Trim glyph size in ImageFont.getmask()

Tests/test_imagefontpil.py

@@ -1,14 +1,22 @@
 from __future__ import annotations
+import struct
 import pytest
+from io import BytesIO
 
-from PIL import Image, ImageDraw, ImageFont, features
+from PIL import Image, ImageDraw, ImageFont, features, _util
 
 from .helper import assert_image_equal_tofile
 
-pytestmark = pytest.mark.skipif(
-    features.check_module("freetype2"),
-    reason="PILfont superseded if FreeType is supported",
-)
+original_core = ImageFont.core
+
+
+def setup_module():
+    if features.check_module("freetype2"):
+        ImageFont.core = _util.DeferredError(ImportError)
+
+
+def teardown_module():
+    ImageFont.core = original_core
 
 
 def test_default_font():
@@ -44,3 +52,23 @@ def test_textbbox():
     default_font = ImageFont.load_default()
     assert d.textlength("test", font=default_font) == 24
     assert d.textbbox((0, 0), "test", font=default_font) == (0, 0, 24, 11)
+
+
+def test_decompression_bomb():
+    glyph = struct.pack(">hhhhhhhhhh", 1, 0, 0, 0, 256, 256, 0, 0, 256, 256)
+    fp = BytesIO(b"PILfont\n\nDATA\n" + glyph * 256)
+
+    font = ImageFont.ImageFont()
+    font._load_pilfont_data(fp, Image.new("L", (256, 256)))
+    with pytest.raises(Image.DecompressionBombError):
+        font.getmask("A" * 1_000_000)
+
+
+@pytest.mark.timeout(4)
+def test_oom():
+    glyph = struct.pack(">hhhhhhhhhh", 1, 0, 0, 0, 32767, 32767, 0, 0, 32767, 32767)
+    fp = BytesIO(b"PILfont\n\nDATA\n" + glyph * 256)
+
+    font = ImageFont.ImageFont()
+    font._load_pilfont_data(fp, Image.new("L", (1, 1)))
+    font.getmask("A" * 1_000_000)

docs/releasenotes/10.2.0.rst

@@ -77,6 +77,16 @@ Pillow will now raise a :py:exc:`ValueError` if the number of characters passed
 This threshold can be changed by setting :py:data:`PIL.ImageFont.MAX_STRING_LENGTH`. It
 can be disabled by setting ``ImageFont.MAX_STRING_LENGTH = None``.
 
+A decompression bomb check has also been added to
+:py:meth:`PIL.ImageFont.ImageFont.getmask`.
+
+ImageFont.getmask: Trim glyph size
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+To protect against potential DOS attacks when using PIL fonts,
+:py:class:`PIL.ImageFont.ImageFont` now trims the size of individual glyphs so that
+they do not extend beyond the bitmap image.
+
 ImageMath.eval: Restricted environment keys
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 

src/PIL/ImageFont.py

@@ -150,6 +150,7 @@ def getmask(self, text, mode="", *args, **kwargs):
                  :py:mod:`PIL.Image.core` interface module.
         """
         _string_length_check(text)
+        Image._decompression_bomb_check(self.font.getsize(text))
         return self.font.getmask(text, mode)
 
     def getbbox(self, text, *args, **kwargs):

src/_imaging.c

@@ -2649,6 +2649,18 @@
         self->glyphs[i].sy0 = S16(B16(glyphdata, 14));
         self->glyphs[i].sx1 = S16(B16(glyphdata, 16));
         self->glyphs[i].sy1 = S16(B16(glyphdata, 18));
+
+        // Do not allow glyphs to extend beyond bitmap image
+        // Helps prevent DOS by stopping cropped images being larger than the original
+        if (self->glyphs[i].sx1 > self->bitmap->xsize) {
+            self->glyphs[i].dx1 -= self->glyphs[i].sx1 - self->bitmap->xsize;
+            self->glyphs[i].sx1 = self->bitmap->xsize;
+        }
+        if (self->glyphs[i].sy1 > self->bitmap->ysize) {
+            self->glyphs[i].dy1 -= self->glyphs[i].sy1 - self->bitmap->ysize;
+            self->glyphs[i].sy1 = self->bitmap->ysize;
+        }
+
         if (self->glyphs[i].dy0 < y0) {
             y0 = self->glyphs[i].dy0;
         }
@@ -2721,7 +2733,7 @@
 static PyObject *
 _font_getmask(ImagingFontObject *self, PyObject *args) {
     Imaging im;
-    Imaging bitmap;
+    Imaging bitmap = NULL;
     int x, b;
     int i = 0;
     int status;
@@ -2730,7 +2742,7 @@
     PyObject *encoded_string;
 
     unsigned char *text;
-    char *mode = "";
+    char *mode;
 
     if (!PyArg_ParseTuple(args, "O|s:getmask", &encoded_string, &mode)) {
         return NULL;
@@ -2753,10 +2765,13 @@
     b = self->baseline;
     for (x = 0; text[i]; i++) {
         glyph = &self->glyphs[text[i]];
-        bitmap =
-            ImagingCrop(self->bitmap, glyph->sx0, glyph->sy0, glyph->sx1, glyph->sy1);
-        if (!bitmap) {
-            goto failed;
+        if (i == 0 || text[i] != text[i - 1]) {
+            ImagingDelete(bitmap);
+            bitmap =
+                ImagingCrop(self->bitmap, glyph->sx0, glyph->sy0, glyph->sx1, glyph->sy1);
+            if (!bitmap) {
+                goto failed;
+            }
         }
         status = ImagingPaste(
             im,
@@ -2766,17 +2781,18 @@
             glyph->dy0 + b,
             glyph->dx1 + x,
             glyph->dy1 + b);
-        ImagingDelete(bitmap);
         if (status < 0) {
             goto failed;
         }
         x = x + glyph->dx;
         b = b + glyph->dy;
     }
+    ImagingDelete(bitmap);
     free(text);
     return PyImagingNew(im);
 
 failed:
+    ImagingDelete(bitmap);
     free(text);
     ImagingDelete(im);
     Py_RETURN_NONE;