handle backfilled attestation subjects correctly

Signed-off-by: William Woodruff <william@yossarian.net>

Library/Homebrew/attestation.rb

@@ -63,9 +63,9 @@ module Homebrew
     # @api private
     sig {
       params(bottle: Bottle, signing_repo: String,
-             signing_workflow: T.nilable(String)).returns(T::Hash[T.untyped, T.untyped])
+             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)
+    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"]
 
@@ -86,9 +86,9 @@ module Homebrew
       # `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.
-      bottle_name = bottle.filename.to_s
+      subject = bottle.filename.to_s if subject.blank?
       attestation = attestations.find do |a|
-        a.dig("verificationResult", "statement", "subject", 0, "name") == bottle_name
+        a.dig("verificationResult", "statement", "subject", 0, "name") == subject
       end
 
       raise InvalidAttestationError, "no attestation matches subject" if attestation.blank?
@@ -112,7 +112,16 @@ module Homebrew
         return attestation
       rescue InvalidAttestationError
         odebug "falling back on backfilled attestation for #{bottle}"
-        backfill_attestation = check_attestation bottle, BACKFILL_REPO, BACKFILL_REPO_CI_URI
+
+        # 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")
 

Library/Homebrew/test/attestation_spec.rb

@@ -6,7 +6,10 @@ 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) { instance_double(Bottle, cached_download:, filename: fake_bottle_filename) }
+  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: {
@@ -15,6 +18,16 @@ RSpec.describe Homebrew::Attestation do
       } },
     ])
   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: {
@@ -113,7 +126,7 @@ RSpec.describe Homebrew::Attestation do
         .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)
+        .and_return(fake_json_resp_backfill)
 
       described_class.check_core_attestation fake_bottle
     end