block/dmg: validate chunk size to avoid overflow

Previously the chunk size was not checked, allowing for a large memory
allocation. This patch checks whether the chunks size is within the
resource fork length, and whether the resource fork is below the
trailer of the dmg file.
Signed-off-by: NPeter Wu <peter@lekensteyn.nl>
Reviewed-by: NJohn Snow <jsnow@redhat.com>
Message-id: 1420566495-13284-6-git-send-email-peter@lekensteyn.nl
Signed-off-by: NStefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: NKevin Wolf <kwolf@redhat.com>

block/dmg.c

@@ -317,7 +317,7 @@ static int dmg_read_resource_fork(BlockDriverState *bs, DmgHeaderState *ds,
         ret = read_uint32(bs, offset, &count);
         if (ret < 0) {
             goto fail;
-        } else if (count == 0) {
+        } else if (count == 0 || count > info_end - offset) {
             ret = -EINVAL;
             goto fail;
         }
@@ -377,6 +377,11 @@ static int dmg_open(BlockDriverState *bs, QDict *options, int flags,
     if (ret < 0) {
         goto fail;
     }
+    if (rsrc_fork_offset >= offset ||
+        rsrc_fork_length > offset - rsrc_fork_offset) {
+        ret = -EINVAL;
+        goto fail;
+    }
     if (rsrc_fork_length != 0) {
         ret = dmg_read_resource_fork(bs, &ds,
                                      rsrc_fork_offset, rsrc_fork_length);