port upstream fix for local buffer overflow

r1425

Fix Huffman local buffer overrun discovered by Debian developers when
attempting to transform a junk image using ImageMagick:
https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=768369

ChangeLog.txt

@@ -16,6 +16,18 @@ instead of -1 if width was < 1.
 [8] cjpeg, djpeg, and jpegtran now accept an argument of -version, which will
 print the library version and exit.
 
+[9] Referring to 1.4 beta1 [15], another extremely rare circumstance was
+discovered under which the Huffman encoder's local buffer can be overrun
+when a buffered destination manager is being used and an
+extremely-high-frequency block (basically junk image data) is being encoded.
+Even though the Huffman local buffer was increased from 128 bytes to 136 bytes
+to address the previous issue, the new issue caused even the larger buffer to
+be overrun.  Further analysis reveals that, in the absolute worst case (such as
+setting alternating AC coefficients to 32767 and -32768 in the JPEG scanning
+order), the Huffman encoder can produce encoded blocks that approach double the
+size of the unencoded blocks.  Thus, the Huffman local buffer was increased to
+256 bytes, which should prevent any such issue from re-occurring in the future.
+
 
 1.3.90 (1.4 beta1)
 ==================

jchuff.c

@@ -408,7 +408,16 @@ dump_buffer (working_state * state)
 #endif
 
 
-#define BUFSIZE (DCTSIZE2 * 2) + 8
+/* Although it is exceedingly rare, it is possible for a Huffman-encoded
+ * coefficient block to be larger than the 128-byte unencoded block.  For each
+ * of the 64 coefficients, PUT_BITS is invoked twice, and each invocation can
+ * theoretically store 16 bits (for a maximum of 2048 bits or 256 bytes per
+ * encoded block.)  If, for instance, one artificially sets the AC
+ * coefficients to alternating values of 32767 and -32768 (using the JPEG
+ * scanning order-- 1, 8, 16, etc.), then this will produce an encoded block
+ * larger than 200 bytes.
+ */
+#define BUFSIZE (DCTSIZE2 * 4)
 
 #define LOAD_BUFFER() { \
   if (state->free_in_buffer < BUFSIZE) { \