Fix specific overflow in qtextlayout (CVE-2023-32763)

Fixes: QTBUG-113337
Pick-to: 6.5 6.5.1 6.2 5.15
Change-Id: I13579306defceaccdc0fbb1ec0e9b77c6f8d1af9
Reviewed-by: Eirik Aavitsland <eirik.aavitsland@qt.io>
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
(cherry picked from commit 7b7a01c)

* asturmlechner 2023-05-23: Upstream backport to 5.15 taken from
  https://www.qt.io/blog/security-advisory-qt-svg-1

src/gui/painting/qfixed_p.h

@@ -54,6 +54,7 @@
 #include <QtGui/private/qtguiglobal_p.h>
 #include "QtCore/qdebug.h"
 #include "QtCore/qpoint.h"
+#include <QtCore/private/qnumeric_p.h>
 #include "QtCore/qsize.h"
 
 QT_BEGIN_NAMESPACE
@@ -182,6 +183,14 @@ Q_DECL_CONSTEXPR inline bool operator<(int i, const QFixed &f) { return i * 64 <
 Q_DECL_CONSTEXPR inline bool operator>(const QFixed &f, int i) { return f.value() > i * 64; }
 Q_DECL_CONSTEXPR inline bool operator>(int i, const QFixed &f) { return i * 64 > f.value(); }
 
+inline bool qAddOverflow(QFixed v1, QFixed v2, QFixed *r)
+{
+    int val;
+    bool result = add_overflow(v1.value(), v2.value(), &val);
+    r->setValue(val);
+    return result;
+}
+
 #ifndef QT_NO_DEBUG_STREAM
 inline QDebug &operator<<(QDebug &dbg, const QFixed &f)
 { return dbg << f.toReal(); }

src/gui/text/qtextlayout.cpp

@@ -2163,11 +2163,14 @@ void QTextLine::layout_helper(int maxGlyphs)
         eng->maxWidth = qMax(eng->maxWidth, line.textWidth);
     } else {
         eng->minWidth = qMax(eng->minWidth, lbh.minw);
-        eng->maxWidth += line.textWidth;
+        if (qAddOverflow(eng->maxWidth, line.textWidth, &eng->maxWidth))
+            eng->maxWidth = QFIXED_MAX;
     }
 
-    if (line.textWidth > 0 && item < eng->layoutData->items.size())
-        eng->maxWidth += lbh.spaceData.textWidth;
+    if (line.textWidth > 0 && item < eng->layoutData->items.size()) {
+        if (qAddOverflow(eng->maxWidth, lbh.spaceData.textWidth, &eng->maxWidth))
+            eng->maxWidth = QFIXED_MAX;
+    }
 
     line.textWidth += trailingSpace;
     if (lbh.spaceData.length) {