Avoid out of bounds memory reads when scaling images

The calculation of the width/height required for the
scaling algorithm was prone to floating point rounding
issues, where the lower value got rounded down, the higher
one rounded up. This could lead to a situation where we
iterated over one more line/pixel in the line than we have
in the source image.

Correct this by passing the dimension of the source image into
the function and bounds checking the values before iterating.

Task-number: QTBUG-35927
Change-Id: If44b2235a479224660d508a0504fec40d724763a
Reviewed-by: Laszlo Agocs <laszlo.agocs@digia.com>

1 files changed, 22 insertions, 2 deletions

diff --git a/src/gui/painting/qblendfunctions_p.h b/src/gui/painting/qblendfunctions_p.h
index 97b89c8313..98e9e10bd8 100644
--- a/src/gui/painting/qblendfunctions_p.h
+++ b/src/gui/painting/qblendfunctions_p.h
@@ -60,7 +60,7 @@ QT_BEGIN_NAMESPACE
 
 template <typename SRC, typename T>
 void qt_scale_image_16bit(uchar *destPixels, int dbpl,
-                          const uchar *srcPixels, int sbpl,
+                          const uchar *srcPixels, int sbpl, int srch,
                           const QRectF &targetRect,
                           const QRectF &srcRect,
                           const QRect &clip,
@@ -136,6 +136,15 @@ void qt_scale_image_16bit(uchar *destPixels, int dbpl,
 
     quint16 *dst = ((quint16 *) (destPixels + ty1 * dbpl)) + tx1;
 
+    // this bounds check here is required as floating point rounding above might in some cases lead to
+    // w/h values that are one pixel too large, falling outside of the valid image area.
+    int yend = (srcy + iy * (h - 1)) >> 16;
+    if (yend < 0 || yend >= srch)
+        --h;
+    int xend = (basex + ix * (w - 1)) >> 16;
+    if (xend < 0 || xend >= (int)(sbpl/sizeof(quint32)))
+        --w;
+
     while (h--) {
         const SRC *src = (const SRC *) (srcPixels + (srcy >> 16) * sbpl);
         int srcx = basex;
@@ -161,7 +170,7 @@ void qt_scale_image_16bit(uchar *destPixels, int dbpl,
 }
 
 template <typename T> void qt_scale_image_32bit(uchar *destPixels, int dbpl,
-                                                const uchar *srcPixels, int sbpl,
+                                                const uchar *srcPixels, int sbpl, int srch,
                                                 const QRectF &targetRect,
                                                 const QRectF &srcRect,
                                                 const QRect &clip,
@@ -215,6 +224,8 @@ template <typename T> void qt_scale_image_32bit(uchar *destPixels, int dbpl,
 
     int h = ty2 - ty1;
     int w = tx2 - tx1;
+    if (!w || !h)
+        return;
 
     quint32 basex;
     quint32 srcy;
@@ -236,6 +247,15 @@ template <typename T> void qt_scale_image_32bit(uchar *destPixels, int dbpl,
 
     quint32 *dst = ((quint32 *) (destPixels + ty1 * dbpl)) + tx1;
 
+    // this bounds check here is required as floating point rounding above might in some cases lead to
+    // w/h values that are one pixel too large, falling outside of the valid image area.
+    int yend = (srcy + iy * (h - 1)) >> 16;
+    if (yend < 0 || yend >= srch)
+        --h;
+    int xend = (basex + ix * (w - 1)) >> 16;
+    if (xend < 0 || xend >= (int)(sbpl/sizeof(quint32)))
+        --w;
+
     while (h--) {
         const uint *src = (const quint32 *) (srcPixels + (srcy >> 16) * sbpl);
         int srcx = basex;