QCborValue: avoid signed integer oveflows when decoding time_t

QDateTime::fromSecsSinceEpoch() multiplies by 1000 but does not check
for overflow. That means we must do so in QCborValue validation. We
can't use mul_overflow<qint64> on 32-bit platforms, so we do a compare-
and-branch there. For 64-bit platforms, we prefer to do the
multiplication with checked overflow, as the common case is that it will
not overflow and we'll need the multiplication anyway.

Change-Id: Ibdc95e9af7bd456a94ecfffd16060cba6f1c86b8
Reviewed-by: Edward Welbourne <edward.welbourne@qt.io>

1 files changed, 19 insertions, 4 deletions

diff --git a/src/corelib/serialization/qcborvalue.cpp b/src/corelib/serialization/qcborvalue.cpp
index a3729b4ef9..f5cccf1be1 100644
--- a/src/corelib/serialization/qcborvalue.cpp
+++ b/src/corelib/serialization/qcborvalue.cpp
@@ -788,10 +788,25 @@ static QCborValue::Type convertToExtendedType(QCborContainerPrivate *d)
             // The data is supposed to be US-ASCII. If it isn't (contains UTF-8),
             // QDateTime::fromString will fail anyway.
             dt = QDateTime::fromString(b->asLatin1(), Qt::ISODateWithMs);
-        } else if (tag == qint64(QCborKnownTags::UnixTime_t) && e.type == QCborValue::Integer) {
-            dt = QDateTime::fromSecsSinceEpoch(e.value, Qt::UTC);
-        } else if (tag == qint64(QCborKnownTags::UnixTime_t) && e.type == QCborValue::Double) {
-            dt = QDateTime::fromMSecsSinceEpoch(qint64(e.fpvalue() * 1000), Qt::UTC);
+        } else if (tag == qint64(QCborKnownTags::UnixTime_t)) {
+            qint64 msecs;
+            bool ok = false;
+            if (e.type == QCborValue::Integer) {
+#if QT_POINTER_SIZE == 8
+                // we don't have a fast 64-bit mul_overflow implementation on
+                // 32-bit architectures.
+                ok = !mul_overflow(e.value, qint64(1000), &msecs);
+#else
+                static const qint64 Limit = std::numeric_limits<qint64>::max() / 1000;
+                ok = (e.value > -Limit && e.value < Limit);
+                if (ok)
+                    msecs = e.value * 1000;
+#endif
+            } else if (e.type == QCborValue::Double) {
+                ok = convertDoubleTo(round(e.fpvalue() * 1000), &msecs);
+            }
+            if (ok)
+                dt = QDateTime::fromMSecsSinceEpoch(msecs, Qt::UTC);
         }
         if (dt.isValid()) {
             QByteArray text = dt.toString(Qt::ISODateWithMs).toLatin1();