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


git-svn-id: svn+ssh://svn.code.sf.net/p/libjpeg-turbo/code/branches/1.3.x@1427 632fc199-4ca6-4c93-a231-07263d6284db

ChangeLog.txt

@@ -44,6 +44,18 @@ for such images are ignored by the decompressor.  However, the TurboJPEG API
 was being too rigid and was expecting the sampling factors to be equal to 1
 before it treated the image as a grayscale JPEG.
 
+[9] Referring to [5] above, 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.
+
 
 1.3.1
 =====

jchuff.c

@@ -391,7 +391,16 @@ 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) { \