ssh: prevent malformed exit-status panic

A malicious server can crash any client by sending
a truncated exit-status payload, which triggers a
panic in binary.BigEndian.Uint32.

Change-Id: Iff4fe182b40210c99d46562f6c3b5ecf38bb98b4
Reviewed-on: https://go-review.googlesource.com/c/crypto/+/780181
LUCI-TryBot-Result: golang-scoped@luci-project-accounts.iam.gserviceaccount.com <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Nicola Murino <nicola.murino@gmail.com>
Reviewed-by: Junyang Shao <shaojunyang@google.com>
Reviewed-by: David Chase <drchase@google.com>

ssh/session.go

@@ -423,6 +423,9 @@ func (s *Session) wait(reqs <-chan *Request) error {
 	for msg := range reqs {
 		switch msg.Type {
 		case "exit-status":
+			if len(msg.Payload) < 4 {
+				return errors.New("ssh: malformed exit-status request")
+			}
 			wm.status = int(binary.BigEndian.Uint32(msg.Payload))
 		case "exit-signal":
 			var sigval struct {

ssh/session_test.go

@@ -334,6 +334,25 @@ func TestExitWithoutStatusOrSignal(t *testing.T) {
 	}
 }
 
+func TestExitStatusShortPayload(t *testing.T) {
+	for _, payload := range [][]byte{nil, {0}, {0, 0}, {0, 0, 0}} {
+		conn := dial(exitStatusShortPayloadHandler(payload), t)
+		session, err := conn.NewSession()
+		if err != nil {
+			t.Fatalf("unable to request new session: %v", err)
+		}
+		if err := session.Shell(); err != nil {
+			t.Fatalf("unable to execute command: %v", err)
+		}
+		err = session.Wait()
+		if err == nil {
+			t.Fatalf("expected error for payload length %d", len(payload))
+		}
+		session.Close()
+		conn.Close()
+	}
+}
+
 // windowTestBytes is the number of bytes that we'll send to the SSH server.
 const windowTestBytes = 16000 * 200
 
@@ -482,6 +501,17 @@ func exitWithoutSignalOrStatus(ch Channel, in <-chan *Request, t *testing.T) {
 	readLine(shell, t)
 }
 
+func exitStatusShortPayloadHandler(payload []byte) serverType {
+	return func(ch Channel, in <-chan *Request, t *testing.T) {
+		defer ch.Close()
+		shell := newServerShell(ch, in, "> ")
+		readLine(shell, t)
+		if _, err := ch.SendRequest("exit-status", false, payload); err != nil {
+			t.Errorf("unable to send status: %v", err)
+		}
+	}
+}
+
 func shellHandler(ch Channel, in <-chan *Request, t *testing.T) {
 	defer ch.Close()
 	// this string is returned to stdout