QSGOpenGLDistanceFieldGlyphCache: fix UB (ordering of pointers not from the same array)

The code performed out of bounds checks by adding the size of the
buffer to a pointer and comparing the result to the the
one-past-the-end pointer of the buffer.

This is UB, for three reasons:

- in one case, a qint64 is added to a pointer, silently truncating the
  result on 32bit platforms

- if the buffer overflow is large, the pointer value may wrap around,
  yielding a result that is numerically less than the end pointer, but
  still out-of-bounds.

- pointer order is only defined within a C array, plus one past the
  end. On failure, pointers outside that range are compared.

Fix by comparing distance(it, end) with the required size for the
chunk to be written instead.

Change-Id: I356bb8c8a65a93b8b1c1eb7bac381dd64bea719e
Reviewed-by: Fabian Kosmale <fabian.kosmale@qt.io>
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
(cherry picked from commit 8d9bd6b381bfc759d575954801b683354ad6a790)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

1 files changed, 3 insertions, 3 deletions

diff --git a/src/quick/scenegraph/qsgrhidistancefieldglyphcache.cpp b/src/quick/scenegraph/qsgrhidistancefieldglyphcache.cpp
index a47b35210b..b139c3428a 100644
--- a/src/quick/scenegraph/qsgrhidistancefieldglyphcache.cpp
+++ b/src/quick/scenegraph/qsgrhidistancefieldglyphcache.cpp
@@ -449,7 +449,7 @@ bool QSGRhiDistanceFieldGlyphCache::loadPregeneratedCache(const QRawFont &font)
 
         const char *textureRecord = allocatorData;
         for (int i = 0; i < textureCount; ++i, textureRecord += Qtdf::TextureRecordSize) {
-            if (textureRecord + Qtdf::TextureRecordSize > qtdfTableEnd) {
+            if (qtdfTableEnd - textureRecord < Qtdf::TextureRecordSize) {
                 qWarning("qtdf table too small in font '%s'.",
                          qPrintable(font.familyName()));
                 return false;
@@ -465,7 +465,7 @@ bool QSGRhiDistanceFieldGlyphCache::loadPregeneratedCache(const QRawFont &font)
 
         const char *glyphRecord = textureRecord;
         for (quint32 i = 0; i < glyphCount; ++i, glyphRecord += Qtdf::GlyphRecordSize) {
-            if (glyphRecord + Qtdf::GlyphRecordSize > qtdfTableEnd) {
+            if (qtdfTableEnd - glyphRecord < Qtdf:: GlyphRecordSize) {
                 qWarning("qtdf table too small in font '%s'.",
                          qPrintable(font.familyName()));
                 return false;
@@ -516,7 +516,7 @@ bool QSGRhiDistanceFieldGlyphCache::loadPregeneratedCache(const QRawFont &font)
             int width = texInfo->allocatedArea.width();
             int height = texInfo->allocatedArea.height();
             qint64 size = qint64(width) * height;
-            if (reinterpret_cast<const char *>(textureData + size) > qtdfTableEnd) {
+            if (qtdfTableEnd - reinterpret_cast<const char *>(textureData) < size) {
                 qWarning("qtdf table too small in font '%s'.",
                          qPrintable(font.familyName()));
                 return false;