Use the correct token from the client

This fixes the case where AcceptSecContext is always called with the
first token sent by the client instead of the most recently sent one.

Fixes golang/go#43875

Author: bodgit

ssh/server.go

@@ -321,7 +321,7 @@ func checkSourceAddress(addr net.Addr, sourceAddrs string) error {
 	return fmt.Errorf("ssh: remote address %v is not allowed because of source-address restriction", addr)
 }
 
-func gssExchangeToken(gssapiConfig *GSSAPIWithMICConfig, firstToken []byte, s *connection,
+func gssExchangeToken(gssapiConfig *GSSAPIWithMICConfig, token []byte, s *connection,
 	sessionID []byte, userAuthReq userAuthRequestMsg) (authErr error, perms *Permissions, err error) {
 	gssAPIServer := gssapiConfig.Server
 	defer gssAPIServer.DeleteSecContext()
@@ -331,7 +331,7 @@ func gssExchangeToken(gssapiConfig *GSSAPIWithMICConfig, firstToken []byte, s *c
 			outToken     []byte
 			needContinue bool
 		)
-		outToken, srcName, needContinue, err = gssAPIServer.AcceptSecContext(firstToken)
+		outToken, srcName, needContinue, err = gssAPIServer.AcceptSecContext(token)
 		if err != nil {
 			return err, nil, nil
 		}
@@ -353,6 +353,7 @@ func gssExchangeToken(gssapiConfig *GSSAPIWithMICConfig, firstToken []byte, s *c
 		if err := Unmarshal(packet, userAuthGSSAPITokenReq); err != nil {
 			return nil, nil, err
 		}
+		token = userAuthGSSAPITokenReq.Token
 	}
 	packet, err := s.transport.readPacket()
 	if err != nil {