policy: verify BuildKit builder images

build/opt.go

@@ -65,18 +65,6 @@ var sendGitQueryAsInput = sync.OnceValue(func() bool {
 	return false
 })
 
-// defaultPolicyEnabled reports whether the builtin default source policy is
-// enabled via the BUILDX_DEFAULT_POLICY environment variable. It is opt-in
-// for now; a future release may flip the default to on.
-var defaultPolicyEnabled = sync.OnceValue(func() bool {
-	if v, ok := os.LookupEnv("BUILDX_DEFAULT_POLICY"); ok {
-		if vv, err := strconv.ParseBool(v); err == nil {
-			return vv
-		}
-	}
-	return false
-})
-
 // policyExplicitlyDisabled reports whether the user passed `--policy
 // disabled=true`, which suppresses both user-defined and builtin default
 // policies.
@@ -664,7 +652,7 @@ func configureSourcePolicy(ctx context.Context, np *noderesolver.ResolvedNode, o
 	// (docker/dockerfile, docker/dockerfile-upstream) that may be implicitly
 	// loaded during a build, and passes through any other source so user
 	// policies retain full control.
-	if defaultPolicyEnabled() && !policyExplicitlyDisabled(opt.Policy) {
+	if policy.DefaultPolicyEnabled() && !policyExplicitlyDisabled(opt.Policy) {
 		builtin := policyOpt{
 			Files: []policyFileSpec{{
 				Filename: policy.DefaultPolicyFilename,

builder/node.go

@@ -8,15 +8,19 @@ import (
 
 	"github.com/containerd/platforms"
 	"github.com/docker/buildx/driver"
+	"github.com/docker/buildx/policy"
 	"github.com/docker/buildx/store"
 	"github.com/docker/buildx/store/storeutil"
+	"github.com/docker/buildx/util/confutil"
 	"github.com/docker/buildx/util/dockerutil"
 	"github.com/docker/buildx/util/imagetools"
 	"github.com/docker/buildx/util/platformutil"
 	"github.com/moby/buildkit/client"
 	"github.com/moby/buildkit/util/grpcerrors"
+	digest "github.com/opencontainers/go-digest"
 	ocispecs "github.com/opencontainers/image-spec/specs-go/v1"
 	"github.com/pkg/errors"
+	"github.com/sirupsen/logrus"
 	"golang.org/x/sync/errgroup"
 	"google.golang.org/grpc/codes"
 )
@@ -109,6 +113,19 @@ func (b *Builder) LoadNodes(ctx context.Context, opts ...LoadNodesOption) (_ []N
 		}
 	}
 
+	var imageVerifier driver.ImageVerifier
+	if policy.DefaultPolicyEnabled() {
+		pol := policy.DefaultPolicy(policy.Opt{
+			Log: func(_ logrus.Level, msg string) {
+				logrus.Debug(msg)
+			},
+			VerifierProvider: policy.SignatureVerifier(confutil.NewConfig(b.opts.dockerCli)),
+		})
+		imageVerifier = func(ctx context.Context, ref string, platform *ocispecs.Platform, resolver policy.SourceMetadataResolver) (digest.Digest, error) {
+			return pol.CheckSource(ctx, ref, platform, resolver)
+		}
+	}
+
 	for i, n := range b.NodeGroup.Nodes {
 		func(i int, n store.Node) {
 			eg.Go(func() error {
@@ -137,6 +154,7 @@ func (b *Builder) LoadNodes(ctx context.Context, opts ...LoadNodesOption) (_ []N
 					Files:           n.Files,
 					DriverOpts:      n.DriverOpts,
 					Auth:            imageopt.Auth,
+					ImageVerifier:   imageVerifier,
 					Platforms:       n.Platforms,
 					ContextPathHash: b.opts.contextPathHash,
 					DialMeta:        lno.dialMeta,

docs/reference/buildx.md

@@ -45,3 +45,11 @@ Extended build capabilities with BuildKit
 ### <a name="builder"></a> Override the configured builder instance (--builder)
 
 You can also use the `BUILDX_BUILDER` environment variable.
+
+### Enable the default policy
+
+Set `BUILDX_DEFAULT_POLICY=1` to enable Buildx's built-in source policy. The
+policy verifies signed tags for images managed by Docker, including BuildKit
+builder images and Dockerfile frontends. Untagged digest references and images
+outside the managed repositories are allowed unchanged. Tagged references
+that also contain a digest still have their release identity verified.

docs/reference/buildx_create.md

@@ -173,6 +173,18 @@ documentation for the specific driver:
 * [`kubernetes` driver](https://docs.docker.com/build/builders/drivers/kubernetes/)
 * [`remote` driver](https://docs.docker.com/build/builders/drivers/remote/)
 
+With `BUILDX_DEFAULT_POLICY=1`, the `docker-container` driver verifies signed
+`moby/buildkit` builder image tags before creating the builder. To explicitly
+bypass this verification, set `allow-untrusted-image=true`. For example:
+
+```console
+$ BUILDX_DEFAULT_POLICY=1 docker buildx create --driver docker-container \
+    --driver-opt allow-untrusted-image=true
+```
+
+Only use this option for an image that you trust. It disables builder image
+verification for the new builder node.
+
 ### <a name="leave"></a> Remove a node from a builder (--leave)
 
 The `--leave` flag changes the action of the command to remove a node from a

driver/bkimage/bkimage.go

@@ -1,7 +1,14 @@
 package bkimage
 
 const (
-	DefaultImage         = "moby/buildkit:buildx-stable-1" // TODO: make this verified
-	QemuImage            = "tonistiigi/binfmt:latest"      // TODO: make this verified
+	DefaultImage         = "moby/buildkit:buildx-stable-1"
+	QemuImage            = "tonistiigi/binfmt:latest" // TODO: make this verified
 	DefaultRootlessImage = DefaultImage + "-rootless"
+
+	// TrustedRepo is the fully-qualified repository whose tags are verified
+	// against the builtin default policy before a builder is created from
+	// them. Images from other repositories pass through unverified; the
+	// allow-untrusted-image driver-opt is only needed when a TrustedRepo tag
+	// that the policy covers does not verify correctly.
+	TrustedRepo = "docker.io/moby/buildkit"
 )

driver/docker-container/driver.go

@@ -13,12 +13,15 @@ import (
 	"time"
 
 	cerrdefs "github.com/containerd/errdefs"
+	"github.com/containerd/platforms"
+	"github.com/distribution/reference"
 	"github.com/docker/buildx/driver"
 	"github.com/docker/buildx/driver/bkimage"
 	"github.com/docker/buildx/util/confutil"
 	"github.com/docker/buildx/util/ghutil"
 	"github.com/docker/buildx/util/imagetools"
 	"github.com/docker/buildx/util/progress"
+	"github.com/docker/buildx/util/sourcemeta"
 	"github.com/docker/cli/cli/context/docker"
 	contextstore "github.com/docker/cli/cli/context/store"
 	"github.com/docker/cli/opts"
@@ -29,6 +32,7 @@ import (
 	"github.com/moby/moby/api/types/mount"
 	dockerclient "github.com/moby/moby/client"
 	"github.com/moby/moby/client/pkg/security"
+	ocispecs "github.com/opencontainers/image-spec/specs-go/v1"
 	"github.com/pkg/errors"
 )
 
@@ -43,21 +47,22 @@ type Driver struct {
 
 	// if you add fields, remember to update docs:
 	// https://github.com/docker/docs/blob/main/content/build/drivers/docker-container.md
-	netMode            string
-	image              string
-	memory             opts.MemBytes
-	memorySwap         opts.MemSwapBytes
-	cpuQuota           int64
-	cpuPeriod          int64
-	cpuShares          int64
-	cpusetCpus         string
-	cpusetMems         string
-	cgroupParent       string
-	restartPolicy      container.RestartPolicy
-	env                []string
-	defaultLoad        bool
-	gpus               []container.DeviceRequest
-	writeProvenanceGHA bool
+	netMode             string
+	image               string
+	allowUntrustedImage bool
+	memory              opts.MemBytes
+	memorySwap          opts.MemSwapBytes
+	cpuQuota            int64
+	cpuPeriod           int64
+	cpuShares           int64
+	cpusetCpus          string
+	cpusetMems          string
+	cgroupParent        string
+	restartPolicy       container.RestartPolicy
+	env                 []string
+	defaultLoad         bool
+	gpus                []container.DeviceRequest
+	writeProvenanceGHA  bool
 }
 
 func (d *Driver) IsMobyDriver() bool {
@@ -92,27 +97,59 @@ func (d *Driver) create(ctx context.Context, l progress.SubLogger) error {
 		imageName = d.image
 	}
 
-	if err := l.Wrap("pulling image "+imageName, func() error {
-		ra, err := imagetools.RegistryAuthForRef(imageName, d.Auth)
+	imageRef := imageName
+	pullErr := l.Wrap("pulling image "+imageRef, func() error {
+		ra, err := imagetools.RegistryAuthForRef(imageRef, d.Auth)
 		if err != nil {
 			return err
 		}
-		resp, err := d.DockerAPI.ImagePull(ctx, imageName, dockerclient.ImagePullOptions{
+		resp, err := d.DockerAPI.ImagePull(ctx, imageRef, dockerclient.ImagePullOptions{
 			RegistryAuth: ra,
 		})
 		if err != nil {
 			return err
 		}
 		return resp.Wait(ctx)
-	}); err != nil {
-		// image pulling failed, check if it exists in local image store.
-		// if not, return pulling error. otherwise log it.
-		_, errInspect := d.DockerAPI.ImageInspect(ctx, imageName)
-		found := errInspect == nil
-		if !found {
+	})
+	image, inspectErr := d.DockerAPI.ImageInspect(ctx, imageRef)
+	if inspectErr != nil {
+		if pullErr != nil {
+			return pullErr
+		}
+		return errors.Wrapf(inspectErr, "failed to inspect pulled image %s", imageRef)
+	}
+
+	imageName = imageRef
+	if image.Descriptor != nil && d.ImageVerifier != nil && !d.allowUntrustedImage {
+		named, err := reference.ParseNormalizedNamed(imageRef)
+		if err != nil {
+			return errors.Wrapf(err, "failed to parse image reference %s", imageRef)
+		}
+		if named.Name() == bkimage.TrustedRepo {
+			if _, canonical := named.(reference.Canonical); !canonical {
+				named = reference.TagNameOnly(named)
+			}
+			pinned, err := reference.WithDigest(named, image.Descriptor.Digest)
+			if err != nil {
+				return errors.Wrapf(err, "failed to construct image reference for %s", imageRef)
+			}
+			imageName = pinned.String()
+		}
+	}
+
+	// Policy verification requires the immutable descriptor exposed by the
+	// containerd image store. Classic-store images are allowed without it.
+	if image.Descriptor != nil {
+		var err error
+		imageName, err = d.verifiedImageRef(ctx, l, imageName)
+		if err != nil {
+			return err
+		}
+	}
+	if pullErr != nil {
+		if err := l.Wrap("using local image "+imageName, func() error { return nil }); err != nil {
 			return err
 		}
-		l.Wrap("pulling failed, using local image "+imageName, func() error { return nil })
 	}
 
 	cfg := &container.Config{
@@ -229,6 +266,65 @@ func (d *Driver) create(ctx context.Context, l progress.SubLogger) error {
 	})
 }
 
+// verifiedImageRef evaluates ref against the builtin default policy and
+// returns the canonical reference carrying the digest that verification
+// resolved. Source metadata is resolved through the BuildKit embedded in the
+// Docker daemon that hosts the builder, so registry access follows the
+// daemon configuration. The reference is returned unchanged when the policy
+// does not apply to it: policy disabled, allow-untrusted-image set, the
+// image pinned by digest without a tag, or an image outside the managed
+// moby/buildkit repository (which the default policy passes through).
+func (d *Driver) verifiedImageRef(ctx context.Context, l progress.SubLogger, ref string) (string, error) {
+	if d.ImageVerifier == nil || d.allowUntrustedImage {
+		return ref, nil
+	}
+
+	c, err := d.buildkitClient(ctx)
+	if err != nil {
+		return "", errors.Wrap(err, "failed to connect to BuildKit for image verification")
+	}
+	defer c.Close()
+	mr := sourcemeta.NewResolver(c)
+	defer mr.Close()
+
+	pinned, applied, err := driver.VerifyImageRef(ctx, l, ref, d.daemonPlatform(ctx), mr, d.ImageVerifier)
+	if err != nil {
+		if !applied {
+			return "", err
+		}
+		return "", errors.Wrapf(err, "failed to verify image %s", ref)
+	}
+	if !applied {
+		return ref, nil
+	}
+	return pinned, nil
+}
+
+// buildkitClient returns a client to the BuildKit embedded in the Docker
+// daemon that hosts the builder container.
+func (d *Driver) buildkitClient(ctx context.Context) (*client.Client, error) {
+	return client.New(ctx, "",
+		client.WithContextDialer(func(context.Context, string) (net.Conn, error) {
+			return d.DockerAPI.DialHijack(ctx, "/grpc", "h2c", d.DialMeta)
+		}),
+		client.WithSessionDialer(func(ctx context.Context, proto string, meta map[string][]string) (net.Conn, error) {
+			return d.DockerAPI.DialHijack(ctx, "/session", proto, meta)
+		}),
+	)
+}
+
+// daemonPlatform returns the platform of the images the daemon pulls,
+// falling back to the client platform when daemon info is unavailable.
+func (d *Driver) daemonPlatform(ctx context.Context) *ocispecs.Platform {
+	if resp, err := d.DockerAPI.Info(ctx, dockerclient.InfoOptions{}); err == nil {
+		if p, err := platforms.Parse(resp.Info.OSType + "/" + resp.Info.Architecture); err == nil {
+			return &p
+		}
+	}
+	p := platforms.Normalize(platforms.DefaultSpec())
+	return &p
+}
+
 func (d *Driver) wait(ctx context.Context, l progress.SubLogger) error {
 	try := 1
 	for {

driver/docker-container/factory.go

@@ -117,6 +117,11 @@ func (f *factory) New(ctx context.Context, cfg driver.InitConfig) (driver.Driver
 			if err != nil {
 				return nil, err
 			}
+		case k == "allow-untrusted-image":
+			d.allowUntrustedImage, err = strconv.ParseBool(v)
+			if err != nil {
+				return nil, err
+			}
 		default:
 			return nil, errors.Errorf("invalid driver option %s for docker-container driver", k)
 		}

driver/image.go

@@ -0,0 +1,54 @@
+package driver
+
+import (
+	"context"
+
+	"github.com/distribution/reference"
+	"github.com/docker/buildx/driver/bkimage"
+	"github.com/docker/buildx/policy"
+	"github.com/docker/buildx/util/progress"
+	digest "github.com/opencontainers/go-digest"
+	ocispecs "github.com/opencontainers/image-spec/specs-go/v1"
+	"github.com/pkg/errors"
+)
+
+// VerifyImageRef evaluates ref against the builtin default policy through
+// verify and returns the reference pinned to the digest that verification
+// resolved. The boolean result reports whether the policy applied: it is
+// false, with ref returned unchanged, when there is no verifier, when ref is
+// pinned by digest without a tag, or when ref is outside the managed
+// moby/buildkit repository (unmanaged images pass through the default policy
+// unchanged). A tagged canonical reference is still verified because its tag
+// carries the release identity checked by the policy.
+func VerifyImageRef(ctx context.Context, l progress.SubLogger, ref string, platform *ocispecs.Platform, resolver policy.SourceMetadataResolver, verify ImageVerifier) (string, bool, error) {
+	if verify == nil {
+		return ref, false, nil
+	}
+	named, err := reference.ParseNormalizedNamed(ref)
+	if err != nil {
+		return "", false, errors.Wrapf(err, "failed to parse image reference %s", ref)
+	}
+	_, isCanonical := named.(reference.Canonical)
+	_, isTagged := named.(reference.Tagged)
+	if isCanonical && !isTagged {
+		return ref, false, nil
+	}
+	if named.Name() != bkimage.TrustedRepo {
+		return ref, false, nil
+	}
+	named = reference.TagNameOnly(named)
+
+	var dgst digest.Digest
+	if err := l.Wrap("verifying image "+named.String(), func() error {
+		var err error
+		dgst, err = verify(ctx, named.String(), platform, resolver)
+		return err
+	}); err != nil {
+		return "", true, err
+	}
+	canonical, err := reference.WithDigest(named, dgst)
+	if err != nil {
+		return "", true, errors.Wrapf(err, "failed to construct canonical reference for %s", ref)
+	}
+	return canonical.String(), true, nil
+}