Fix segfault in host-ctr

Bottlerocket invokes the host-ctr executable with an optional
"--registry-config" parameter. If this parameter was omitted (for
instance, in customer code), host-ctr would segfault. Now it will
behave as it would for an empty registry-config file.

sources/host-ctr/cmd/host-ctr/main.go

@@ -1033,6 +1033,8 @@ func pullImage(ctx context.Context, source string, client *containerd.Client, re
 				Error("failed to read registry config")
 			return nil, err
 		}
+	} else {
+		registryConfig = NewBlankRegistryConfig()
 	}
 
 	// Pull the image
@@ -1152,9 +1154,12 @@ func withDynamicResolver(ctx context.Context, ref string, registryConfig *Regist
 		}
 	// For Amazon ECR Public registries, we should try and fetch credentials before resolving the image reference
 	case strings.HasPrefix(ref, "public.ecr.aws/"):
-		// ... not if the user has specified their own registry credentials for 'public.ecr.aws'; In that case we use the default resolver.
-		if _, found := registryConfig.Credentials["public.ecr.aws"]; found {
-			return defaultResolver
+		// if we have registryConfig, and the user specified credentials for
+		// 'public.ecr.aws', then use defaultResolver.
+		if registryConfig != nil {
+			if _, found := registryConfig.Credentials["public.ecr.aws"]; found {
+				return defaultResolver
+			}
 		}
 
 		// Try to get credentials for authenticated pulls from ECR Public

sources/host-ctr/cmd/host-ctr/registry.go

@@ -45,6 +45,13 @@ func NewRegistryConfig(registryConfigFile string) (*RegistryConfig, error) {
 	return &config, toml.Unmarshal(raw, &config)
 }
 
+// NewBlankRegistryConfig sets up a RegistryConfig without unmarshalling any toml
+func NewBlankRegistryConfig() *RegistryConfig {
+	config := RegistryConfig{}
+	// no config to unmarshal
+	return &config
+}
+
 // registryHosts returns the registry hosts to be used by the resolver.
 // Heavily borrowed from containerd CRI plugin's implementation.
 // See https://github.com/containerd/containerd/blob/1407cab509ff0d96baa4f0eb6ff9980270e6e620/pkg/cri/server/image_pull.go#L332-L405
@@ -58,15 +65,19 @@ func registryHosts(registryConfig *RegistryConfig, authorizerOverride *docker.Au
 			authConfig runtime.AuthConfig
 		)
 		// Set up endpoints for the registry
-		if _, ok := registryConfig.Mirrors[host]; ok {
-			endpoints = registryConfig.Mirrors[host].Endpoints
-		} else {
-			endpoints = registryConfig.Mirrors["*"].Endpoints
+		// The endpoints array may still be empty after this step
+		if registryConfig != nil {
+			if _, ok := registryConfig.Mirrors[host]; ok {
+				endpoints = registryConfig.Mirrors[host].Endpoints
+			} else if _, ok := registryConfig.Mirrors["*"]; ok {
+				endpoints = registryConfig.Mirrors["*"].Endpoints
+			}
 		}
 		defaultHost, err := docker.DefaultHost(host)
 		if err != nil {
 			return nil, errors.Wrap(err, "get default host")
 		}
+		// append this default host to the possibly-empty endpoints array
 		endpoints = append(endpoints, defaultHost)
 
 		for _, endpoint := range endpoints {