dmg: prevent chunk buffer overflow (CVE-2014-0145)

Both compressed and uncompressed I/O is buffered.  dmg_open() calculates
the maximum buffer size needed from the metadata in the image file.

There is currently a buffer overflow since ->lengths[] is accounted
against the maximum compressed buffer size but actually uses the
uncompressed buffer:

  switch (s->types[chunk]) {
  case 1: /* copy */
      ret = bdrv_pread(bs->file, s->offsets[chunk],
                       s->uncompressed_chunk, s->lengths[chunk]);

We must account against the maximum uncompressed buffer size for type=1
chunks.

This patch fixes the maximum buffer size calculation to take into
account the chunk type.  It is critical that we update the correct
maximum since there are two buffers ->compressed_chunk and
->uncompressed_chunk.

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
