matplotlib · efiring · Jul 5, 2019 · Jun 7, 2019 · Jun 10, 2019 · Jun 10, 2019
diff --git a/lib/matplotlib/testing/conftest.py b/lib/matplotlib/testing/conftest.py
@@ -26,6 +26,8 @@ def mpl_test_settings(request):
             assert len(backend_marker.args) == 1, \
                 "Marker 'backend' must specify 1 backend."
             backend, = backend_marker.args
+            skip_on_importerror = backend_marker.kwargs.get(
+                'skip_on_importerror', False)
             prev_backend = matplotlib.get_backend()
 
         style = '_classic_test'  # Default of cleanup and image_comparison too.
@@ -45,7 +47,7 @@ def mpl_test_settings(request):
             except ImportError as exc:
                 # Should only occur for the cairo backend tests, if neither
                 # pycairo nor cairocffi are installed.
-                if 'cairo' in backend.lower():
+                if 'cairo' in backend.lower() or skip_on_importerror:
                     pytest.skip("Failed to switch to backend {} ({})."
                                 .format(backend, exc))
                 else:

diff --git a/lib/matplotlib/tests/test_backend_tk.py b/lib/matplotlib/tests/test_backend_tk.py
@@ -0,0 +1,28 @@
+import pytest
+import numpy as np
+from matplotlib import pyplot as plt
+
+
+@pytest.mark.backend('TkAgg', skip_on_importerror=True)
+def test_blit():
+    from matplotlib.backends import _tkagg
+    def evil_blit(photoimage, aggimage, offsets, bboxptr):
+        data = np.asarray(aggimage)
+        height, width = data.shape[:2]
+        dataptr = (height, width, data.ctypes.data)
+        _tkagg.blit(
+            photoimage.tk.interpaddr(), str(photoimage), dataptr, offsets,
+            bboxptr)
+
+    fig, ax = plt.subplots()
+    for bad_boxes in ((-1, 2, 0, 2),
+                      (2, 0, 0, 2),
+                      (1, 6, 0, 2),
+                      (0, 2, -1, 2),
+                      (0, 2, 2, 0),
+                      (0, 2, 1, 6)):
+        with pytest.raises(ValueError):
+            evil_blit(fig.canvas._tkphoto,
+                      np.ones((4, 4, 4)),
+                      (0, 1, 2, 3),
+                      bad_boxes)
diff --git a/src/_tkagg.cpp b/src/_tkagg.cpp
@@ -67,6 +67,11 @@ static PyObject *mpl_tk_blit(PyObject *self, PyObject *args)
         PyErr_SetString(PyExc_ValueError, "Failed to extract Tk_PhotoHandle");
         goto exit;
     }
+    if (0 > y1 || y1 > y2 || y2 > height || 0 > x1 || x1 > x2 || x2 > width) {
-    if (0 > y1 || y1 > y2 || y2 > height || 0 > x1 || x1 > x2 || x2 > width) {
+    if (y1 < 0 || y1 > y2 || y2 > height || x1 < 0 || x1 > x2 || x2 > width) {
-    if (0 > y1 || y1 > y2 || y2 > height || 0 > x1 || x1 > x2 || x2 > width) {
+    if (y1 < 0 || y1 > y2 || y2 > height || x1 < 0 || x1 > x2 || x2 > width) {
+        PyErr_SetString(PyExc_ValueError, "Attempting to draw out of bounds");
+        goto exit;
+    }
+
     block.pixelPtr = data_ptr + 4 * ((height - y2) * width + x1);
     block.width = x2 - x1;
     block.height = y2 - y1;