pull: Validate layer digest format

Otherwise a malformed or empty digest may cause a panic.

Signed-off-by: Brian Goff <cpuguy83@gmail.com>

builder/builder-next/adapters/containerimage/pull.go

@@ -524,6 +524,9 @@ func (p *puller) Snapshot(ctx context.Context, g session.Group) (cache.Immutable
 	layers := make([]xfer.DownloadDescriptor, 0, len(mfst.Layers))
 
 	for i, desc := range mfst.Layers {
+		if err := desc.Digest.Validate(); err != nil {
+			return nil, errors.Wrap(err, "layer digest could not be validated")
+		}
 		ongoing.add(desc)
 		layers = append(layers, &layerDescriptor{
 			desc:    desc,

distribution/pull_v2.go

@@ -528,6 +528,9 @@ func (p *v2Puller) pullSchema1(ctx context.Context, ref reference.Reference, unv
 	// to top-most, so that the downloads slice gets ordered correctly.
 	for i := len(verifiedManifest.FSLayers) - 1; i >= 0; i-- {
 		blobSum := verifiedManifest.FSLayers[i].BlobSum
+		if err = blobSum.Validate(); err != nil {
+			return "", "", errors.Wrapf(err, "could not validate layer digest %q", blobSum)
+		}
 
 		var throwAway struct {
 			ThrowAway bool `json:"throwaway,omitempty"`
@@ -626,6 +629,9 @@ func (p *v2Puller) pullSchema2Layers(ctx context.Context, target distribution.De
 	// Note that the order of this loop is in the direction of bottom-most
 	// to top-most, so that the downloads slice gets ordered correctly.
 	for _, d := range layers {
+		if err := d.Digest.Validate(); err != nil {
+			return "", errors.Wrapf(err, "could not validate layer digest %q", d.Digest)
+		}
 		layerDescriptor := &v2LayerDescriptor{
 			digest:            d.Digest,
 			repo:              p.repo,