Fix BLP DOS -- CVE-2021-28678

* BlpImagePlugin did not properly check that reads after jumping to
  file offsets returned data. This could lead to a DOS where the
  decoder could be run a large number of times on empty data
* This dates to Pillow 5.1.0

Tests/test_file_blp.py

@@ -17,3 +17,22 @@ def test_load_blp2_dxt1():
 def test_load_blp2_dxt1a():
     with Image.open("Tests/images/blp/blp2_dxt1a.blp") as im:
         assert_image_equal_tofile(im, "Tests/images/blp/blp2_dxt1a.png")
+
+
+@pytest.mark.parametrize(
+    "test_file",
+    [
+        "Tests/images/timeout-060745d3f534ad6e4128c51d336ea5489182c69d.blp",
+        "Tests/images/timeout-31c8f86233ea728339c6e586be7af661a09b5b98.blp",
+        "Tests/images/timeout-60d8b7c8469d59fc9ffff6b3a3dc0faeae6ea8ee.blp",
+        "Tests/images/timeout-8073b430977660cdd48d96f6406ddfd4114e69c7.blp",
+        "Tests/images/timeout-bba4f2e026b5786529370e5dfe9a11b1bf991f07.blp",
+        "Tests/images/timeout-d6ec061c4afdef39d3edf6da8927240bb07fe9b7.blp",
+        "Tests/images/timeout-ef9112a065e7183fa7faa2e18929b03e44ee16bf.blp",
+    ],
+)
+def test_crashes(test_file):
+    with open(test_file, "rb") as f:
+        with Image.open(f) as im:
+            with pytest.raises(OSError):
+                im.load()

src/PIL/BlpImagePlugin.py

@@ -286,33 +286,36 @@ def decode(self, buffer):
             raise OSError("Truncated Blp file") from e
         return 0, 0
 
+    def _safe_read(self, length):
+        return ImageFile._safe_read(self.fd, length)
+
     def _read_palette(self):
         ret = []
         for i in range(256):
             try:
-                b, g, r, a = struct.unpack("<4B", self.fd.read(4))
+                b, g, r, a = struct.unpack("<4B", self._safe_read(4))
             except struct.error:
                 break
             ret.append((b, g, r, a))
         return ret
 
     def _read_blp_header(self):
-        (self._blp_compression,) = struct.unpack("<i", self.fd.read(4))
+        (self._blp_compression,) = struct.unpack("<i", self._safe_read(4))
 
-        (self._blp_encoding,) = struct.unpack("<b", self.fd.read(1))
-        (self._blp_alpha_depth,) = struct.unpack("<b", self.fd.read(1))
-        (self._blp_alpha_encoding,) = struct.unpack("<b", self.fd.read(1))
-        (self._blp_mips,) = struct.unpack("<b", self.fd.read(1))
+        (self._blp_encoding,) = struct.unpack("<b", self._safe_read(1))
+        (self._blp_alpha_depth,) = struct.unpack("<b", self._safe_read(1))
+        (self._blp_alpha_encoding,) = struct.unpack("<b", self._safe_read(1))
+        (self._blp_mips,) = struct.unpack("<b", self._safe_read(1))
 
-        self.size = struct.unpack("<II", self.fd.read(8))
+        self.size = struct.unpack("<II", self._safe_read(8))
 
         if self.magic == b"BLP1":
             # Only present for BLP1
-            (self._blp_encoding,) = struct.unpack("<i", self.fd.read(4))
-            (self._blp_subtype,) = struct.unpack("<i", self.fd.read(4))
+            (self._blp_encoding,) = struct.unpack("<i", self._safe_read(4))
+            (self._blp_subtype,) = struct.unpack("<i", self._safe_read(4))
 
-        self._blp_offsets = struct.unpack("<16I", self.fd.read(16 * 4))
-        self._blp_lengths = struct.unpack("<16I", self.fd.read(16 * 4))
+        self._blp_offsets = struct.unpack("<16I", self._safe_read(16 * 4))
+        self._blp_lengths = struct.unpack("<16I", self._safe_read(16 * 4))
 
 
 class BLP1Decoder(_BLPBaseDecoder):
@@ -324,7 +327,7 @@ def _load(self):
             if self._blp_encoding in (4, 5):
                 data = bytearray()
                 palette = self._read_palette()
-                _data = BytesIO(self.fd.read(self._blp_lengths[0]))
+                _data = BytesIO(self._safe_read(self._blp_lengths[0]))
                 while True:
                     try:
                         (offset,) = struct.unpack("<B", _data.read(1))
@@ -346,10 +349,10 @@ def _load(self):
     def _decode_jpeg_stream(self):
         from PIL.JpegImagePlugin import JpegImageFile
 
-        (jpeg_header_size,) = struct.unpack("<I", self.fd.read(4))
-        jpeg_header = self.fd.read(jpeg_header_size)
-        self.fd.read(self._blp_offsets[0] - self.fd.tell())  # What IS this?
-        data = self.fd.read(self._blp_lengths[0])
+        (jpeg_header_size,) = struct.unpack("<I", self._safe_read(4))
+        jpeg_header = self._safe_read(jpeg_header_size)
+        self._safe_read(self._blp_offsets[0] - self.fd.tell())  # What IS this?
+        data = self._safe_read(self._blp_lengths[0])
         data = jpeg_header + data
         data = BytesIO(data)
         image = JpegImageFile(data)
@@ -370,7 +373,7 @@ def _load(self):
             # Uncompressed or DirectX compression
 
             if self._blp_encoding == BLP_ENCODING_UNCOMPRESSED:
-                _data = BytesIO(self.fd.read(self._blp_lengths[0]))
+                _data = BytesIO(self._safe_read(self._blp_lengths[0]))
                 while True:
                     try:
                         (offset,) = struct.unpack("<B", _data.read(1))
@@ -384,20 +387,20 @@ def _load(self):
                     linesize = (self.size[0] + 3) // 4 * 8
                     for yb in range((self.size[1] + 3) // 4):
                         for d in decode_dxt1(
-                            self.fd.read(linesize), alpha=bool(self._blp_alpha_depth)
+                            self._safe_read(linesize), alpha=bool(self._blp_alpha_depth)
                         ):
                             data += d
 
                 elif self._blp_alpha_encoding == BLP_ALPHA_ENCODING_DXT3:
                     linesize = (self.size[0] + 3) // 4 * 16
                     for yb in range((self.size[1] + 3) // 4):
-                        for d in decode_dxt3(self.fd.read(linesize)):
+                        for d in decode_dxt3(self._safe_read(linesize)):
                             data += d
 
                 elif self._blp_alpha_encoding == BLP_ALPHA_ENCODING_DXT5:
                     linesize = (self.size[0] + 3) // 4 * 16
                     for yb in range((self.size[1] + 3) // 4):
-                        for d in decode_dxt5(self.fd.read(linesize)):
+                        for d in decode_dxt5(self._safe_read(linesize)):
                             data += d
                 else:
                     raise BLPFormatError(