fix: sign release checksums with a sigstore bundle (#5362)

* fix: sign release checksums with a sigstore bundle

The cosign update rejects the legacy `--output-certificate` / `--output-signature` flags alongside the new bundle format, which broke signing during the v1.53.0 release.

- checksums are now signed into a single `checksums.txt.sigstore.json` bundle (signature, certificate, and tlog proof)
- `install.sh -v` verifies with `--bundle` for releases >= v1.53.0 and keeps the `.sig` / `.pem` flow for older releases

Signed-off-by: Alex Goodman <wagoodman@users.noreply.github.com>

* chore: smooth the cutover to sigstore bundle verification

- `install.sh -v` now hints that bundle verification needs cosign v2.5.0 or newer when verification of a bundle-signed release fails
- install tests keep exercising the legacy `.sig` / `.pem` path against v1.52.0 once the latest release is bundle-signed
- touch the install test environment Dockerfiles so the cached images rebuild with a current cosign

Signed-off-by: Alex Goodman <wagoodman@users.noreply.github.com>

* fix: only hint at the cosign minimum when cosign is actually too old

- `install.sh -v` used to print the cosign v2.5.0 note on any failed bundle verification, including a missing bundle or a bad signature. It now only prints it when cosign reports it couldn't read the bundle (`bundle does not contain cert`), which is how v2.2.4 through v2.4.1 fail on a sigstore bundle.
- `verify_sign` is now covered with a stub cosign, so the bundle path is exercised before any bundle-signed release is latest

Signed-off-by: Alex Goodman <wagoodman@users.noreply.github.com>

* fix: stop the install when a release asset is missing or fails its checksum

- `install.sh` now stops when the checksums file, the signature bundle, or the asset can't be downloaded, instead of handing an empty or error-page file to the next step
- a sha256 mismatch between the asset and the checksums file now stops the install. Before this the mismatch was logged and the asset was installed anyway, so `-v` only ever vouched for `checksums.txt`, not the binary

Signed-off-by: Alex Goodman <wagoodman@users.noreply.github.com>

* chore: pin cosign in the install test images

The install test images pulled whatever cosign was latest, unverified. They now install cosign v3.1.3 checked against its published sha256, so the install tests always run with a cosign that can verify sigstore bundles (releases >= v1.53.0 need v2.5.0 or newer).

Signed-off-by: Alex Goodman <wagoodman@users.noreply.github.com>

---------

Signed-off-by: Alex Goodman <wagoodman@users.noreply.github.com>
Co-authored-by: Keith Zantow <kzantow@gmail.com>

install.sh

@@ -19,6 +19,8 @@ VERIFY_SIGN=false
 VERIFY_SIGN_SUPPORTED_VERSION=v0.104.0
 # this is the earliest tag in the repo where the -v flag was introduced to this install.sh script
 VERIFY_SIGN_FLAG_VERSION=v1.6.0
+# releases at or after this version are signed with a sigstore bundle (.sigstore.json); earlier releases ship a separate .sig and .pem
+VERIFY_SIGN_BUNDLE_VERSION=v1.53.0
 
 # do not change the name of this parameter (this must always be backwards compatible)
 DOWNLOAD_TAG_INSTALL_SCRIPT=${DOWNLOAD_TAG_INSTALL_SCRIPT:-true}
@@ -386,8 +388,8 @@ download_github_release_checksums_files() (
   complete_url=$(github_release_asset_url "${download_url}" "${name}" "${version}" "${filename}")
   output_path="${output_dir}/${complete_filename}"
 
-  http_download "${output_path}" "${complete_url}" ""
-  asset_file_exists "${output_path}"
+  http_download "${output_path}" "${complete_url}" "" || return 1
+  asset_file_exists "${output_path}" || return 1
 
   log_trace "download_github_release_checksums_files() returned '${output_path}' for file '${complete_filename}'"
 
@@ -585,28 +587,31 @@ download_and_install_asset() (
   install_asset "${asset_filepath}" "${install_path}" "${binary}"
 )
 
-# verify_sign [checksums-file-path] [certificate-reference] [signature-reference] [version]
+# verify_sign [checksums-file-path] [verification-material-flags...]
 #
 # attempts verify the signature of the checksums file from the release workflow in Github Actions run against the main branch.
+# the verification material is either "--bundle <path>" or (for older releases) "--certificate <ref> --signature <ref>".
 #
 verify_sign() {
   checksums_file=$1
-  cert_reference=$2
-  sig_reference=$3
+  shift
 
-  log_trace "verifying artifact $1"
+  log_trace "verifying artifact ${checksums_file}"
 
   log_file=$(mktemp)
 
   ${COSIGN_BINARY} \
     verify-blob "$checksums_file" \
-      --certificate "$cert_reference" \
-      --signature "$sig_reference" \
+      "$@" \
       --certificate-identity "https://github.com/${OWNER}/${REPO}/.github/workflows/release.yaml@refs/heads/main" \
       --certificate-oidc-issuer "https://token.actions.githubusercontent.com" > "${log_file}" 2>&1
 
   if [ $? -ne 0 ]; then
     log_err "$(cat "${log_file}")"
+    # cosign older than v2.4.2 can't read the bundle's verification material and reports a missing cert
+    if [ "$1" = "--bundle" ] && grep -q "bundle does not contain cert" "${log_file}"; then
+      log_err "note: releases >= ${VERIFY_SIGN_BUNDLE_VERSION} are verified with a sigstore bundle, which requires cosign v2.5.0 or newer (check '${COSIGN_BINARY} version')"
+    fi
     rm -f "${log_file}"
     return 1
   fi
@@ -630,7 +635,10 @@ download_asset() (
 
   log_trace "download_asset(url=${download_url}, destination=${destination}, name=${name}, os=${os}, arch=${arch}, version=${version}, format=${format})"
 
-  checksums_filepath=$(download_github_release_checksums "${download_url}" "${name}" "${version}" "${destination}")
+  if ! checksums_filepath=$(download_github_release_checksums "${download_url}" "${name}" "${version}" "${destination}"); then
+    log_err "unable to download checksums file"
+    return 1
+  fi
 
   log_trace "checksums content:\n$(cat ${checksums_filepath})"
 
@@ -642,13 +650,25 @@ download_asset() (
   fi
 
   if [ "$VERIFY_SIGN" = true ]; then
-    checksum_sig_file_url=$(github_release_checksums_sig_url "${download_url}" "${name}" "${version}")
-    log_trace "checksums signature url: ${checksum_sig_file_url}"
+    if compare_semver "${version}" "${VERIFY_SIGN_BUNDLE_VERSION}"; then
+      if ! checksums_bundle_filepath=$(download_github_release_checksums_files "${download_url}" "${name}" "${version}" "${destination}" "checksums.txt.sigstore.json"); then
+        log_err "unable to download checksums signature bundle"
+        return 1
+      fi
+      log_trace "checksums bundle: ${checksums_bundle_filepath}"
 
-    checksums_cert_file_url=$(github_release_checksums_cert_url "${download_url}" "${name}" "${version}")
-    log_trace "checksums certificate url: ${checksums_cert_file_url}"
+      set -- --bundle "${checksums_bundle_filepath}"
+    else
+      checksum_sig_file_url=$(github_release_checksums_sig_url "${download_url}" "${name}" "${version}")
+      log_trace "checksums signature url: ${checksum_sig_file_url}"
 
-    if ! verify_sign "${checksums_filepath}" "${checksums_cert_file_url}" "${checksum_sig_file_url}"; then
+      checksums_cert_file_url=$(github_release_checksums_cert_url "${download_url}" "${name}" "${version}")
+      log_trace "checksums certificate url: ${checksums_cert_file_url}"
+
+      set -- --certificate "${checksums_cert_file_url}" --signature "${checksum_sig_file_url}"
+    fi
+
+    if ! verify_sign "${checksums_filepath}" "$@"; then
       log_err "signature verification failed"
       return 1
     fi
@@ -657,9 +677,15 @@ download_asset() (
 
   asset_url="${download_url}/${asset_filename}"
   asset_filepath="${destination}/${asset_filename}"
-  http_download "${asset_filepath}" "${asset_url}" ""
+  if ! http_download "${asset_filepath}" "${asset_url}" ""; then
+    log_err "unable to download asset '${asset_url}'"
+    return 1
+  fi
 
-  hash_sha256_verify "${asset_filepath}" "${checksums_filepath}"
+  # this is what ties the asset to the (optionally signature verified) checksums file, so a mismatch must stop the install
+  if ! hash_sha256_verify "${asset_filepath}" "${checksums_filepath}"; then
+    return 1
+  fi
 
   log_trace "download_asset_by_checksums_file() returned '${asset_filepath}'"