ksmbd: do not sign response to session request for guest login

If ksmbd.mountd is configured to assign unknown users to the guest account
("map to guest = bad user" in the config), ksmbd signs the response.

This is wrong according to MS-SMB2 3.3.5.5.3:
   12. If the SMB2_SESSION_FLAG_IS_GUEST bit is not set in the SessionFlags
   field, and Session.IsAnonymous is FALSE, the server MUST sign the
   final session setup response before sending it to the client, as
   follows:
    [...]

This fixes libsmb2 based applications failing to establish a session
("Wrong signature in received").

Fixes: e2f34481 ("cifsd: add server-side procedures for SMB3")
Cc: stable@vger.kernel.org
Signed-off-by: Marios Makassikis <mmakassikis@freebox.fr>
Acked-by: Namjae Jeon <linkinjeon@kernel.org>
Signed-off-by: Steve French <stfrench@microsoft.com>

fs/ksmbd/smb2pdu.c

@@ -8663,6 +8663,7 @@ int smb3_decrypt_req(struct ksmbd_work *work)
 bool smb3_11_final_sess_setup_resp(struct ksmbd_work *work)
 {
 	struct ksmbd_conn *conn = work->conn;
+	struct ksmbd_session *sess = work->sess;
 	struct smb2_hdr *rsp = smb2_get_msg(work->response_buf);
 
 	if (conn->dialect < SMB30_PROT_ID)
@@ -8672,6 +8673,7 @@ bool smb3_11_final_sess_setup_resp(struct ksmbd_work *work)
 		rsp = ksmbd_resp_buf_next(work);
 
 	if (le16_to_cpu(rsp->Command) == SMB2_SESSION_SETUP_HE &&
+	    sess->user && !user_guest(sess->user) &&
 	    rsp->Status == STATUS_SUCCESS)
 		return true;
 	return false;