Slightly modified from this patch in order to apply against SDDM master git:
https://github.com/AOSC-Dev/sddm/commit/c6be72636f43a28f1b6aebff9d682d0182dd7a1e.patch

From c6be72636f43a28f1b6aebff9d682d0182dd7a1e Mon Sep 17 00:00:00 2001
From: Leslie Zhai <xiangzhai83@gmail.com>
Date: Mon, 31 Aug 2015 14:04:17 +0800
Subject: [PATCH] Disable pam backend because loginFailed

---
diff -uar sddm-d42700a_20150822git.orig/CMakeLists.txt sddm-d42700a_20150822git/CMakeLists.txt
--- sddm-d42700a_20150822git.orig/CMakeLists.txt	2015-08-22 16:17:37.000000000 +0200
+++ sddm-d42700a_20150822git/CMakeLists.txt	2015-08-31 12:01:47.342810382 +0200
@@ -31,6 +31,7 @@
 # Options
 option(BUILD_MAN_PAGES "Build man pages" OFF)
 option(ENABLE_JOURNALD "Enable logging to journald" ON)
+option(ENABLE_PAM "Enable pam" OFF)
 
 # Definitions
 add_definitions(-Wall -std=c++11 -DQT_NO_CAST_FROM_ASCII)
@@ -66,7 +67,7 @@
 find_package(PkgConfig)
 
 # PAM
-if(NOT NO_PAM)
+if(ENABLE_PAM)
     find_package(PAM)
 
     if(PAM_FOUND)
diff -uar sddm-d42700a_20150822git.orig/src/helper/backend/PasswdBackend.cpp sddm-d42700a_20150822git/src/helper/backend/PasswdBackend.cpp
--- sddm-d42700a_20150822git.orig/src/helper/backend/PasswdBackend.cpp	2015-08-22 16:17:37.000000000 +0200
+++ sddm-d42700a_20150822git/src/helper/backend/PasswdBackend.cpp	2015-08-31 12:00:43.995324823 +0200
@@ -38,7 +38,7 @@
         if (m_autologin)
             return true;
 
-        if (m_user == "sddm") {
+        if (m_user == QStringLiteral("sddm")) {
             if (m_greeter)
                 return true;
             else
@@ -49,17 +49,17 @@
         QString password;
 
         if (m_user.isEmpty())
-            r.prompts << Prompt(AuthPrompt::LOGIN_USER, "Login", false);
-        r.prompts << Prompt(AuthPrompt::LOGIN_PASSWORD, "Password", true);
+            r.prompts << Prompt(AuthPrompt::LOGIN_USER, QStringLiteral("Login"), false);
+        r.prompts << Prompt(AuthPrompt::LOGIN_PASSWORD, QStringLiteral("Password"), true);
 
         Request response = m_app->request(r);
         Q_FOREACH(const Prompt &p, response.prompts) {
             switch (p.type) {
                 case AuthPrompt::LOGIN_USER:
-                    m_user = p.response;
+                    m_user = QString::fromUtf8(p.response);
                     break;
                 case AuthPrompt::LOGIN_PASSWORD:
-                    password = p.response;
+                    password = QString::fromUtf8(p.response);
                     break;
                 default:
                     break;
@@ -68,13 +68,13 @@
 
         struct passwd *pw = getpwnam(qPrintable(m_user));
         if (!pw) {
-            m_app->error(QString("Wrong user/password combination"), Auth::ERROR_AUTHENTICATION);
+            m_app->error(QStringLiteral("Wrong user/password combination"), Auth::ERROR_AUTHENTICATION);
             return false;
         }
 
         struct spwd *spw = getspnam(pw->pw_name);
         if (!spw) {
-            qWarning() << "[Passwd] Could get passwd but not shadow";
+            qWarning() << QStringLiteral("[Passwd] Could get passwd but not shadow");
             return false;
         }
 
@@ -86,7 +86,7 @@
             return true;
         }
 
-        m_app->error(QString("Wrong user/password combination"), Auth::ERROR_AUTHENTICATION);
+        m_app->error(QStringLiteral("Wrong user/password combination"), Auth::ERROR_AUTHENTICATION);
         return false;
     }