QProcess/Unix: ensure we don't accidentally execute something from CWD

Unless "." (or the empty string) is in $PATH, we're not supposed to find
executables in the current directory. This is how the Unix shells behave
and we match their behavior. It's also the behavior Qt had prior to 5.9
(commit 28666d167a). On Windows, searching
the current directory is the norm, so we keep that behavior.

This commit does not add an explicit check for an empty return from
QStandardPaths::findExecutable(). Instead, we allow that empty string to
go all the way to execve(2), which will fail with ENOENT. We could catch
it early, before fork(2), but why add code for the error case?

See https://kde.org/info/security/advisory-20220131-1.txt

[ChangeLog][Important Behavior Changes] When passed a simple program
name with no slashes, QProcess on Unix systems will now only search the
current directory if "." is one of the entries in the PATH environment
variable. This bug fix restores the behavior QProcess had before Qt 5.9.
If launching an executable in the directory set by setWorkingDirectory()
or inherited from the parent is intended, pass a program name starting
with "./". For more information and best practices about finding an
executable, see QProcess' documentation.

Pick-to: 5.15 6.2 6.3
Change-Id: I54f205f6b7314351b078fffd16cf7013c97ee9fb
Reviewed-by: Qt CI Bot <qt_ci_bot@qt-project.org>
Reviewed-by: Mårten Nordheim <marten.nordheim@qt.io>
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

src/corelib/io/qprocess_unix.cpp

@@ -1,7 +1,7 @@
 /****************************************************************************
 **
 ** Copyright (C) 2020 The Qt Company Ltd.
-** Copyright (C) 2021 Intel Corporation.
+** Copyright (C) 2022 Intel Corporation.
 ** Copyright (C) 2021 Alex Trotsenko.
 ** Contact: https://www.qt.io/licensing/
 **
@@ -438,9 +438,10 @@ static QString resolveExecutable(const QString &program)
 #endif
 
     if (!program.contains(QLatin1Char('/'))) {
-        QString exeFilePath = QStandardPaths::findExecutable(program);
-        if (!exeFilePath.isEmpty())
-            return exeFilePath;
+        // findExecutable() returns its argument if it's an absolute path,
+        // otherwise it searches $PATH; returns empty if not found (we handle
+        // that case much later)
+        return QStandardPaths::findExecutable(program);
     }
     return program;
 }

tests/auto/corelib/io/qprocess/tst_qprocess.cpp

@@ -1,7 +1,7 @@
 /****************************************************************************
 **
 ** Copyright (C) 2021 The Qt Company Ltd.
-** Copyright (C) 2020 Intel Corporation.
+** Copyright (C) 2022 Intel Corporation.
 ** Contact: https://www.qt.io/licensing/
 **
 ** This file is part of the test suite of the Qt Toolkit.
@@ -151,6 +151,8 @@ private slots:
     void startStopStartStopBuffers();
     void processEventsInAReadyReadSlot_data();
     void processEventsInAReadyReadSlot();
+    void startFromCurrentWorkingDir_data();
+    void startFromCurrentWorkingDir();
 
     // keep these at the end, since they use lots of processes and sometimes
     // caused obscure failures to occur in tests that followed them (esp. on the Mac)
@@ -2524,5 +2526,94 @@ void tst_QProcess::processEventsInAReadyReadSlot()
         QVERIFY(process.waitForFinished());
 }
 
+enum class ChdirMode {
+    None = 0,
+    InParent,
+    InChild
+};
+Q_DECLARE_METATYPE(ChdirMode)
+
+void tst_QProcess::startFromCurrentWorkingDir_data()
+{
+    qRegisterMetaType<ChdirMode>();
+    QTest::addColumn<QString>("programPrefix");
+    QTest::addColumn<ChdirMode>("chdirMode");
+    QTest::addColumn<bool>("success");
+
+    constexpr bool IsWindows = true
+#ifdef Q_OS_UNIX
+            && false
+#endif
+            ;
+
+    // baseline: trying to execute the directory, this can't possibly succeed!
+    QTest::newRow("plain-same-cwd") << QString() << ChdirMode::None << false;
+
+    // cross-platform behavior: neither OS searches the setWorkingDirectory()
+    // dir without "./"
+    QTest::newRow("plain-child-chdir") << QString() << ChdirMode::InChild << false;
+
+    // cross-platform behavior: both OSes search the parent's CWD with "./"
+    QTest::newRow("prefixed-parent-chdir") << "./" << ChdirMode::InParent << true;
+
+    // opposite behaviors: Windows searches the parent's CWD and Unix searches
+    // the child's with "./"
+    QTest::newRow("prefixed-child-chdir") << "./" << ChdirMode::InChild << !IsWindows;
+
+    // Windows searches the parent's CWD without "./"
+    QTest::newRow("plain-parent-chdir") << QString() << ChdirMode::InParent << IsWindows;
+}
+
+void tst_QProcess::startFromCurrentWorkingDir()
+{
+    QFETCH(QString, programPrefix);
+    QFETCH(ChdirMode, chdirMode);
+    QFETCH(bool, success);
+
+    QProcess process;
+    qRegisterMetaType<QProcess::ProcessError>();
+    QSignalSpy errorSpy(&process, &QProcess::errorOccurred);
+    QVERIFY(errorSpy.isValid());
+
+    // both the dir name and the executable name
+    const QString target = QStringLiteral("testProcessNormal");
+    process.setProgram(programPrefix + target);
+
+#ifdef Q_OS_UNIX
+    // Reset PATH, to be sure it doesn't contain . or the empty path.
+    // We can't do this on Windows because DLLs are searched in PATH
+    // and Windows always searches "." anyway.
+    auto restoreEnv = qScopeGuard([old = qgetenv("PATH")] {
+        qputenv("PATH", old);
+    });
+    qputenv("PATH", "/");
+#endif
+
+    switch (chdirMode) {
+    case ChdirMode::InParent: {
+        auto restoreCwd = qScopeGuard([old = QDir::currentPath()] {
+            QDir::setCurrent(old);
+        });
+        QVERIFY(QDir::setCurrent(target));
+        process.start();
+        break;
+    }
+    case ChdirMode::InChild:
+        process.setWorkingDirectory(target);
+        Q_FALLTHROUGH();
+    case ChdirMode::None:
+        process.start();
+        break;
+    }
+
+    QCOMPARE(process.waitForStarted(), success);
+    QCOMPARE(errorSpy.count(), int(!success));
+    if (success) {
+        QVERIFY(process.waitForFinished());
+    } else {
+        QCOMPARE(process.error(), QProcess::FailedToStart);
+    }
+}
+
 QTEST_MAIN(tst_QProcess)
 #include "tst_qprocess.moc"

tests/auto/widgets/kernel/qapplication/tst_qapplication.cpp

@@ -1530,7 +1530,7 @@ void tst_QApplication::desktopSettingsAware()
     environment += QLatin1String("QT_MAC_DISABLE_FOREGROUND_APPLICATION_TRANSFORM=1");
     testProcess.setEnvironment(environment);
 #endif
-    testProcess.start("desktopsettingsaware_helper");
+    testProcess.start("./desktopsettingsaware_helper");
     QVERIFY2(testProcess.waitForStarted(),
              qPrintable(QString::fromLatin1("Cannot start 'desktopsettingsaware_helper': %1").arg(testProcess.errorString())));
     QVERIFY(testProcess.waitForFinished(10000));
@@ -2470,7 +2470,7 @@ void tst_QApplication::qtbug_12673()
 #if QT_CONFIG(process)
     QProcess testProcess;
     QStringList arguments;
-    testProcess.start("modal_helper", arguments);
+    testProcess.start("./modal_helper", arguments);
     QVERIFY2(testProcess.waitForStarted(),
              qPrintable(QString::fromLatin1("Cannot start 'modal_helper': %1").arg(testProcess.errorString())));
     QVERIFY(testProcess.waitForFinished(20000));