File 0001-Redesign-Xauth-handling.patch of Package sddm

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File 0001-Redesign-Xauth-handling.patch of Package sddm (Revision 57f56f8cb81e7a2128ee3b2861934f68)

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From fbdf20d59d1c63cd2b8fd78efb3125478a2ea07c Mon Sep 17 00:00:00 2001
From: Fabian Vogt <fabian@ritter-vogt.de>
Date: Wed, 21 Aug 2019 16:32:03 +0200
Subject: [PATCH] Redesign Xauth handling

This commit moves Xauthority handling over to libXau.
Advantage is that this allows use of FamilyWild, is faster, more reliable
and easier to read. However, we lose the ability to merge the new cookie into
an existing Xauthority file, so support for using a non-temporary file is
dropped. Even if merging was implemented manually, use of FamilyWild would
"infect" such a file and break it for DMs which don't write it.

Unfortunately, a hack in UserSession is required to get XAUTHORITY into
the process environment. The Xauthority file has to be created as the target
user, but that's only possible in setupChildProcess(). In there, changes to
processEnvironment() to set XAUTHORITY to the generated path are not effective,
so configure the process to inherit the environment instead and use qputenv.
---
 CMakeLists.txt                   |  3 ++
 data/man/sddm.conf.rst.in        |  8 ----
 src/auth/Auth.cpp                |  6 +--
 src/auth/Auth.h                  |  6 +--
 src/common/Configuration.h       |  2 -
 src/common/XauthUtils.cpp        | 82 ++++++++++++++++++++++++++++++++
 src/common/XauthUtils.h          | 16 +++++++
 src/daemon/CMakeLists.txt        |  3 ++
 src/daemon/XorgDisplayServer.cpp | 45 ++----------------
 src/daemon/XorgDisplayServer.h   |  4 +-
 src/helper/Backend.cpp           |  7 ---
 src/helper/CMakeLists.txt        |  8 +++-
 src/helper/HelperApp.cpp         |  4 +-
 src/helper/HelperApp.h           |  4 +-
 src/helper/UserSession.cpp       | 53 +++++++++++----------
 src/helper/UserSession.h         |  9 ++++
 16 files changed, 165 insertions(+), 95 deletions(-)
 create mode 100644 src/common/XauthUtils.cpp
 create mode 100644 src/common/XauthUtils.h

diff --git a/CMakeLists.txt b/CMakeLists.txt
index e52e0e9..fc3f2ce 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -88,6 +88,9 @@ add_feature_info("PAM" PAM_FOUND "PAM support")
 include(CheckFunctionExists)
 check_function_exists(getspnam HAVE_GETSPNAM)
 
+# XAU
+pkg_check_modules(LIBXAU REQUIRED "xau")
+
 # XCB
 find_package(XCB REQUIRED)
 
diff --git a/data/man/sddm.conf.rst.in b/data/man/sddm.conf.rst.in
index bee0768..e91280a 100644
--- a/data/man/sddm.conf.rst.in
+++ b/data/man/sddm.conf.rst.in
@@ -110,10 +110,6 @@ OPTIONS
 	Path of the Xephyr.
 	Default value is "/usr/bin/Xephyr".
 
-`XauthPath=`
-	Path of the Xauth.
-	Default value is "/usr/bin/xauth".
-
 `SessionDir=`
 	Path of the directory containing session files.
 	Default value is "/usr/share/xsessions".
@@ -128,10 +124,6 @@ OPTIONS
         Path to the user session log file, relative to the home directory.
         Default value is ".local/share/sddm/xorg-session.log".
 
-`UserAuthFile=`
-        Path to the Xauthority file, relative to the home directory.
-        Default value is ".Xauthority".
-
 `DisplayCommand=`
 	Path of script to execute when starting the display server.
 	Default value is "@DATA_INSTALL_DIR@/scripts/Xsetup".
diff --git a/src/auth/Auth.cpp b/src/auth/Auth.cpp
index caca314..c2228ae 100644
--- a/src/auth/Auth.cpp
+++ b/src/auth/Auth.cpp
@@ -64,7 +64,7 @@ namespace SDDM {
         QLocalSocket *socket { nullptr };
         QString sessionPath { };
         QString user { };
-        QString cookie { };
+        QByteArray cookie { };
         bool autologin { false };
         bool greeter { false };
         QProcessEnvironment environment { };
@@ -266,7 +266,7 @@ namespace SDDM {
         return d->greeter;
     }
 
-    const QString& Auth::cookie() const {
+    const QByteArray& Auth::cookie() const {
         return d->cookie;
     }
 
@@ -298,7 +298,7 @@ namespace SDDM {
         d->environment.insert(key, value);
     }
 
-    void Auth::setCookie(const QString& cookie) {
+    void Auth::setCookie(const QByteArray& cookie) {
         if (cookie != d->cookie) {
             d->cookie = cookie;
             Q_EMIT cookieChanged();
diff --git a/src/auth/Auth.h b/src/auth/Auth.h
index 87f5f44..38d63fc 100644
--- a/src/auth/Auth.h
+++ b/src/auth/Auth.h
@@ -54,7 +54,7 @@ namespace SDDM {
         Q_PROPERTY(bool autologin READ autologin WRITE setAutologin NOTIFY autologinChanged)
         Q_PROPERTY(bool greeter READ isGreeter WRITE setGreeter NOTIFY greeterChanged)
         Q_PROPERTY(bool verbose READ verbose WRITE setVerbose NOTIFY verboseChanged)
-        Q_PROPERTY(QString cookie READ cookie WRITE setCookie NOTIFY cookieChanged)
+        Q_PROPERTY(QByteArray cookie READ cookie WRITE setCookie NOTIFY cookieChanged)
         Q_PROPERTY(QString user READ user WRITE setUser NOTIFY userChanged)
         Q_PROPERTY(QString session READ session WRITE setSession NOTIFY sessionChanged)
         Q_PROPERTY(AuthRequest* request READ request NOTIFY requestChanged)
@@ -90,7 +90,7 @@ namespace SDDM {
         bool autologin() const;
         bool isGreeter() const;
         bool verbose() const;
-        const QString &cookie() const;
+        const QByteArray &cookie() const;
         const QString &user() const;
         const QString &session() const;
         AuthRequest *request();
@@ -149,7 +149,7 @@ namespace SDDM {
          * Set the display server cookie, to be inserted into the user's $XAUTHORITY
          * @param cookie cookie data
          */
-        void setCookie(const QString &cookie);
+        void setCookie(const QByteArray &cookie);
 
     public Q_SLOTS:
         /**
diff --git a/src/common/Configuration.h b/src/common/Configuration.h
index cf44a62..a7e0585 100644
--- a/src/common/Configuration.h
+++ b/src/common/Configuration.h
@@ -63,11 +63,9 @@ namespace SDDM {
             Entry(ServerPath,          QString,     _S("/usr/bin/X"),                           _S("Path to X server binary"));
             Entry(ServerArguments,     QString,     _S("-nolisten tcp"),                        _S("Arguments passed to the X server invocation"));
             Entry(XephyrPath,          QString,     _S("/usr/bin/Xephyr"),                      _S("Path to Xephyr binary"));
-            Entry(XauthPath,           QString,     _S("/usr/bin/xauth"),                       _S("Path to xauth binary"));
             Entry(SessionDir,          QString,     _S("/usr/share/xsessions"),                 _S("Directory containing available X sessions"));
             Entry(SessionCommand,      QString,     _S(SESSION_COMMAND),                        _S("Path to a script to execute when starting the desktop session"));
 	    Entry(SessionLogFile,      QString,     _S(".local/share/sddm/xorg-session.log"),   _S("Path to the user session log file"));
-	    Entry(UserAuthFile,        QString,     _S(".Xauthority"),                          _S("Path to the Xauthority file"));
             Entry(DisplayCommand,      QString,     _S(DATA_INSTALL_DIR "/scripts/Xsetup"),     _S("Path to a script to execute when starting the display server"));
             Entry(DisplayStopCommand,  QString,     _S(DATA_INSTALL_DIR "/scripts/Xstop"),      _S("Path to a script to execute when stopping the display server"));
             Entry(MinimumVT,           int,         MINIMUM_VT,                                 _S("The lowest virtual terminal number that will be used."));
diff --git a/src/common/XauthUtils.cpp b/src/common/XauthUtils.cpp
new file mode 100644
index 0000000..da1c691
--- /dev/null
+++ b/src/common/XauthUtils.cpp
@@ -0,0 +1,82 @@
+/****************************************************************************
+ * SPDX-FileCopyrightText: 2020 Fabian Vogt <fvogt@suse.de>
+ *
+ * SPDX-License-Identifier: GPL-2.0-or-later
+ ***************************************************************************/
+
+#include <limits.h>
+#include <unistd.h>
+#include <X11/Xauth.h>
+
+#include <random>
+
+#include <QString>
+
+#include "XauthUtils.h"
+
+namespace SDDM { namespace Xauth {
+    QByteArray generateCookie()
+    {
+        std::random_device rd;
+        std::mt19937 gen(rd());
+        std::uniform_int_distribution<> dis(0, 0xFF);
+
+        QByteArray cookie;
+        cookie.reserve(16);
+
+        for(int i = 0; i < 16; i++)
+            cookie[i] = dis(gen);
+
+        return cookie;
+    }
+
+    bool writeCookieToFile(const QString &filename, const QString &display, QByteArray cookie)
+    {
+        if(display.size() < 2 || display[0] != QLatin1Char(':') || cookie.count() != 16)
+            return false;
+
+        // Truncate the file. We don't support merging like the xauth tool does.
+        FILE * const authFp = fopen(qPrintable(filename), "wb");
+        if (authFp == nullptr)
+            return false;
+
+        char localhost[HOST_NAME_MAX + 1] = "";
+        if (gethostname(localhost, HOST_NAME_MAX) < 0)
+            strcpy(localhost, "localhost");
+
+        ::Xauth auth = {};
+        char cookieName[] = "MIT-MAGIC-COOKIE-1";
+
+        // Skip the ':'
+        QByteArray displayNumberUtf8 = display.midRef(1).toUtf8();
+
+        auth.family = FamilyLocal;
+        auth.address = localhost;
+        auth.address_length = strlen(auth.address);
+        auth.number = displayNumberUtf8.data();
+        auth.number_length = displayNumberUtf8.size();
+        auth.name = cookieName;
+        auth.name_length = sizeof(cookieName) - 1;
+        auth.data = cookie.data();
+        auth.data_length = cookie.count();
+
+        if (XauWriteAuth(authFp, &auth) == 0) {
+            fclose(authFp);
+            return false;
+        }
+
+        // Write the same entry again, just with FamilyWild
+        auth.family = FamilyWild;
+        auth.address_length = 0;
+        if (XauWriteAuth(authFp, &auth) == 0) {
+            fclose(authFp);
+            return false;
+        }
+
+        bool success = fflush(authFp) != EOF;
+
+        fclose(authFp);
+
+        return success;
+    }
+}}
diff --git a/src/common/XauthUtils.h b/src/common/XauthUtils.h
new file mode 100644
index 0000000..112d003
--- /dev/null
+++ b/src/common/XauthUtils.h
@@ -0,0 +1,16 @@
+// SPDX-License-Identifier: GPL-2.0-or-later
+
+#ifndef SDDM_XAUTHUTILS_H
+#define SDDM_XAUTHUTILS_H
+
+class QString;
+class QByteArray;
+
+namespace SDDM {
+    namespace Xauth {
+        QByteArray generateCookie();
+        bool writeCookieToFile(const QString &filename, const QString &display, QByteArray cookie);
+    }
+}
+
+#endif // SDDM_XAUTHUTILS_H
diff --git a/src/daemon/CMakeLists.txt b/src/daemon/CMakeLists.txt
index 86d014b..9145607 100644
--- a/src/daemon/CMakeLists.txt
+++ b/src/daemon/CMakeLists.txt
@@ -2,6 +2,7 @@ include_directories(
     "${CMAKE_SOURCE_DIR}/src/common"
     "${CMAKE_SOURCE_DIR}/src/auth"
     "${CMAKE_BINARY_DIR}/src/common"
+    ${LIBXAU_INCLUDE_DIRS}
     "${LIBXCB_INCLUDE_DIR}"
 )
 
@@ -13,6 +14,7 @@ set(DAEMON_SOURCES
     ${CMAKE_SOURCE_DIR}/src/common/ThemeMetadata.cpp
     ${CMAKE_SOURCE_DIR}/src/common/Session.cpp
     ${CMAKE_SOURCE_DIR}/src/common/SocketWriter.cpp
+    ${CMAKE_SOURCE_DIR}/src/common/XauthUtils.cpp
     ${CMAKE_SOURCE_DIR}/src/auth/Auth.cpp
     ${CMAKE_SOURCE_DIR}/src/auth/AuthPrompt.cpp
     ${CMAKE_SOURCE_DIR}/src/auth/AuthRequest.cpp
@@ -64,6 +66,7 @@ target_link_libraries(sddm
                       Qt5::DBus
                       Qt5::Network
                       Qt5::Qml
+                      ${LIBXAU_LIBRARIES}
                       ${LIBXCB_LIBRARIES})
 if(PAM_FOUND)
     target_link_libraries(sddm ${PAM_LIBRARIES})
diff --git a/src/daemon/XorgDisplayServer.cpp b/src/daemon/XorgDisplayServer.cpp
index 5f93a1b..148ebcc 100644
--- a/src/daemon/XorgDisplayServer.cpp
+++ b/src/daemon/XorgDisplayServer.cpp
@@ -25,6 +25,7 @@
 #include "Display.h"
 #include "SignalHandler.h"
 #include "Seat.h"
+#include "XauthUtils.h"
 
 #include <QDebug>
 #include <QFile>
@@ -55,17 +56,7 @@ namespace SDDM {
         m_authPath = QStringLiteral("%1/%2").arg(authDir).arg(QUuid::createUuid().toString());
 
         // generate cookie
-        std::random_device rd;
-        std::mt19937 gen(rd());
-        std::uniform_int_distribution<> dis(0, 15);
-
-        // resever 32 bytes
-        m_cookie.reserve(32);
-
-        // create a random hexadecimal number
-        const char *digits = "0123456789abcdef";
-        for (int i = 0; i < 32; ++i)
-            m_cookie[i] = digits[dis(gen)];
+        m_cookie = Xauth::generateCookie();
     }
 
     XorgDisplayServer::~XorgDisplayServer() {
@@ -84,35 +75,10 @@ namespace SDDM {
         return QStringLiteral("x11");
     }
 
-    const QString &XorgDisplayServer::cookie() const {
+    const QByteArray &XorgDisplayServer::cookie() const {
         return m_cookie;
     }
 
-    bool XorgDisplayServer::addCookie(const QString &file) {
-        // log message
-        qDebug() << "Adding cookie to" << file;
-
-        // Touch file
-        QFile file_handler(file);
-        file_handler.open(QIODevice::Append);
-        file_handler.close();
-
-        QString cmd = QStringLiteral("%1 -f %2 -q").arg(mainConfig.X11.XauthPath.get()).arg(file);
-
-        // execute xauth
-        FILE *fp = popen(qPrintable(cmd), "w");
-
-        // check file
-        if (!fp)
-            return false;
-        fprintf(fp, "remove %s\n", qPrintable(m_display));
-        fprintf(fp, "add %s . %s\n", qPrintable(m_display), qPrintable(m_cookie));
-        fprintf(fp, "exit\n");
-
-        // close pipe
-        return pclose(fp) == 0;
-    }
-
     bool XorgDisplayServer::start() {
         // check flag
         if (m_started)
@@ -130,8 +96,7 @@ namespace SDDM {
         // generate auth file.
         // For the X server's copy, the display number doesn't matter.
         // An empty file would result in no access control!
-        m_display = QStringLiteral(":0");
-        if(!addCookie(m_authPath)) {
+        if(!Xauth::writeCookieToFile(m_authPath, QStringLiteral(":0"), m_cookie)) {
             qCritical() << "Failed to write xauth file";
             return false;
         }
@@ -229,7 +194,7 @@ namespace SDDM {
         // The file is also used by the greeter, which does care about the
         // display number. Write the proper entry, if it's different.
         if(m_display != QStringLiteral(":0")) {
-            if(!addCookie(m_authPath)) {
+            if(!Xauth::writeCookieToFile(m_authPath, m_display, m_cookie)) {
                 qCritical() << "Failed to write xauth file";
                 return false;
             }
diff --git a/src/daemon/XorgDisplayServer.h b/src/daemon/XorgDisplayServer.h
index e97a0b5..05e0a4c 100644
--- a/src/daemon/XorgDisplayServer.h
+++ b/src/daemon/XorgDisplayServer.h
@@ -38,7 +38,7 @@ namespace SDDM {
 
         QString sessionType() const;
 
-        const QString &cookie() const;
+        const QByteArray &cookie() const;
 
         bool addCookie(const QString &file);
 
@@ -50,7 +50,7 @@ namespace SDDM {
 
     private:
         QString m_authPath;
-        QString m_cookie;
+        QByteArray m_cookie;
 
         QProcess *process { nullptr };
 
diff --git a/src/helper/Backend.cpp b/src/helper/Backend.cpp
index a324b39..a053310 100644
--- a/src/helper/Backend.cpp
+++ b/src/helper/Backend.cpp
@@ -68,13 +68,6 @@ namespace SDDM {
             env.insert(QStringLiteral("SHELL"), QString::fromLocal8Bit(pw->pw_shell));
             env.insert(QStringLiteral("USER"), QString::fromLocal8Bit(pw->pw_name));
             env.insert(QStringLiteral("LOGNAME"), QString::fromLocal8Bit(pw->pw_name));
-            if (env.contains(QStringLiteral("DISPLAY")) && !env.contains(QStringLiteral("XAUTHORITY"))) {
-                // determine Xauthority path
-                QString value = QStringLiteral("%1/%2")
-                        .arg(QString::fromLocal8Bit(pw->pw_dir))
-                        .arg(mainConfig.X11.UserAuthFile.get());
-                env.insert(QStringLiteral("XAUTHORITY"), value);
-            }
 #if defined(Q_OS_FREEBSD)
         /* get additional environment variables via setclassenvironment();
             this needs to be done here instead of in UserSession::setupChildProcess
diff --git a/src/helper/CMakeLists.txt b/src/helper/CMakeLists.txt
index 8914ea7..81b939b 100644
--- a/src/helper/CMakeLists.txt
+++ b/src/helper/CMakeLists.txt
@@ -3,6 +3,7 @@ include(CheckLibraryExists)
 include_directories(
     "${CMAKE_SOURCE_DIR}/src/common"
     "${CMAKE_SOURCE_DIR}/src/auth"
+    ${LIBXAU_INCLUDE_DIRS}
 )
 include_directories("${CMAKE_BINARY_DIR}/src/common")
 
@@ -10,6 +11,7 @@ set(HELPER_SOURCES
     ${CMAKE_SOURCE_DIR}/src/common/Configuration.cpp
     ${CMAKE_SOURCE_DIR}/src/common/ConfigReader.cpp
     ${CMAKE_SOURCE_DIR}/src/common/SafeDataStream.cpp
+    ${CMAKE_SOURCE_DIR}/src/common/XauthUtils.cpp
     Backend.cpp
     HelperApp.cpp
     UserSession.cpp
@@ -41,7 +43,11 @@ else()
 endif()
 
 add_executable(sddm-helper ${HELPER_SOURCES})
-target_link_libraries(sddm-helper Qt5::Network Qt5::DBus Qt5::Qml)
+target_link_libraries(sddm-helper
+                      Qt5::Network
+                      Qt5::DBus
+                      Qt5::Qml
+                      ${LIBXAU_LIBRARIES})
 if("${CMAKE_SYSTEM_NAME}" STREQUAL "FreeBSD")
     # On FreeBSD (possibly other BSDs as well), we want to use
     # setusercontext() to set up the login configuration from login.conf
diff --git a/src/helper/HelperApp.cpp b/src/helper/HelperApp.cpp
index 672359a..4ad9cbd 100644
--- a/src/helper/HelperApp.cpp
+++ b/src/helper/HelperApp.cpp
@@ -237,7 +237,7 @@ namespace SDDM {
         str >> m >> env >> m_cookie;
         if (m != AUTHENTICATED) {
             env = QProcessEnvironment();
-            m_cookie = QString();
+            m_cookie = {};
             qCritical() << "Received a wrong opcode instead of AUTHENTICATED:" << m;
         }
         return env;
@@ -263,7 +263,7 @@ namespace SDDM {
         return m_user;
     }
 
-    const QString& HelperApp::cookie() const {
+    const QByteArray& HelperApp::cookie() const {
         return m_cookie;
     }
 
diff --git a/src/helper/HelperApp.h b/src/helper/HelperApp.h
index 3742df1..d417494 100644
--- a/src/helper/HelperApp.h
+++ b/src/helper/HelperApp.h
@@ -40,7 +40,7 @@ namespace SDDM {
 
         UserSession *session();
         const QString &user() const;
-        const QString &cookie() const;
+        const QByteArray &cookie() const;
 
     public slots:
         Request request(const Request &request);
@@ -62,7 +62,7 @@ namespace SDDM {
         QLocalSocket *m_socket { nullptr };
         QString m_user { };
         // TODO: get rid of this in a nice clean way along the way with moving to user session X server
-        QString m_cookie { };
+        QByteArray m_cookie { };
 
         /*!
          \brief Write utmp/wtmp/btmp records when a user logs in
diff --git a/src/helper/UserSession.cpp b/src/helper/UserSession.cpp
index c9a8a20..e55e69e 100644
--- a/src/helper/UserSession.cpp
+++ b/src/helper/UserSession.cpp
@@ -23,6 +23,7 @@
 #include "UserSession.h"
 #include "HelperApp.h"
 #include "VirtualTerminal.h"
+#include "XauthUtils.h"
 
 #include <sys/types.h>
 #include <sys/ioctl.h>
@@ -35,6 +36,8 @@
 #include <fcntl.h>
 #include <sched.h>
 
+#include <QStandardPaths>
+
 namespace SDDM {
     UserSession::UserSession(HelperApp *parent)
             : QProcess(parent) {
@@ -260,38 +263,38 @@ namespace SDDM {
             qWarning() << "Could not redirect stdout";
         }
 
+        // Drop the current environment
+        for (QString key : QProcessEnvironment::systemEnvironment().keys())
+            qunsetenv(qPrintable(key));
+
+        // Apply the new one. This has the nice effect that XDG_RUNTIME_DIR etc are effective
+        for (QString key : processEnvironment().keys())
+            qputenv(qPrintable(key), processEnvironment().value(key).toLocal8Bit());
+
         // set X authority for X11 sessions only
         if (sessionType != QLatin1String("x11"))
             return;
-        QString cookie = qobject_cast<HelperApp*>(parent())->cookie();
-        if (!cookie.isEmpty()) {
-            QString file = processEnvironment().value(QStringLiteral("XAUTHORITY"));
-            QString display = processEnvironment().value(QStringLiteral("DISPLAY"));
-            qDebug() << "Adding cookie to" << file;
-
-
-            // create the path
-            QFileInfo finfo(file);
-            QDir().mkpath(finfo.absolutePath());
 
-            QFile file_handler(file);
-            file_handler.open(QIODevice::Append);
-            file_handler.close();
-
-            QString cmd = QStringLiteral("%1 -f %2 -q").arg(mainConfig.X11.XauthPath.get()).arg(file);
+        QByteArray cookie = qobject_cast<HelperApp*>(parent())->cookie();
+        if (cookie.isEmpty())
+            return;
 
-            // execute xauth
-            FILE *fp = popen(qPrintable(cmd), "w");
+        QString dir = QStandardPaths::writableLocation(QStandardPaths::RuntimeLocation);
+        if (!dir.isEmpty()) {
+            m_xauthFile.setFileTemplate(dir + QStringLiteral("/xauth_XXXXXX"));
+            m_xauthFile.open();
+        }
 
-            // check file
-            if (!fp)
-                return;
-            fprintf(fp, "remove %s\n", qPrintable(display));
-            fprintf(fp, "add %s . %s\n", qPrintable(display), qPrintable(cookie));
-            fprintf(fp, "exit\n");
+        if (m_xauthFile.fileName().isEmpty())
+            qWarning() << "Could not create the Xauthority file";
+        else {
+            QString display = processEnvironment().value(QStringLiteral("DISPLAY"));
+            qDebug() << "Adding cookie to" << m_xauthFile.fileName();
 
-            // close pipe
-            pclose(fp);
+            if (!Xauth::writeCookieToFile(m_xauthFile.fileName(), display, cookie))
+                qWarning() << "Failed to write the Xauthority file";
+            else
+                qputenv("XAUTHORITY", m_xauthFile.fileName().toUtf8());
         }
     }
 
diff --git a/src/helper/UserSession.h b/src/helper/UserSession.h
index 7069084..23776f4 100644
--- a/src/helper/UserSession.h
+++ b/src/helper/UserSession.h
@@ -25,6 +25,7 @@
 #include <QtCore/QObject>
 #include <QtCore/QString>
 #include <QtCore/QProcess>
+#include <QtCore/QTemporaryFile>
 
 namespace SDDM {
     class HelperApp;
@@ -53,12 +54,20 @@ namespace SDDM {
         */
         qint64 cachedProcessId();
 
+        /* Hack! QProcess does not allow changing processEnvironment by
+           setupChildProcess(), but this is needed for creating XAUTHORITY.
+           So inherit the caller's environment and apply the assignments manually. */
+        QProcessEnvironment processEnvironment() { return m_processEnv; }
+        void setProcessEnvironment(QProcessEnvironment env) { m_processEnv = env; }
+
     protected:
         void setupChildProcess();
 
     private:
         QString m_path { };
         qint64 m_cachedProcessId;
+        QProcessEnvironment m_processEnv;
+        QTemporaryFile m_xauthFile;
     };
 }
 
-- 
2.25.1

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File 0001-Redesign-Xauth-handling.patch of Package sddm (Revision 57f56f8cb81e7a2128ee3b2861934f68)

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