QTzTimeZonePrivate: fix UB (data race on m_icu)

The fallback m_icu QIcuTimeZonePrivate is lazily constructed, which
means that two threads each with their own copy of a QTimeZone with a
shared QTzTimeZonePrivate will race over who gets to set m_icu,
e.g. when concurrently calling QTimeZone::displayName().

Fix by protecting m_icu with a mutex. For simplicity, use a static
mutex, not a per-instance one (which would delete the
QTzTimeZonePrivate copy constructor, which clone() relies on). This is
sufficient for 5.15. For Qt 6, going forward, we could make this
lock-less, too.

[ChangeLog][QtCore][QTimeZone] Fixed a data race on Unix platforms when
implicitly-shared copies of QTimeZone objects were used in certain ways
(e.g. calling displayName()) from different threads and Qt was
configured with ICU support.

Pick-to: 6.3 6.2 5.15
Change-Id: I7e57aef3dd44a90289ad86d0578ece1e54920730
Reviewed-by: Qt CI Bot <qt_ci_bot@qt-project.org>
Reviewed-by: Lars Knoll <lars.knoll@qt.io>
Reviewed-by: Edward Welbourne <edward.welbourne@qt.io>

1 files changed, 12 insertions, 0 deletions

diff --git a/src/corelib/time/qtimezoneprivate_tz.cpp b/src/corelib/time/qtimezoneprivate_tz.cpp
index b23ccfb3b2..8dfd88b281 100644
--- a/src/corelib/time/qtimezoneprivate_tz.cpp
+++ b/src/corelib/time/qtimezoneprivate_tz.cpp
@@ -42,6 +42,7 @@
 #include "qtimezone.h"
 #include "qtimezoneprivate_p.h"
 #include "private/qlocale_tools_p.h"
+#include "private/qlocking_p.h"
 
 #include <QtCore/QDataStream>
 #include <QtCore/QDateTime>
@@ -62,6 +63,10 @@
 
 QT_BEGIN_NAMESPACE
 
+#if QT_CONFIG(icu)
+static QBasicMutex s_icu_mutex;
+#endif
+
 /*
     Private
 
@@ -729,6 +734,9 @@ QTzTimeZonePrivate::~QTzTimeZonePrivate()
 
 QTzTimeZonePrivate *QTzTimeZonePrivate::clone() const
 {
+#if QT_CONFIG(icu)
+    const auto lock = qt_scoped_lock(s_icu_mutex);
+#endif
     return new QTzTimeZonePrivate(*this);
 }
 
@@ -989,12 +997,14 @@ QString QTzTimeZonePrivate::displayName(qint64 atMSecsSinceEpoch,
                                         const QLocale &locale) const
 {
 #if QT_CONFIG(icu)
+    auto lock = qt_unique_lock(s_icu_mutex);
     if (!m_icu)
         m_icu = new QIcuTimeZonePrivate(m_id);
     // TODO small risk may not match if tran times differ due to outdated files
     // TODO Some valid TZ names are not valid ICU names, use translation table?
     if (m_icu->isValid())
         return m_icu->displayName(atMSecsSinceEpoch, nameType, locale);
+    lock.unlock();
 #else
     Q_UNUSED(nameType);
     Q_UNUSED(locale);
@@ -1008,12 +1018,14 @@ QString QTzTimeZonePrivate::displayName(QTimeZone::TimeType timeType,
                                         const QLocale &locale) const
 {
 #if QT_CONFIG(icu)
+    auto lock = qt_unique_lock(s_icu_mutex);
     if (!m_icu)
         m_icu = new QIcuTimeZonePrivate(m_id);
     // TODO small risk may not match if tran times differ due to outdated files
     // TODO Some valid TZ names are not valid ICU names, use translation table?
     if (m_icu->isValid())
         return m_icu->displayName(timeType, nameType, locale);
+    lock.unlock();
 #else
     Q_UNUSED(timeType);
     Q_UNUSED(nameType);