrew/formula_installer.rb

@@ -22,6 +22,7 @@
 require "deprecate_disable"
 require "unlink"
 require "service"
+require "attestation"
 
 # Installer for a formula.
 #
@@ -1256,6 +1257,25 @@
 
  sig { void }
  def pour
+ if Homebrew::EnvConfig.verify_attestations? && formula.tap&.core_tap?
+ ohai "Verifying attestation for #{formula.name}"
+ begin
+ Homebrew::Attestation.check_core_attestation formula.bottle
+ rescue Homebrew::Attestation::InvalidAttestationError => e
+ raise CannotInstallFormulaError, <<~EOS
+ The bottle for #{formula.name} has an invalid build provenance attestation.
+
+ This may indicate that the bottle was not produced by the expected
+ tap, or was maliciously inserted into the expected tap's bottle
+ storage.
+
+ Additional context:
+
+ #{e}
+ EOS
+ end
+ end
+
  HOMEBREW_CELLAR.cd do
  downloader.stage
  end

Library/Homebrew/software_spec.rb

@@ -423,6 +423,11 @@ def tab_attributes
  github_packages_manifest_resource_tab(github_packages_manifest_resource)
  end
 
+ sig { returns(Filename) }
+ def filename
+ Filename.create(resource.owner, @tag, @spec.rebuild)
+ end
+
  private
 
  def github_packages_manifest_resource_tab(github_packages_manifest_resource)

Library/Homebrew/test/attestation_spec.rb

@@ -0,0 +1,153 @@
+# frozen_string_literal: true
+
+require "diagnostic"
+
+RSpec.describe Homebrew::Attestation do
+ let(:fake_gh) { Pathname.new("/extremely/fake/gh") }
+ let(:cached_download) { "/fake/cached/download" }
+ let(:fake_bottle_filename) { instance_double(Bottle::Filename, to_s: "fakebottle--1.0.faketag.bottle.tar.gz") }
+ let(:fake_bottle_url) { "https://example.com/#{fake_bottle_filename}" }
+ let(:fake_bottle) do
+ instance_double(Bottle, cached_download:, filename: fake_bottle_filename, url: fake_bottle_url)
+ end
+ let(:fake_json_resp) do
+ JSON.dump([
+ { verificationResult: {
+ verifiedTimestamps: [{ timestamp: "2024-03-13T00:00:00Z" }],
+ statement: { subject: [{ name: fake_bottle_filename.to_s }] },
+ } },
+ ])
+ end
+ let(:fake_json_resp_backfill) do
+ JSON.dump([
+ { verificationResult: {
+ verifiedTimestamps: [{ timestamp: "2024-03-13T00:00:00Z" }],
+ statement: {
+ subject: [{ name: "#{Digest::SHA256.hexdigest(fake_bottle_url)}--#{fake_bottle_filename}" }],
+ },
+ } },
+ ])
+ end
+ let(:fake_json_resp_too_new) do
+ JSON.dump([
+ { verificationResult: {
+ verifiedTimestamps: [{ timestamp: "2024-03-15T00:00:00Z" }],
+ statement: { subject: [{ name: fake_bottle_filename.to_s }] },
+ } },
+ ])
+ end
+ let(:fake_json_resp_wrong_sub) do
+ JSON.dump([
+ { verificationResult: {
+ verifiedTimestamps: [{ timestamp: "2024-03-13T00:00:00Z" }],
+ statement: { subject: [{ name: "wrong-subject.tar.gz" }] },
+ } },
+ ])
+ end
+
+ describe "::gh_executable" do
+ it "calls ensure_executable" do
+ expect(described_class).to receive(:ensure_executable!)
+ .with("gh")
+ .and_return(fake_gh)
+
+ described_class.gh_executable
+ end
+ end
+
+ describe "::check_attestation" do
+ before do
+ allow(described_class).to receive(:gh_executable)
+ .and_return(fake_gh)
+ end
+
+ it "raises when gh subprocess fails" do
+ expect(Utils).to receive(:safe_popen_read)
+ .with(fake_gh, "attestation", "verify", cached_download, "--repo",
+ described_class::HOMEBREW_CORE_REPO, "--format", "json")
+ .and_raise(ErrorDuringExecution.new(["foo"], status: 1))
+
+ expect do
+ described_class.check_attestation fake_bottle,
+ described_class::HOMEBREW_CORE_REPO
+ end.to raise_error(described_class::InvalidAttestationError)
+ end
+
+ it "raises when gh returns invalid JSON" do
+ expect(Utils).to receive(:safe_popen_read)
+ .with(fake_gh, "attestation", "verify", cached_download, "--repo",
+ described_class::HOMEBREW_CORE_REPO, "--format", "json")
+ .and_return("\"invalid json")
+
+ expect do
+ described_class.check_attestation fake_bottle,
+ described_class::HOMEBREW_CORE_REPO
+ end.to raise_error(described_class::InvalidAttestationError)
+ end
+
+ it "raises when gh returns other subjects" do
+ expect(Utils).to receive(:safe_popen_read)
+ .with(fake_gh, "attestation", "verify", cached_download, "--repo",
+ described_class::HOMEBREW_CORE_REPO, "--format", "json")
+ .and_return(fake_json_resp_wrong_sub)
+
+ expect do
+ described_class.check_attestation fake_bottle,
+ described_class::HOMEBREW_CORE_REPO
+ end.to raise_error(described_class::InvalidAttestationError)
+ end
+ end
+
+ describe "::check_core_attestation" do
+ before do
+ allow(described_class).to receive(:gh_executable)
+ .and_return(fake_gh)
+ end
+
+ it "calls gh with args for homebrew-core" do
+ expect(Utils).to receive(:safe_popen_read)
+ .with(fake_gh, "attestation", "verify", cached_download, "--repo",
+ described_class::HOMEBREW_CORE_REPO, "--format", "json", "--cert-identity",
+ described_class::HOMEBREW_CORE_CI_URI)
+ .and_return(fake_json_resp)
+
+ described_class.check_core_attestation fake_bottle
+ end
+
+ it "calls gh with args for backfill when homebrew-core fails" do
+ expect(Utils).to receive(:safe_popen_read)
+ .with(fake_gh, "attestation", "verify", cached_download, "--repo",
+ described_class::HOMEBREW_CORE_REPO, "--format", "json", "--cert-identity",
+ described_class::HOMEBREW_CORE_CI_URI)
+ .once
+ .and_raise(described_class::InvalidAttestationError)
+
+ expect(Utils).to receive(:safe_popen_read)
+ .with(fake_gh, "attestation", "verify", cached_download, "--repo",
+ described_class::BACKFILL_REPO, "--format", "json", "--cert-identity",
+ described_class::BACKFILL_REPO_CI_URI)
+ .and_return(fake_json_resp_backfill)
+
+ described_class.check_core_attestation fake_bottle
+ end
+
+ it "raises when the backfilled attestation is too new" do
+ expect(Utils).to receive(:safe_popen_read)
+ .with(fake_gh, "attestation", "verify", cached_download, "--repo",
+ described_class::HOMEBREW_CORE_REPO, "--format", "json", "--cert-identity",
+ described_class::HOMEBREW_CORE_CI_URI)
+ .once
+ .and_raise(described_class::InvalidAttestationError)
+
+ expect(Utils).to receive(:safe_popen_read)
+ .with(fake_gh, "attestation", "verify", cached_download, "--repo",
+ described_class::BACKFILL_REPO, "--format", "json", "--cert-identity",
+ described_class::BACKFILL_REPO_CI_URI)
+ .and_return(fake_json_resp_too_new)
+
+ expect do
+ described_class.check_core_attestation fake_bottle
+ end.to raise_error(described_class::InvalidAttestationError)
+ end
+ end
+end