remotecache: honor gha cache transparency log thresholds

Pass explicit cache log thresholds to policy-helpers during signature
verification. Preserve the distinction between an omitted threshold and
zero. Omission uses the bundle-derived default, while zero skips log
verification and requires a verified TSA timestamp.

Remove the local log threshold check, which counted log timestamps rather
than verified logs and could reject valid inclusion-proof-only entries.
Keep timestamp and certificate policy checks in place.

Signed-off-by: CrazyMax <1951866+crazy-max@users.noreply.github.com>

cache/remotecache/gha/gha.go

@@ -364,7 +364,12 @@ func verifySignature(ctx context.Context, dgst digest.Digest, bundle []byte, con
 		return errors.New("no verifier available for signed github actions cache")
 	}
 
-	sig, err := v.VerifyArtifact(ctx, dgst, bundle, policy.WithSLSANotRequired())
+	policyRules := config.Verify.Policy
+	opts := []policy.ArtifactVerifyOpt{policy.WithSLSANotRequired()}
+	if policyRules.TlogThreshold != nil {
+		opts = append(opts, policy.WithTlogThreshold(*policyRules.TlogThreshold))
+	}
+	sig, err := v.VerifyArtifact(ctx, dgst, bundle, opts...)
 	if err != nil {
 		return err
 	}
@@ -372,19 +377,9 @@ func verifySignature(ctx context.Context, dgst digest.Digest, bundle []byte, con
 		return errors.New("signature verification failed: no signer found")
 	}
 	numTimestamps := len(sig.Timestamps)
-	numTlog := 0
-	for _, t := range sig.Timestamps {
-		if t.Type == "Tlog" {
-			numTlog++
-		}
-	}
-	policyRules := config.Verify.Policy
 	if policyRules.TimestampThreshold > numTimestamps {
 		return errors.Errorf("signature verification failed: not enough timestamp authorities: have %d, need %d", numTimestamps, policyRules.TimestampThreshold)
 	}
-	if policyRules.TlogThreshold > numTlog {
-		return errors.Errorf("signature verification failed: not enough tlog authorities: have %d, need %d", numTlog, policyRules.TlogThreshold)
-	}
 
 	certRules, err := certToStringMap(&config.Verify.Policy.Summary)
 	if err != nil {

cache/remotecache/gha/gha_test.go

@@ -9,13 +9,33 @@ import (
 	"testing"
 	"time"
 
+	"github.com/moby/buildkit/cache/remotecache/gha/ghatypes"
 	"github.com/moby/buildkit/client"
 	"github.com/moby/buildkit/client/llb"
 	"github.com/moby/buildkit/util/testutil/integration"
 	"github.com/moby/buildkit/util/testutil/workers"
+	policy "github.com/moby/policy-helpers"
+	digest "github.com/opencontainers/go-digest"
 	"github.com/stretchr/testify/require"
 )
 
+func TestVerifySignatureNegativeTlogThreshold(t *testing.T) {
+	config := &Config{
+		CacheConfig: &ghatypes.CacheConfig{
+			Verify: ghatypes.VerifyConfig{
+				Policy: ghatypes.VerifyPolicy{
+					TlogThreshold: new(-1),
+				},
+			},
+		},
+		verifier: func() (*policy.Verifier, error) {
+			return &policy.Verifier{}, nil
+		},
+	}
+	err := verifySignature(t.Context(), digest.FromString("cache index"), nil, config)
+	require.EqualError(t, err, "transparency log threshold must not be negative")
+}
+
 func init() {
 	if workers.IsTestDockerd() {
 		workers.InitDockerdWorker()

cache/remotecache/gha/ghatypes/config.go

@@ -17,7 +17,7 @@ type VerifyConfig struct {
 }
 
 type VerifyPolicy struct {
-	TimestampThreshold int `toml:"timestampThreshold"`
-	TlogThreshold      int `toml:"tlogThreshold"`
+	TimestampThreshold int  `toml:"timestampThreshold"`
+	TlogThreshold      *int `toml:"tlogThreshold"`
 	certificate.Summary
 }

docs/buildkitd.toml.md

@@ -267,6 +267,8 @@ provenanceEnvDir = "/etc/buildkit/provenance.d"
 #
 # [cache.gha.verify.policy]
 #   timestampThreshold = 1
+#   # If omitted, require all distinct transparency logs present in the bundle.
+#   # Zero skips log verification and requires a verified TSA timestamp instead.
 #   tlogThreshold = 1
 #   # certificate properties that need to match. Simple wildcards (*) are supported.
 #   certificateIssuer = ""