Fix crash when reading corrupt font data

When loading the font data, we had some unprotected reads. To harden
this, we check everything against the length of the font data
before reading.

[ChangeLog][QtGui][Windows] Fixed a possible crash that could happen
when loading corrupted font data.

Fixes: QTBUG-116773
Change-Id: I156df3b8833c9ed785fcc690821a7a74d9a51126
Reviewed-by: Lars Knoll <lars@knoll.priv.no>
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
Reviewed-by: Qt CI Bot <qt_ci_bot@qt-project.org>
(cherry picked from commit 9fe47cf2e11d7c9ad4f72e6fc5e53f10a9743b03)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>
(cherry picked from commit d0ed5db78160ce7c16d9fffef79aa11a7783e34e)

1 files changed, 51 insertions, 16 deletions

diff --git a/src/gui/text/windows/qwindowsfontdatabase.cpp b/src/gui/text/windows/qwindowsfontdatabase.cpp
index 2de53be6a8..3f27466df6 100644
--- a/src/gui/text/windows/qwindowsfontdatabase.cpp
+++ b/src/gui/text/windows/qwindowsfontdatabase.cpp
@@ -868,36 +868,70 @@ QT_WARNING_POP
     return fontEngine;
 }
 
-static QList<quint32> getTrueTypeFontOffsets(const uchar *fontData)
+static QList<quint32> getTrueTypeFontOffsets(const uchar *fontData, const uchar *fileEndSentinel)
 {
     QList<quint32> offsets;
-    const quint32 headerTag = *reinterpret_cast<const quint32 *>(fontData);
+    if (fileEndSentinel - fontData < 12) {
+        qCWarning(lcQpaFonts) << "Corrupted font data detected";
+        return offsets;
+    }
+
+    const quint32 headerTag = qFromUnaligned<quint32>(fontData);
     if (headerTag != MAKE_TAG('t', 't', 'c', 'f')) {
         if (headerTag != MAKE_TAG(0, 1, 0, 0)
             && headerTag != MAKE_TAG('O', 'T', 'T', 'O')
             && headerTag != MAKE_TAG('t', 'r', 'u', 'e')
-            && headerTag != MAKE_TAG('t', 'y', 'p', '1'))
+            && headerTag != MAKE_TAG('t', 'y', 'p', '1')) {
             return offsets;
+        }
         offsets << 0;
         return offsets;
     }
+
+    const quint32 maximumNumFonts = 0xffff;
     const quint32 numFonts = qFromBigEndian<quint32>(fontData + 8);
-    for (uint i = 0; i < numFonts; ++i) {
-        offsets << qFromBigEndian<quint32>(fontData + 12 + i * 4);
+    if (numFonts > maximumNumFonts) {
+        qCWarning(lcQpaFonts) << "Font collection of" << numFonts << "fonts is too large. Aborting.";
+        return offsets;
+    }
+
+    if (quintptr(fileEndSentinel - fontData) > 12 + (numFonts - 1) * 4) {
+        for (quint32 i = 0; i < numFonts; ++i)
+            offsets << qFromBigEndian<quint32>(fontData + 12 + i * 4);
+    } else {
+        qCWarning(lcQpaFonts) << "Corrupted font data detected";
     }
+
     return offsets;
 }
 
-static void getFontTable(const uchar *fileBegin, const uchar *data, quint32 tag, const uchar **table, quint32 *length)
+static void getFontTable(const uchar *fileBegin, const uchar *fileEndSentinel, const uchar *data, quint32 tag, const uchar **table, quint32 *length)
 {
-    const quint16 numTables = qFromBigEndian<quint16>(data + 4);
-    for (uint i = 0; i < numTables; ++i) {
-        const quint32 offset = 12 + 16 * i;
-        if (*reinterpret_cast<const quint32 *>(data + offset) == tag) {
-            *table = fileBegin + qFromBigEndian<quint32>(data + offset + 8);
-            *length = qFromBigEndian<quint32>(data + offset + 12);
-            return;
+    if (fileEndSentinel - data >= 6) {
+        const quint16 numTables = qFromBigEndian<quint16>(data + 4);
+        if (fileEndSentinel - data >= 28 + 16 * (numTables - 1)) {
+            for (quint32 i = 0; i < numTables; ++i) {
+                const quint32 offset = 12 + 16 * i;
+                if (qFromUnaligned<quint32>(data + offset) == tag) {
+                    const quint32 tableOffset = qFromBigEndian<quint32>(data + offset + 8);
+                    if (quintptr(fileEndSentinel - fileBegin) <= tableOffset) {
+                        qCWarning(lcQpaFonts) << "Corrupted font data detected";
+                        break;
+                    }
+                    *table = fileBegin + tableOffset;
+                    *length = qFromBigEndian<quint32>(data + offset + 12);
+                    if (quintptr(fileEndSentinel - *table) < *length) {
+                        qCWarning(lcQpaFonts) << "Corrupted font data detected";
+                        break;
+                    }
+                    return;
+                }
+            }
+        } else {
+            qCWarning(lcQpaFonts) << "Corrupted font data detected";
         }
+    } else {
+        qCWarning(lcQpaFonts) << "Corrupted font data detected";
     }
     *table = 0;
     *length = 0;
@@ -910,8 +944,9 @@ static void getFamiliesAndSignatures(const QByteArray &fontData,
                                      QList<QFontValues> *values)
 {
     const uchar *data = reinterpret_cast<const uchar *>(fontData.constData());
+    const uchar *dataEndSentinel = data + fontData.size();
 
-    QList<quint32> offsets = getTrueTypeFontOffsets(data);
+    QList<quint32> offsets = getTrueTypeFontOffsets(data, dataEndSentinel);
     if (offsets.isEmpty())
         return;
 
@@ -919,7 +954,7 @@ static void getFamiliesAndSignatures(const QByteArray &fontData,
         const uchar *font = data + offsets.at(i);
         const uchar *table;
         quint32 length;
-        getFontTable(data, font, MAKE_TAG('n', 'a', 'm', 'e'), &table, &length);
+        getFontTable(data, dataEndSentinel, font, MAKE_TAG('n', 'a', 'm', 'e'), &table, &length);
         if (!table)
             continue;
         QFontNames names = qt_getCanonicalFontNames(table, length);
@@ -929,7 +964,7 @@ static void getFamiliesAndSignatures(const QByteArray &fontData,
         families->append(std::move(names));
 
         if (values || signatures)
-            getFontTable(data, font, MAKE_TAG('O', 'S', '/', '2'), &table, &length);
+            getFontTable(data, dataEndSentinel, font, MAKE_TAG('O', 'S', '/', '2'), &table, &length);
 
         if (values) {
             QFontValues fontValues;