ssh: allow server auth callbacks to send additional banners

Add a new BannerError error type that auth callbacks can return to send
banner to the client. While the BannerCallback can send the initial
banner message, auth callbacks might want to communicate more
information to the client to help them diagnose failures.

Updates golang/go#64962

Change-Id: I97a26480ff4064b95a0a26042b0a5e19737cfb62
Reviewed-on: https://go-review.googlesource.com/c/crypto/+/558695
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Roland Shoemaker <roland@golang.org>
Reviewed-by: Nicola Murino <nicola.murino@gmail.com>
Auto-Submit: Nicola Murino <nicola.murino@gmail.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>

ssh/server.go

@@ -462,6 +462,24 @@ func (p *PartialSuccessError) Error() string {
 // It is returned in ServerAuthError.Errors from NewServerConn.
 var ErrNoAuth = errors.New("ssh: no auth passed yet")
 
+// BannerError is an error that can be returned by authentication handlers in
+// ServerConfig to send a banner message to the client.
+type BannerError struct {
+	Err     error
+	Message string
+}
+
+func (b *BannerError) Unwrap() error {
+	return b.Err
+}
+
+func (b *BannerError) Error() string {
+	if b.Err == nil {
+		return b.Message
+	}
+	return b.Err.Error()
+}
+
 func (s *connection) serverAuthenticate(config *ServerConfig) (*Permissions, error) {
 	sessionID := s.transport.getSessionID()
 	var cache pubKeyCache
@@ -734,6 +752,18 @@ userAuthLoop:
 			config.AuthLogCallback(s, userAuthReq.Method, authErr)
 		}
 
+		var bannerErr *BannerError
+		if errors.As(authErr, &bannerErr) {
+			if bannerErr.Message != "" {
+				bannerMsg := &userAuthBannerMsg{
+					Message: bannerErr.Message,
+				}
+				if err := s.transport.writePacket(Marshal(bannerMsg)); err != nil {
+					return nil, err
+				}
+			}
+		}
+
 		if authErr == nil {
 			break userAuthLoop
 		}