terminal+subserver: validate subserver macaroons

When running in the same process, lnd will not know what to do with a
macaroon that is meant for a subserver. With this commit we instruct the
interceptor in the RPC proxy to delegate the validation of any macaroon
unknown to lnd back to the terminal instance which then passes it on to
the subserver it was meant for.

Author: guggero

terminal.go

@@ -28,6 +28,8 @@ import (
 	"github.com/lightningnetwork/lnd/signal"
 	"github.com/rakyll/statik/fs"
 	"google.golang.org/grpc"
+	"gopkg.in/macaroon-bakery.v2/bakery"
+
 	// Import generated go package that contains all static files for the
 	// UI in a compressed format.
 	_ "github.com/lightninglabs/lightning-terminal/statik"
@@ -136,7 +138,7 @@ func (g *LightningTerminal) Run() error {
 	g.cfg.frdrpcCfg = &frdrpc.Config{}
 	g.faradayServer = frdrpc.NewRPCServer(g.cfg.frdrpcCfg)
 	g.loopServer = loopd.New(g.cfg.Loop, nil)
-	g.rpcProxy = newRpcProxy(g.cfg, nil, getAllPermissions())
+	g.rpcProxy = newRpcProxy(g.cfg, g, getAllPermissions())
 
 	// Hook interceptor for os signals.
 	err = signal.Intercept()
@@ -358,6 +360,36 @@ func (g *LightningTerminal) RegisterRestSubserver(ctx context.Context,
 	)
 }
 
+// ValidateMacaroon extracts the macaroon from the context's gRPC metadata,
+// checks its signature, makes sure all specified permissions for the called
+// method are contained within and finally ensures all caveat conditions are
+// met. A non-nil error is returned if any of the checks fail.
+func (g *LightningTerminal) ValidateMacaroon(ctx context.Context,
+	requiredPermissions []bakery.Op, fullMethod string) error {
+
+	// Validate all macaroons for services that are running in the local
+	// process. Calls that we proxy to a remote host don't need to be
+	// checked as they'll have their own interceptor.
+	switch {
+	case isLoopURI(fullMethod):
+		return g.loopServer.ValidateMacaroon(
+			ctx, requiredPermissions, fullMethod,
+		)
+
+	case isFaradayURI(fullMethod):
+		return g.faradayServer.ValidateMacaroon(
+			ctx, requiredPermissions, fullMethod,
+		)
+	}
+
+	// Because lnd will spin up its own gRPC server with macaroon
+	// interceptors if it is running in this process, it will check its
+	// macaroons there. If lnd is running remotely, that process will check
+	// the macaroons. So we don't need to worry about anything other than
+	// the subservers that are running in the local process.
+	return nil
+}
+
 // shutdown stops all subservers that were started and attached to lnd.
 func (g *LightningTerminal) shutdown() error {
 	var returnErr error