Fix SherClockHolmes#27: Panic on large notification payloads

This commit fixes a case where, with large notification payloads, pad
would call make with a negative length, triggering a panic.

webpush.go

@@ -166,7 +166,9 @@ func SendNotification(message []byte, s *Subscription, options *Options) (*http.
 	// Pad content to max record size - 16 - header
 	// Padding ending delimeter
 	dataBuf.Write([]byte("\x02"))
-	pad(dataBuf, recordLength-recordBuf.Len())
+	if err := pad(dataBuf, recordLength-recordBuf.Len()); err != nil {
+		return nil, err
+	}
 
 	// Compose the ciphertext
 	ciphertext := gcm.Seal([]byte{}, nonce, dataBuf.Bytes(), nil)
@@ -243,10 +245,16 @@ func getHKDFKey(hkdf io.Reader, length int) ([]byte, error) {
 	return key, nil
 }
 
-func pad(payload *bytes.Buffer, maxPadLen int) {
+func pad(payload *bytes.Buffer, maxPadLen int) error {
 	payloadLen := payload.Len()
+	if payloadLen > maxPadLen {
+		return errors.New("payload is too large")
+	}
+
 	padLen := maxPadLen - payloadLen
 
 	padding := make([]byte, padLen)
 	payload.Write(padding)
+
+	return nil
 }

webpush_test.go

@@ -2,6 +2,7 @@ package webpush
 
 import (
 	"net/http"
+	"strings"
 	"testing"
 )
 
@@ -76,3 +77,17 @@ func TestSendNotificationToStandardEncodedSubscription(t *testing.T) {
 		)
 	}
 }
+
+func TestSendTooLargeNotification(t *testing.T) {
+	_, err := SendNotification([]byte(strings.Repeat("Test", int(MaxRecordSize))), getStandardEncodedTestSubscription(), &Options{
+		HTTPClient:      &testHTTPClient{},
+		Subscriber:      "<[email protected]>",
+		Topic:           "test_topic",
+		TTL:             0,
+		Urgency:         "low",
+		VAPIDPrivateKey: "testKey",
+	})
+	if err == nil {
+		t.Fatal("Expected error with too large payload")
+	}
+}